Elementary Meteorology Online

Cold spring

2011-03-29T11:36:00.000-04:00

It has been a cold spring in Vermont, even by Northeast Kingdom standards. Yesterday's climate report for Burlington shows a high temperature of 32 F and a low of 22 F compared with normals of 44 and 27 F for this date. It has been like this for a week.

The reason for this is a large-scale high pressure system stalled over the middle of North America. Because wind around high pressure centers blow clockwise and outward from highs, it means we have been getting northwesterly winds across the northeastern U.S. for the entire week. These northwesterly winds have been advecting cold arctic air over us non-stop. The springtime sun is no match for this cold arctic air.

Overview of warm and cold front passage

2009-04-30T04:26:00.012-04:00

Tuesday and Wednesday 27 and 28 April, 2009 saw temperatures fluctuate wildly as a warm and cold front passed over Vermont.The images at right document that passage (see Unisys archive for more maps).

At 0Z 28 April (Fig. 1) a warm front was slowly approaching from the west as soutwesterly winds brought warm, dry air from the southwestern desert states across New York State and into Canada.

Overnight, the warm front would slowly move across Vermont, and by 12 Z was located on the Maine-New Hampshire border (fig. 2). A cold front associated with the same Low pressure system over Quebec approached New England from the west.

This cold front would pass over Vermont within 6 hours, ushering in a much cooler air mass. By 0Z 29 April (Tuesday evening), the cold front had swept over Maine into the Atlantic (Fig. 3).

More links to graphics for this case can be found here.

2009-04-15T16:13:00.003-04:00

Anticyclone center tracks for month of July for North America.

Source: (Zishka and Smith 1980)

2009-04-15T16:11:00.005-04:00

Anticyclone (high pressure) center tracks for month of January for North America.

Most high pressure systems track southeastward across the middle of the continent from Alaska and the Yukon
Most anticycles end their track near Bermuda

Source: (Zishka and Smith 1980)

2009-04-15T16:07:00.005-04:00

Cyclone center tracks for month of July for North America. Four main cyclone tracks are indicated according to their regions of origin.

Cyclones have a more northerly track mostly across northern Canada.
Cyclone tracks generally ends near Greenland and Iceland

Source: (Zishka and Smith 1980)

2009-04-15T16:00:00.004-04:00

Cyclone center tracks for month of January for North America. Four main cyclone tracks are inicated according to their regions of origin:

Pacific lows
Alberta clippers
Colorado lows
Hatteras lows (Atlantic cyclones)

Source: (Zishka and Smith 1980)

Polar and subtropical jets

2009-04-15T15:00:00.005-04:00

A jet is defined as any core of fast moving air, often indicated by a curved arrows on maps and by television meteorologists. The line is drawn in the location of the strongest winds. Jetstreams are typically wider and not as distinct, but more of a region where the wind increases toward a core of the strongest wind. One way of visualizing this is consider a river. The river's current is generally the strongest in the center with decreasing strength as one approaches the river's bank. It can be said that jetstreams are "rivers of air".

Observation of winds from rawinsondes show that there are two principle regions in the upper troposphere where winds are strongest. The first is in the 50°-60° N/S region and is called the polar jet and is typically associates with the polar front. The second is called the subtropical jet and is located around 30°N. Jet streams vary in height of four to eight miles (about 500 to 200 mb) and reach a maximum at the tropopause, reaching speeds of more than 275 mph.

The actual appearance of jet streams result from the complex interaction between many variables - such as the location of high and low pressure systems, warm and cold air, and seasonal changes. They meander around the globe, dipping and rising in altitude/latitude, splitting at times and forming eddies, and even disappearing altogether to appear somewhere else. Jet streams also "follow the sun" in that as the sun's elevation increases each day in the spring, the jet streams shifts north moving into Canada by Summer. As Autumn approaches and the sun's elevation decreases, the jet stream moves south into the United States helping to bring cooler air to the country.

Source: NWS

Midlatitude Surface systems

2009-04-15T14:48:00.007-04:00

Midlatitude surface systems consist of continental-scale high and low pressure systems.

1. Can you summarize the three most important aspects of the wind circulation around high and low pressure systems?

2. What are the results of these wind circulations for cloud and precipitation formation of these characteristics associated with high and low pressure systems?

3. What are some the consequences of temperature advection associated with each of these systems?

Solution:

	High (anticyclone)	Low (cyclone)
1.	a) clockwise b) outward c) downward	a) counterclockwise b) inward c) upward
2.	downward circulation associated with clear conditions	upward circulation associated with cloud/precipitation
3.	warm advection to west (left in picture) cold advection to east (right in picture)	warm advection to east (right in picture) cold advection to west (left in picture)

A greenhouse gas

2009-03-26T02:41:00.004-04:00

Water vapor is a greenhouse gas, meaning that it absorbs outgoing longwave (IR) radiation and reemits it to the surfac of the Earth. The image at left shows the absorptivity of water vapor with respect to various wavelengths of ultraviolet, visible, and infrared radiation. Low values indicate transparency. For example, to visible light, water vapor is invisible. It is also almost invisible to the IR4 channel, the channel used to generate most IR satellite imagery. However, to wavelengths in the IR3 (water vapor) channel, about 50% of the IR is absorbed.

The IR3 channel is used to generate water vapor imagery. Please read the details at the following links:

Water vapor equilibrium

2009-03-25T22:43:00.006-04:00

Given that water boils at 100 C (212 F) and that no place on Earth ever reaches that temperature under normal atmospheric conditions, an astute student might ask why there is any water vapor in the atmosphere at all? The answer is that just because the oceans don't boil, doesn;t mean there is no evaporation going on.

To picture and understand why, consider a "dry" atmosphere (without water vapor) above a warm tropical ocean at 300 K (27 C or 90 F) . Random motions will result in collisions between water molecules and with air molecules. There will always be some water molecules that pick up enough energy from these collisions to escape into the atmosphere. In other words, these water molecules evaporate, and the number of water molecules in the atmosphere increases, adding to the atmosphere's humidity.

At a certain point, because the energy of the system is limited, a number of water molecules in the atmosphere will begin to lose enough energy to condense back to liquid form. If condensation occurs in the atmosphere, falling precipitation will return the water to the ocean surface. Eventually, the atmosphere-ocean system reaches an equilibrium, where evaporation is balanced by condensation and precipitation. Here, the the atmosphere will become saturated with respect to water vapor, unable to sustain more water molecules. This balance occurs in the marine boundary layer, a small humid layer above the ocean or large body of water. Ascending updrafts are required to remove humidity from this layer to moisten the layer above the marine layer.

Adding heat to the system and increasing the temperature of the ocean (and atmosphere) to 305 K changes the equilibrium. With more energy, more water molecules will escape the ocean into the marine boundary layer, increasing evaporation. The number of water molecules in the marine boundary layer will increase. Because energy is still limited, precipitation and condensation increase to maintain a new equilibrium. Warmer temepratures mean a more active water cycle.

Global Water Reservoirs

2009-03-25T16:30:00.002-04:00

This table shows that most water in the planetary system is stored in the world's oceans and polar ice. The atmosphere only holds a small fraction if the earth's water, mostly in the form of water vapor and cloud.

Note that rivers and streams contain less water than the atmosphere. This makes sense when you consider that rivers and streams are fed exclusively by water vapor in the atmosphere, so that the atmosphere actually represents an ultimate reservoir for rivers and streams.

Monitoring weather and forecasts

2009-03-18T12:11:00.004-04:00

The following forecast for 12Z tomorrow morning map shows the evolution of the surface systems at this time.

You can monitor surface conditions for today by looking outside the window or using meteograms and observations at these two links:

Charts used in midterm

2009-03-18T11:56:00.009-04:00

The following charts for this morning (12Z 18 March 2009) will be used for questions on the exam. We will be analyzing the effects of an approaching cold from associated with a low pressure system over northeastern Canada and a high pressure system over northwestern Canada.

Passage of a cold front: Meteogram

2009-03-11T23:17:00.006-04:00

The following shows a meteogram for Barre/Montpelier from 2 UTC 11 March and 2 UTC 12 March, 2009 (10 PM Tuesday to 10 PM Wednesday). Wind barb and cloud cover symbols show southerly winds and overcast conditions early in the period. The temperature (top line in top panel) stays relatively constant at 35 F for most of the evening. With sunrise at 11Z the temperature gradually climbs to a little over 40 degrees at 19Z.

At 20Z (4 PM EDT) the clouds begin to dissipate and the winds begin to shift towards the west as the occluded/cold front moves in. The dewpoint (lower line on upper panel) plummets as drier air moves in, although the temperature actually rises to a sharp peak of 49 F at 22 Z. After this, cold advection takes over and the temperature begins to fall rapidly.

The pressure trace is also consistent with the passage of a low pressure trough. Precipitation occurs between 5Z and 13 Z early in the morning as pressures drop. Pressure bottoms out at 20Z with the arrival of the front.

Finally, the weathercam image for Wednesday shows how dramatically the frontal passage changes sky conditions. For most of the day, a low deck of clouds moves rapidly from right to left (south to north) across the screen. At around 21Z, the front arrives and the clouds rapidly dissipate. They also begin to move into the page, indicating that the wind has shifted to the west. The setting sun can be seen briefly lighting Burke montain and the underside of the dissipating cloud. Nice sunset.

Passage of cold front: Local Maps

2009-03-11T23:08:00.003-04:00

Local maps for 18Z (2PM) and 0Z (8PM) show the eastward passage of the occluded front over Vermont. It transforms into a cold front during this period. The winds shifts from southerly ahead of the front to westerly behind the front.

The second cold front approaching the U.S. from Canada weakens during this period. The dashed cold front symbol means that the front is dissipating. Winds behind this front are strong, ranging from 10 to 20 knots. As temperatures are also considerably colder in this region (in the 10's and 20's) than over New England (40's), we can expect cold advection by the westerly winds to cause temperatures to fall at least to these levels.

Passage of cold front: Overview

2009-03-11T22:05:00.013-04:00

Wed. 11 March 2009 analysis shows that the weather maker for the northeast is a deep low pressure system (central pressure 976 mb) moving over Northern Quebec. It is following a track typical for Colorado cyclones (see red arrow). The system formed east of the Colorad0 rockies on Tuesday evening, moving rapidly northeastward across Lake Superior to it's current position. An occluded front (purple) extends southward from the system into New York State at this time. An additional cold front extends to the Great Lakes. The counterclockwise circulation around the low pressure system indicates a northwesterly circulation behind the fronts, suggesting that the front will bring in colder temperatures to the northeast.

2009-03-05T01:53:00.004-05:00

The trace at left shows the effect of radiative disequilibrium between solar and terrestrial radiation ( (i.e. the imbalance between the intensity of visible and infrared light) on daily temperature variations. Note that it is very similar to current conditions as the graph is valid for the equinoxes (March 21 and September 21).

The green line shows the effect of solar radiation. It is only active after sunrise, peaks at noon, and dissappears after 6 PM.
The purple line shows the intensity of terrestrial IR radiation. It closely follows that of surface temperature (red line) since the amount of IR emitted by the earth is directly related to its temperature (Stefan-Boltzman Law).
The red shading shows the time of day when the incoming solar radiation is stronger than the outgoing longwave radiation (OLR). Because of this imbalance, the surface temperature increases during this period.
The blue shading shows the time of day when OLR is more intense than the incoming solar radiation. At this time of day, the surface temperature decreases.
Note that the peak temperature is reached when OLR and solar radiation are equal. This is usually about two hours after the solar noon (2 PM EST or 3PM EDT).
Note that the temperature drops most quickly early in the eveing and gradually tapers off to a minimum just before sunrise.
The temperature rises most quickly just after sunrise owing to the minimum in OLR intensity and the rapid increase in the intensity of solar radiation.

Note that the trace for LSC in the previous posting varies from the ideal situation depicted here. The cloud that pushed in around noon blocked out the sun, weakening the solar radiation and halting the temperature rise. The cloud also absorbs OLR, reemitting heat radiation back to the surface of earth early in the evening (see figure). This slows the dip in temperature early in the evening.

2009-03-05T01:37:00.007-05:00

Sounding data for Albany, NY at 0Z 4 mar, 12Z 4 Mar shows that the daily temperature variation is in fact restricted to the very lowest layer of the atmosphere. At 0Z, the surface temperature at Albany is just about -8 C, whereas at 12 Z it has decreased to about -11 C. Note however, that above 950 mb, the temperature has barely budged. For example, at 950 mb the temperature remains constant at about -16 C.

This is typical of diurnal (daily) temperature fluctuations. These are restricted to a region of the troposphere called the boundary layer. Although the height of this layer can vary depending on weather conditions, it typically is restricted to below 850 mb. It's common to use temperature data from above this layer (usually 850 mb maps) to determine temperature trends based on temperature advection to remove the effect of the sun.

Data is taken from the University of Wyoming website.

2009-03-05T00:11:00.004-05:00

The situation at 11 PM shows that winds have become calm over much of New England, with clears skies dominating the region as the high pressure system begins to dominate. Light winds and clear skies result in rapid temperature falls throughout the region. Where there is a weak wind, it tends to be westerly.

The time series below is for the last 24 hours at Littleton-Whitefield airport (click here for current conditions). Note that after 7 PM, with calm, clear conditions, that the temperature plummets by 26 degrees 15 F to -11 F as outgoing longwave radiation travels into outer space.

	Time EST (UTC)	Temperature F (C)	Dew Point F (C)	Pressure Inches (hPa)	Wind MPH	Weather
Latest	Midnight (5) Mar 05	-6.0 (-21.1)	-11.0 (-23.9)	30.26 (1024)	Calm
	11 PM (4) Mar 04	-2.9 (-19.4)	-9.0 (-22.8)	30.24 (1024)	Calm
	10 PM (3) Mar 04	-0.9 (-18.3)	-6.0 (-21.1)	30.23 (1023)	Calm
	9 PM (2) Mar 04	1.9 (-16.7)	-2.9 (-19.4)	30.22 (1023)	Calm
	8 PM (1) Mar 04	8.1 (-13.3)	1.0 (-17.2)	30.22 (1023)	Calm
	7 PM (0) Mar 04	15.1 (-9.4)	3.0 (-16.1)	30.2 (1022)	W 7
	6 PM (23) Mar 04	18.0 (-7.8)	5.0 (-15.0)	30.18 (1022)	NW 10
	5 PM (22) Mar 04	19.9 (-6.7)	3.9 (-15.6)	30.15 (1020)	WNW 15
	4 PM (21) Mar 04	19.9 (-6.7)	3.9 (-15.6)	30.14 (1020)	W 16
	3 PM (20) Mar 04	21.0 (-6.1)	3.9 (-15.6)	30.13 (1020)	WNW 16
	2 PM (19) Mar 04	19.9 (-6.7)	3.9 (-15.6)	30.14 (1020)	WNW 16
	1 PM (18) Mar 04	19.0 (-7.2)	5.0 (-15.0)	30.15 (1020)	W 10
	Noon (17) Mar 04	18.0 (-7.8)	3.0 (-16.1)	30.17 (1021)	W 12
	11 AM (16) Mar 04	16.0 (-8.9)	1.0 (-17.2)	30.19 (1022)	W 8
	10 AM (15) Mar 04	12.9 (-10.6)	-0.9 (-18.3)	30.21 (1023)	WNW 5
	9 AM (14) Mar 04	3.9 (-15.6)	-0.9 (-18.3)	30.22 (1023)	Calm
	8 AM (13) Mar 04	-9.9 (-23.3)	-11.9 (-24.4)	30.23 (1023)	Calm
	7 AM (12) Mar 04	-17.0 (-27.2)	-23.1 (-30.6)	30.23 (1023)	Calm
	6 AM (11) Mar 04	-18.0 (-27.8)	-23.1 (-30.6)	30.21 (1023)	Calm
	5 AM (10) Mar 04	-17.0 (-27.2)	-22.0 (-30.0)	30.2 (1022)	Calm
	4 AM (9) Mar 04	-15.0 (-26.1)	-22.0 (-30.0)	30.18 (1022)	Calm
	3 AM (8) Mar 04	-14.1 (-25.6)	-20.9 (-29.4)	30.18 (1022)	Calm
	2 AM (7) Mar 04	-14.1 (-25.6)	-18.9 (-28.3)	30.19 (1022)	Calm
Oldest	1 AM (6) Mar 04	-11.9 (-24.4)	-18.0 (-27.8)	30.19 (1022)	Calm
	Time EST (UTC)	Temperature F(C)	Dew Point F(C)	Pressure Inches(hPa)	Wind (MPH)	Weather

2009-03-04T23:11:00.004-05:00

The image at left is for 15Z (10 AM EST) Wed. March 4, 2008. New England is under the influence of a high pressure system over Virginia and a low pressure system over Northern Labrador. Animation loops at archived sites show that these are stationary systems. Note that the counterclockwise flow around the low pressure system reaches into New England leading to north-northwesterly winds. Animation for earlier in the week (see link) shows that this is the same system that brought the northeast coast over a foot of snow.
A closer look at New England at 21Z confirms a trough (dashed orange line) swinging through from the north. This elongated region of weak low pressure extends southward form the low pressure system over Labrador and explains the decrease in pressure and snow showers onserved over LSC in the afternoon. Temperatures directly to the north of Vermont are not appreciably different, but temepratures further to the northwest are slightly colder (in the mid teens). To the west of Vermont in western New York State and Ontario temperatures are slightly warmer. A slight change in wind direction to the west could lead to warm advection.

Temperatue and weather at LSC weather station

2009-03-04T21:57:00.005-05:00

Weather summary for Wed. March 4, 2009 at LSC indicates the following high and low
Hi temp: 22.2F (-5.4C) at 13:55
Low temp: -11.7 F (-24.3C) at 06:10

Overnight temperatures were falling continuously (blue) before the low at 6:10 AM shortly before sunrise. This indicates a relatively clear night in which outgoing longwave radiation (OLR) was able to radiate into outer space without clouds interfering.

The sharp 30 degree rise in temperatures during the morning hours (red) indicates strong solar warming associated with clear skies and higher noontime solar angles.

Finally, temperatures in the early afternoon (green) are relatively constant in the lower 20's after about noon . Indeed, a slight dip in temperatures can be observed at around noon. The pressure trace shows a very slight dip (about 2 or 3 mb) in the pressure trace.

Comparison to temperature and pressure data shows a clear morning with light winds and a sudden appearance of snow over Burke Mountain around noon. This corresponds to pressure fall and constant afternoon temperature. Click here to retrieve larger image at webcam archive.

2009-02-19T04:57:00.003-05:00

The link at the image at left shows a 30 hour loop ending 0Z Thursday 19 Feb, 2008. It shows that the cloud over New England is associated with a low pressure system moving in from Colorado. Systems that follow this track are known as Colorado lows. They usually yield moderate precipitation, although this on seemed a little drier than usually. An occluded front (purple) extends southward from the low centered over Southern Ontario on 0Z Thursday. The warm front appears to further south. A cold front extends until the Texas coast. Ahead of the cold front, thunderstorms break out over the southeastern U.S.

A usual, the flow around the low is counterclockwise and inward. Note that southerly flow east the low pressure system brings moisture and low-level cloud northward. At the same time, the cloud deck appear to move in from the west as the jet stream moves the whole system eastward.

Local satellite images for Wednesday afternoon

2009-02-19T02:29:00.003-05:00

The image at left shows the visible satellite at 21 UTC Wednesday, February 18 2009. Clicking on the image will link you to a a visible satellite loop of the entire afternoon after 17Z.

The loop shows thin, scattered cloud moving in early from the west. Later in the afternoon, a thicker, more extensive wave of cloud moves in, also from the west. This wave of cloud is the one that brought with it light snow.

The second image shows an IR image for the same time. It links to a loop of IR imagery. It confirms the double wave of cloud, and that the second wave appears to be more extensive. In addition, comparing the cloud shading to the temperature scale at the bottom scale at the bottom shows that the cloud tops have a temperature of about -40 C.

Current and archived imagery can be found at the UCAR's satellite page.

Wednesday's WeatherCam

2009-02-18T21:15:00.004-05:00

The above WeatherCam image is for Wed Feb 18, 2008. The time in UTC is displayed at the top right of the animation. A larger animation can be viewed at the LSC Meteorology site webcam archive. It documents a weak storm system moving into the NEK and most of New England from the southwest. The camera faces eastward from the college campus towards Burke Mountain. Clouds moving into the page therefore mover from west to east, and those moving from right to left move form south to north.

The day begins with clear conditions and occasional high level cloud moving across the region from the northwest. Around 11 AM (16 Z) a mid level deck of cloud (at about 5 km or 15 000 feet)moves in from the west with the jet stream. Around 18Z. a lower, thicker deck of clouds moves in rapidly from the south. By sunset around 5 PM (22Z), this deck has lowered to the top of Burke Mountain (about 4000 ft). Snow begins to fall and obstruct the view of the mountain in the final few frames of the animation.

Local data

2009-02-11T23:13:00.006-05:00

LSC data for Wed. 11 Feb. 2008 is posted in the image at right. the temperature reached a balmy 52.2 F (11.2C) at 14:45 in the afternoon. The dewpoint (purple line) rose consistently throughout the day as the air became more humid. The temperature (red line) fell and relative humidity (green line) rose abruptly at around 4 PM *(16:00 EST) . The dewpoint and temperature met at about 35 F as the relative humidity hit 100%, an indication of fog and precipitation.

The approach of the upcoming storm is announced by the increasingly sharp drop in pressure (blue line). The total drop in pressure appears to be about 15 mb for the day. Winds actually appear to have died down in the evening after peaking in the late afternoon.

Data from Littleton/Whitefield Airport, NH (HIE) confirms a high of 52.0 F and the onset of rain and fog at about 5 PM EST. A similar plunge in sealevel pressure is also observed. The rain gauge detected 0.10 inches of precip before 0Z (7PM EST).

Local precipitation

2009-02-11T22:24:00.005-05:00

A close up of the situation over the northeast U.S. at 1Z Thursday 12 Feb, 2008 confirms a broad zone of overcast skies and light rain to the north of warm front cutting right across central New England and New York. In addition, Morrisville, VT (MVL) reports fog and Mount Washington, NH (MWN) reports freezing rain. Winds are either calm or from the southeast.

In contrast, behind the front in Massachusetts and Connecticut winds are southwesterly, with some stations actually report clear skies.

You link to the map of local stations to locate relevant stations.

February showers

2009-02-11T21:08:00.006-05:00

Today's rain is associated with a low pressure system and associated warm front moving in from the southwest. On the surface analyses for 12 Z (7AM) Wednesday 11 February, 2008 (left) the low pressure system was located over the center of the U.S. A warm front extended northeastward into West Virginia. A south-southesterly flow ahead of the storm system (red arrows) transports warm, humid air from the Gulf towards the warm front. Meteorologists refer to this flow as a vapor channel. The humidity falls as precipitation to the north of the front (black circle). This is typical of of midlatitude winter storms. Light to moderate precipitation typically falls in broad regions north of warm fronts to the northeast of the low pressure center.

By 00Z Thursday 12 Feb. 2008, the storm center has moved into the Ohio valley. The vapor channel from the Gulf of Mexico is cut off by the cold front moving eastward across the southeastern states. Moist air from the Atlantic continues to stream northward ahead of the storm system (red arrows). Precipitation continues to fall ahead of the warm front (shown as a stationary front over northern New England (black circle).

This precipitation occurs because warm humid air overruns the cold air ahead of the front when it reaches the front. This leads to cloud formation and precipitation. This falls ahead of the front as frozen precipitation if the surface temperature is below freezing and rain of the surface is above freezing. See "Precipitation along warm front" tutorial for more information.

Record cold for Feb 5?

2009-02-05T01:30:00.004-05:00

Record low for Saint Johnsbury is -36 F. It is currently -6 F as of 1 AM. We're not even close.

High pressure system and eastern cold surge

2009-02-04T22:40:00.009-05:00

The image at right is for 0Z Thursday 05 Feb
2009 (7PM EST Wednesday 04 Feb 2009). A line of high pressure systems over the eastern U.S. dominates the eastern half of the continent. A line of high pressure centers is called a ridge. The circulation around this ridge is clockwise and outward. This means that to the east of the high the winds blow from northwest and push in cold air (blue arrows). To the west of the highs, winds blow from the southeast (red arrows). Note that the cold arctic air is transported all the way to southern Florida behind a cold front. The temperature at Savannah Georgia is approaching freezing (36 F).

The second image shows a forecast valid one day later at 0Z Friday 06 Feb 2009 (7PM EST Thursday 05 Feb 2008). The high pressure centers have moved southeastward to the Florida panhandle. The associated ridge to the north shifts eastward, extending across New York State into Canada, indicating that pressure over Vermont and New England will continue to rise. Cold northwesterly winds will continue through all of Thursday. A low pressure system forms over southern Alberta (an Alberta clipper!).

The third image shows a forecast valid two days later at 0Z Saturday 07 Feb 2009 (7PM EST Friday 06 Feb 2008). The high pressure system has moves souteastward to the coast of northern Florida. As a result, a south or southwesterly wind now dominates much of the eastern part of the U.S., bringing with it warmer air. It appears that this will warm up temperatures over the weekend starting Saturday morning.

The dark black arrow indicates the path of the Alberta Clipper moving into the Dakotas from southern Alberta. The counter clockwise circulation around the low enhances the southerly winds to the east of the system. This low pressure system will bring it with cloud and light precipitation as it moves rapidly across the continent this weekend.

Cold air and high pressure

2009-02-04T22:14:00.005-05:00

The trace of weather data from LSC for Wed. Feb 5, 2008 indicates rising pressures and falling temperatures. As of 10:30 PM, the temperature is already -3 F, colder than the overnight low of 2 F the previous morning. The pressure trace shows a rise in pressure to about 1015 mb currently from about 10 mb from 1005 mb at midnite the previous evening. (Note however that the pressure values are low compared to other stations, indicating possible problems with the barometer). In the bottom panel, sustained winds (blue) and gusts (purple) become calmer and calmer as the high pressure moves in.

Warm layers and precipitation type near Portland, Maine

2008-12-13T14:36:00.000-05:00

The image at left illustrates how a warm layer (above freezing) aloft affects precipitation type. Where temperatures are below freezing everywhere, snow falls. Where the layer is above freezing near the surface, rain falls. In between, the type of frozen precipitation depends on teh depth of warm and cold layer. Warm layers need to be deep enough to melt the snow, and cold surface layers need to be deep enough to freeze the melted precipitation.

The images below show the evolution of the warm layer ahead of the coastal warm front for 24 hours on Friday, 12 December, 2008 near Portland Maine. Note that an extra radiosonde was launched at 6UTC (1 AM EST) to document this exceptional event. The freezing line is marked by a bold blue line at 0C (32 F).

At 0Z (7PM Thursday), a small warm layer above freezing can be seen forming at 850 mb. An inversion can be seen through most of the layer below this level. This would likely produce snow mixed with some ice pellets and freezing rain.

By 6Z (1 AM Friday morning) the warm layer has expanded to include the entire 900-700 mb layer. Southwesterly winds keep this layer warm, whereas the cold layer below 900 mb is sustained by cold northeasterly winds.

By 12Z, the warm layer reaches temperature of 10 C (50 F) while surface temperatures hover around freezing. Saturated conditions can be observed throughout the entire troposphere during this entire period. At rhis point on Friday morning, the warm layer and associated frozen precipitation extended all the way across the Northeast Kingdom, albeit for a very brief period.

By 0Z 13 December (Friday 7PM), cold advection and northwesterly winds are pushing out the warm layer, ending the ice storm. Note that temperatures everywhere are zero, and that conditions are far drier than before.

Ice Storm surface map

2008-12-13T14:26:00.000-05:00

A New England surface map for 12Z (7AM Friday Dec 12, 2008 (click image for larger map) at the height of the storm shows a low pressure center tracking along a coastal front. It is a warm front because it is being advected toward the coast by the southerly winds in the warm sector of the low pressure system. This front had been in stationary for the previous two or three days. These winds overrun the front and the White mountains of New Hampshire. Cold air remains wedged northwest of the front in the region of strong precipitation northeast of the front. Freezing rain and sleet at this point extended all the way across Vermont into Canada. The rawinsonde stations of Albany, NY (ALB), Gray, Maine (GYX) near P0rtland), and Cariboo, Maine (CAR) are marked with boxes. The station in northern New Hampshire is Littleton (HIE).

Cold air damming

2008-12-13T13:39:00.000-05:00

In addition to a warm front, mountains help exasperate the situation through cold air damming. Cold air, which is denser than the warm air advected toward the mountain gets trapped east of North American mountain ranges such as the White Mountains in New England and the Appalachians in the Carolinas. The warm moist air then overruns this cold air, generating precipitation. If the warm air is above freezing, rain will fall from the warm layer and freeze in the cold layer.

References:
USA Today
WW2010

Warm fronts and frozen precipitation

2008-12-13T13:20:00.001-05:00

The structure of warm fronts (stationary fronts are similar) is illustrated at left. Warm, humid air from the warm sector of the system is advected over a wedge of cold air. The lift leads to cloud formation, and a layer of warm, humid air above the cold wedge. However, precipitation falls into the colder air, and will freeze if the air is below freezing. Click here for a more detailed explanation of warm fronts.

Ice Storm hits Maine, New Hampshire, and New York

2008-12-13T11:08:00.000-05:00

Friday's storm hit parts of southern New England much harder than the Northeast Kingdom. There are 400, 000 homes and businesses are without power in New Hampshire, and with the an arctic air mass moving this weekend, people are moving into shelters to escape the cold.

Here are a few news stories covering the story:

American Press
Maine News
Nashua Telegraph
Albany Times Union

Stationary and slow moving warm fronts are generally responsible for most severe ice storm events. They are rare, occurring once or twice a decade. The front provides a) lift to drive ascending motion b) cold air to the north or west to freeze the precipitation c) warm air aloft above the cold sector of the storm to melt the falling the precipitation. Because they are slow moving, they concentrate their precipitation fall over one region.

2008-12-11T19:57:00.000-05:00

Frontal analysis for 21 Z Thursday 11 Dec shows that the front pasing over New England yesterday has stalled off the coast. A coastal cyclone will track along this front tomorrow, intensifying the precipitation over New England. This indicates that the precipitation pattern over New England will not change much until the cyclone has passed tomorrow evening. This will be a nasty ice storm in coastal Maine, southern New Hampshire, and Western Massachusetts. If you were planning to drive into these regions tomorrow, don't. Roads will be a sheet of ice.

2008-12-11T18:42:00.000-05:00

Intellicast provides radar imagery identifying precipitation type. The image at upper left represents radar echoes at 23:15 Z (6:15 PM) Thursday. Blue represents snow, red represents mixed precipitation (wet snow, freezing rain, and sleet), and green represents rain. Click here for a current radar loop.

It is just starting to snow over the NEK. Mixed precipitation extends from along the coast of Maine across southern New Hampshire and Vermont, across the Albany area of New York, and into northern Pennsylvania.

The surface plot for approximately the same time confirms that the mixed precipitation is freezing rain. Subfreezing temperatures extend across this region. Cold air continues to be advected into New England by northerly winds.

Monitoring melting layers aloft

2008-12-11T17:45:00.000-05:00

Freezing rain requires surface temperatures below freezing and a warmer layer aloft that is above freezing. For the NEK, the Mount Washington Observatory has weather sensors at various elevations that allow the weather observer to identify a warm layer up to 6000 ft (approximately 800 mb).

As of 6 PM Thursday, the temperature the base of Mount Washington (1600 ft) was 22.5 F and the summit (6288 ft) was 25.7 F, indicating a weak inversion. Precipitation would be falling as snow under these conditions. If th temperature at the summit were above 32 F, we could expect freezing rain or ice pellets.

Weather observations for Mount Washington can be found at this link.

Ice Storm or Snow Storm?

2008-12-11T15:08:00.000-05:00

With cold air firmly in place over Vermont and a strong front well to the southeast of Vermont, the forecast question for tomorrow is whether tomorrow's precipitation will fall as freezing rain or snow. Here are a few pointers for predicting precipitation type using a modern forecasting technique known as ensemble forecasting.

Ensemble forecasts are numerical (i.e. computer) forecasts that have slight differences in the the way the forecast parameters such as temperature, precipitation, and humidity are calculated. Nobody knows ahead of time which precise method is best, so several (21 to be precise) computer model runs are made to see how variable these predictions are. They give forecasters a feel for how prone a numerical forecast are to error. Taking the average gives you a "best guess" at what may happen .

The image at top left (click on image for larger image) shows a 27 hour forecast of the WRF model ensemble forecast for the NE U.S. It is initialized 12 Z (7 AM) Thursday, 11 December, 2008; it is valid 3Z Friday, 12 December, 2008(10 PM Thursday night). Shown is the precipitation type (snow (blue); freezing rain (pink); rain (green). An ice storm will occur somewhere along a large swath extending from Maine to Pennsylvania. The question is whether this swath will make it to the NEK in Vermont. About 1/3 of the runs at 10 PM Thursday show no precipitation, another 1/3 show snow, and yet another 1/3 show freezing rain.

As the blue band of snow is very narrow, skiers and snowboarders will be disappointed. Any snow we get at Burke will likely be mixed with all sorts of sleet and freezing rain. Because there will be a strong inversion, it could be that mountaintops will even experience mostly rain. New Hampshire ski areas are almost certain to get freezing rain, and Jay is likely to get only a little snow early in the day, major snowfall occuring late in the day. The best skiing will likely be at Jay on Saturday morning, if you can make it up there in strong winds and ice cold temperatures.

Three hourly forecasts out to four days are available at the Penn State Meteorology Map Wall. Mouse over the forecast time (F03, F06, F09, ... ) for the forecast of the given time. This will be a major event that will intensify throughout the afternoon. Stay tuned for possible class cancellations Friday morning!

Observations of Jetstream: Surface Maps

2008-09-08T20:03:00.000-04:00

With a little knowledge of geography, you can observe the jet stream on a surface map too! The circled station (click on the map to see details) is from the Mount Washington observatory high atop the mountain about 1.5 miles above sea-level. Whereas winds are calm almost everywhere in New Hampshire, a steady westerly breeze of 25 knots (a little more than 25 mph) is blowing here. You can see a current surface plot of this map here.

Observations of jet stream: Satellite Imagery

2008-09-08T19:38:00.000-04:00

This high resolution visible satellite imagery of VT and HM shows the sun going down at the end of today. It confirms oue obervations of clouds being moved from west to east by the mid-latitude westerlies. A stratocumulus cloud mass (meteorologists sometimes call it a "blob of cloud" for obvious reasons) passes over the Northeast Kingdom of stratocumulus late in the day.

You can see current high resolution visible satellite imagery of New England at the Meteorology Department's Satellite page.

Observations of jet stream: WebCams

2008-09-08T19:00:00.000-04:00

LSC Webcam imagery for Monday, 8 Sept 2008. Click here for current animation.

The LSC webcam is in the staff room on the 4th floor of the Vail Building looking southeastward over the campus in summer and eastward towards Burke Mountain in the winter. It's a good way of watching clouds develop, storms pass, and the day go by. You can guess at which direction the jet stream is moving too.

The clouds seen in this video are generally stratocumulus clouds: sometimes they look like puffy cottonballs (cumulus clouds), and sometimes they flatten out into a flat cover (a stratus deck). It looks like they are low, maybe a mile or two above the surface.

Note that even though the wind was relatively calm all day on the ground, they are moving at a good clip due to a strong westerly jet stream, pushing them towards the east. This is the norm in the middle latitudes. Winds usually increase with height in the lower 10 miles of the atmosphere due to the jet stream.

Severe Storm Reports

2008-09-03T21:07:00.000-04:00

Severe storm reports of damage associated with thunderstorms (mostly) is compiled by the National Weather Service at the Storm Prediction Center (SPC) in Norman, Oklahoma. You can access their website on the course weatherpage. You can view todays reports by clicking on the "Reports" links on the menu at the top of the SPC site. Click here to view the more detailed updated reports for Sept. 3, 2008 (9 PM version shown above). You can click to previous days' reports at the top of the page. Note that what's left of tropical depression Gustav is still triggering some tornado activity and flooding along the southern Mississippi River.

Closer to home, a line of thundershowers seems to be triggering hailstorms in the Berkshires just to the south of Vermont. The radar below for this Wednesday evening shows storms moving southward from Vermont and into the Berkshires (click on image for animation). You may have seen the towering clouds move past us to our west during sunset today. The red colors show the storms that were probably generating the hail.

The direction of the storms is somewhat unusual. Usually storms will move in from the south. It explains why the SPC missed issuing a watch for these. To be fair, they were also probably watching the tornadoes in Louisiana more closely, because they are much more dangerous.

The southward path of the New England storms can be explained by the conterclockwise circulation around the low pressure system situated off the shores of New England. As discussed in class, this results in northerly winds winds FROM the north over New England (see Sea-level Map below). They move the thunderstorms southward into Massachusetts.

2008-07-25T13:19:00.000-04:00

The image at left shows the frequency of Atlantic Hurricanes over the last 100 years as described by the climatology at the NHC Homepage. It shows that while the hurricane is well underway in June and July (the season officially begins June 1), the season really spins up in August and peaks in mid-September. So we can expect an increase in tropical storm activity and intensity throughout August. This peak is due to the fact that ocean waters actually reach their peak temperatures in September (dry land reaches peak temperatures in late July).

This is also accompanied by a slight change in the track of the storms. The July climatology shows tropical storms developing over the West Indies and tracking either to the Gulf of Mexico or along the coast of the Southeastern States. Note that hurricanes Dolly and Cristobal both followed these paths, respectively, in the past week.The August climatology shows an over all intensification of tropical cyclone activity, but also a new track towards Florida not visible in July. As well, storms appear to be more likely to veer northward in the Gulf of Mexico, unlike July storms like Dolly which hook westward towards the coast of Southern Texas and Mexico.

Vapor channel from tropics

2008-07-24T13:12:00.000-04:00

Warm front advances
The northeastern frontal map for 12Z Thu. 24 July, 2008 (left) shows that the stationary front that was over southern New England on Monday is being pushed northward by southerly winds circulating in a counterclockwise direction around a low pressure system over Lake Ontario. Cloudy conditions. Dewpoints in the high 60's almost everywhere in New England indicate humid conditions. Overcast conditions keep temperatures int he 70's all day.

Transport of humidity
The main weather story today (and all of this week!) is the rain and humidity. This is largely because of a channel of water vapor (i.e. humidity) that has opened up between New England and the Gulf of Mexico. This occurs along the narrow band of clouds ahead of a cold front along the Atlantic coast in the image at left (valid 12Z Thu. 24 July, 2008). A cold front extends southward from the low pressure system over southeastern Ontario all the way to the Gulf of Mexico. Red arrows in the image at left show the direction of the warm, humid flow along the Atlantic coast of the U.S. ahead of the cold front. This flow is pushing the warm humid air northward. The cold front is pushed eastward by a surge of cooler, drier westerly winds behind the front (blue arrows).

Warm conveyer belt
This is pretty typical of a mature phase of mid-latitude cyclone development shown at left (see Jetstream section of cyclone model for details). The image shows a warm, humid conveyer belt (red arrows) overrunning cooler air to the north where the warm flow meets the warm front. This rising motion leads to large amounts of precipitation ahead of the warm front. Note that the occluded front (purple at left) does not appear on todays map. Occluded fronts are rare in the summertime because weather systems evolve more slowly, with fronts moving much more slowly. Your text discusses this in more detail in Chapter 10.

Vapor channel in satellite imagery
The vapor channel is easy to see in satellite animations. Click here to see an animated loop of the warm conveyor belt (or vapor channel) discussed above. The cold front can be discerned along the border between clear and cloudy skies in the eastern half of the U.S. The thunderstorms and clouds along the eastern seaboard ahead of the front all moves northward in the fast moving southerly flow ahead of the front. Note also Tropical Storm Dolly along the Gulf coast of Texas/Mexico.

Hurricane/Tropical Storm Dolly

2008-07-24T11:32:00.000-04:00

The image at left shows the current Atlantic tropical storm outlook. Tropical Storm Dolly made landfall along the south Texas coast as a Category 1 Hurricane but has since been downgraded to a tropical storm. The clouds in the yellow circle represent a tropical depression (weaker than a tropical storm) that is slowly moving westward across the Atlantic. This could develop into a tropical storm or hurricane over the weekend, but it is too early to tell. You can find updates to this graphic along with a brief forecast discussion at the National Hurricane Center's Webpage.

The second image shows the swath of tropical storm and hurricane force winds (winds greater than 38 mph and 64 mph, respectively) associated with Dolly. The National Hurricane Center has introduced this map because it shows the real effects of the storm. Dolly picks up energy shortly before it hits the coast and attains hurricane status. Dolly then loses much of its "punch" as it hits land. This is typical of tropical storms. Increased friction over land puts a break on the winds, and the storm gets cut off from the warm waters that give them their energy. This image is also updated at the NHC webpage.
The final image is the station map of the south-central U.S. taken 18Z Wed 23 July, 2008 as Dolly made landfall. This map illustrates how small the storm actually is. There is no station plot actually showing hurricane force winds. This is because the actual region of hurricane force winds is very small, but also because there is little data in the eyewall of a hurricane (hurricane force winds tend to topple wind towers over land, and ocean going ships avoid hurricane centers). Most data from hurricane centers is taken by dropsondes every six hours (dropsondes are like rawinsondes except that they are dropped from reconnaissance aircraft flying out of the NHC headquarters in Miami). You can find regional maps and loops like this one at the HPC North American Surface Analysis webpage.

Steering flow

2008-07-21T22:28:00.000-04:00

Clicking on the image at left will show a 24-hr animation of the stationary front over New England on Monday 21 July, 2008. It is being distorted by small mesoscale disturbances (or low pressure systems) that advect parts of the front northward and southward as they rapidly move eastward. They also trigger local showers as they pass through a given area. These mesoscale systems are in fact smaller than the large synoptic-scale systems we usually track in the other seasons. Recent animations of New England station plots and fronts can be found and the HPC surface analysis page.

These disturbances are steered over New England by the upper flow. Clicking on the image of left for 8 AM EDT Sunday 20 January 2008 shows the jet stream on a 500 mb map dipping south of the Great Lakes and across New England roughly above the stationary front described above. It is this westerly flow that guides system eastward across New England.

The 24 hr precipitation field for 8 AM Sunday through 8 AM Monday reflects the path of small scale distirbances in the steering flow. Precipitation fall in the northern U.S. follows the same curved pattern around the Great Lakes and into New England as the systems guided by the jet stream. These maps (and more) can by found at the Daily Weather Maps webpage of the HPC.

Stationary Front over New England

2008-07-21T16:30:00.000-04:00

Fronts are usually analyzed at the leading edge of cold air. The past few days have seen a stationary front stalled over New England leading, with a moist polar air mass lying to the north and a moist tropical air mass lying to the south . High humidity in these air masses combined with the front itself has given us lots of rain over the past few days.
The image at left shows temperatures taken at 3PM Monday, July 21 2008. Northern New England and Southern Quebec are in a uniform polar air mass where most temperatures (with a few exceptions in the Burlington VT area) lie in the 70's. South of the front, temperatures are more tropical (generally in the 80's).

The station analysis (click on image below for better view) for 18Z (2 PM EDT) on the same date shows dewpoints (green numbers) in the 60's almost everywhere in the northeast, indicating an abundant supply of water vapor. These high humidity conditions make for sticky, uncomfortable weather everywhere. The nightime temperatures are not likely to sink below the dewpoint, meaning warm, clammy nights where you constantly have to flip your pillow to get the cold side (grrrr ... ).

A stationary front extends along the temperature gradient (the area of contrasting temperatures) across New York State and Connecticut. The strong temperature gradient is actually north of the front. Very weak lows associated with cloud and thundershowers are shown over Massachusetts, southern Ontario, and southern Michigan. Weak winds around these systems blow across the stationary front, leading to warm and cold advection. These weak systems are typical of the summer, when extratropical cyclone development is not as well defined as in the winter, making these systems difficult to track. Tropical cyclones, as we will see this week, are much more frequent and obvious at this time of year!

Flooding in Iowa

2008-06-17T12:22:00.001-04:00

You may have been hearing news stories of the disastrous flooding in Cedar Rapids, Iowa.

This is largely due to a stationary front that persisted over the Iowa region for more than a week. Stationary fronts are a lot like like warm fronts in that they result in moderate precipitation over a broad area

2008-06-11T18:58:00.000-04:00

The visible satellite image at left shows the tops of thunderstorms mushrooming along the trough extending southward from the low pressure system over Quebec described in the previous post below. These storms flipped a truck on the Champlain Bridge in the middle of rush hour in Montreal, leading to a 7 truck pile up. Amazingly, no one was killed. Winds also knocked out power at LSC and at my home in Frelighsburg, QC.

A long line of thunderstorms like this is called a squall line and accounts for most of the severe thunderstorms in our area.

The animated visible satellite image shows how quickly these thunderstorms grow in the hour between 18Z and 19Z. It seems that these storms form as the trough hits the Lake Champlain Valley. This rapid formation is why thunderstorms are so difficult to predict. The trough and cold front which triggered the storms could be seen days before, but if these storms erupted an hour later the Champlain Valley and Montreal would have been missed entirely!

High resolution visible satellite satellite imagery for Vermont can be accessed at the LSC Meteorology Department website.

Air mass advection and fronts

2008-06-11T16:10:00.000-04:00

The image at left from 21 Z (5 PM EDT) Tuesday 10 June 2008 will show how air mass advection and fronts around midlatitude cyclones can change the weather in New England.

A low pressure center can be seen in Quebec to the northwest of Vermont. Wind barbs indicate a counterclockwise and inward circulation around the low that is usually seen around a cyclone. A cold front extends soutwestward and a stationary front extends southeastward from the system center.

Three airmasses can be identified in the map. Over Vermont and most of New England, a marine tropical air mass predominates. It's this air mass that brought us near-record high temperatures between the weekend and Tuesday. It is bordered by the cold front to the west and a stationary front to the east, forming a wedge south of the low pressure center. Temperatures are in the 80's and 90's F and dewpoints are in the high 60's. Southerly winds in this air mass advect warm moist air northward in this region all the way from the Gulf of Mexico.

West and northwest of Vermont lies a continental polar air mass, with both temperatures and dewpoints in the 50's and 70's, the cooler temperatures residing to the north. It's this air mass that has moved in today, giving us cooler and drier conditions outside. Winds all have a westerly, with those over Canada having a more northerly component. They advect this airmass eastward. You probably noticed that it cooled off considerably yesterday evening as the front moved through with the storms yesterday.

Finally, over Maine and the Atlantic Ocean, a marine polar air mass had been destroying the hopes of anyone looking for a summer day at the beach. Temperatures and dewpoints along the coast of Maine and the adjacent ocean are all in the 40's and 50's, indicating the strong influence of cool Atlantic waters. It's these air masses that typically lead to ocean fog. Easterly and south easterly winds advect this cooler air onshore. These winds are enhanced by the afternoon seabreeze circulation.

The final feature to notice on the map is the dashed brown line, representing a trough. You can see this also in the isobar looping southward from the center of the low. This was associated with the line of thunderstorms that swept across northern Vermont and southern Quebec yesterday evening and knocked out our power.

You can access a 24 hr loop of New England fronts and weather station data at the NWS-HPC surface analysis website.

Climate data for yesterday

2008-06-11T14:14:00.000-04:00

Clicking the image at left will give you an idea of the high temperatures reached yesterday. It was in the 90's pretty much everywhere in Vermont, with the hot spots being near the south end of the Connecticut Valley and in the southern Champlain Valley region. Mountain stations were cooler, but the 79 F maximum temperature on Mount Mansfield tied a record set for this date in 1974.

The second image shows the total precipitation that fell yesterday afternoon and evening. The effect of the Green Mountains is strong here. The largest precipitation amounts were in excess of one inch in the area of Mount Mansfield (3.67'' at Underhill, 2.52'' at Jeffersonville) and Jay (1.80'' at Jay Peak, 1.33'' at East Berkshire). In the Northeast Kingdom and Connecticut Valley, less that 1'' fell.

Precipitation is a difficult quantity to get a feeling for. Most areas of the northeastern U.S. get on average about 3 inches of rain in a month. So 3'' is about what you would normally expect to get in month. If that amount falls in a day, you're getting a lot of rain!

Both of these examples illustrate climate data that record weather extremes and totals for a given day. Local climate maps can be retrieved at the NWS-Burlington Office website.

Hot Sunday

2008-06-08T20:34:00.000-04:00

Data from the LSC weather station around noon Sunday reads:

Time: 12:35 EST
Temp: 84.3 °F
Dew Point:67.2 °F
RH: 55.8 %

Summer is not a time of year that starts at the June 21 summer solstice and ends September 21 at the fall equinox. Summer in New England happens when southerly winds advect in a marine Tropical (mT) air mass from the Gulf of Mexico.

Clicking on the the mesoscale analysis of temperature at left (see NWS website for updated map) shows the effect of elevation on these temperatures. Temperatures on top of Jay Peak and Mount Mansfield are 79 F, whereas low elevation stations in the southern Connecticut and Champlain valleys are already in the 90's.

Everyone knows that 84 degrees F is hot in the noon day sun. What about a dewpoint of 67 F? What does that feel like? Well, you might note that it makes working outside a sticky experience. The humidity index chart shows that this humidity (corresponding to 57% relative humidity) makes 85 F feel more like 90 F and 90 F feel more like 100 F.

My personal weather gauge, Spooky the Cat, has climbed down from her current warm spot on the sofa and is sprawled out on the floor wondering when this will all end; she wants to go back to flushing out the small burrowing mammals attacking my cabbage patch. I think this means that it's uncomfortable for all species. More on why this is happening on Mondays blog.

Warm, humid air mass in Vermont

2008-06-08T12:00:00.000-04:00

Data from the LSC weather station around noon Sunday reads:

Time: 12:35 EST
Temp: 84.3 °F
Dew Point:67.2 °F
RH: 55.8 %

Summer is not a time of year that starts at the June 21 summer solstice ends September 21 at the fall equinox. Summer in New England happens when southerly winds advect in a marine Tropical air mass is advected in from the Gulf of Mexico. Those of us inVermont is a warm, humid (mT) air mass.

Clicking on the the mesoscale analysis of temperature at left (see NWS website for updated map) shows the effect of elevation on these temperatures. Temperatures on top of Jay Peak and mount Mansfield are 79 F, whereas low elevation stations in the southern Connecticut and Champlain valleys are already in the 90's.

Everyone knows that 84 degrees F is hot in the noon day sun. What about a dewpoint of 67 F? What does that feel like? Well, you might note that it makes working outside a sticky experience. The humidity index chart shows that at this humidity (57% relative humidity), it makes 85 F feel more like 80 F and 90 F feel more like 100 F, My personal weather gauge, Spooky the Cat, has climbed down from her current warm spot on the sofa and is sprawled out on the floor wondering when this will all end and go back to flushing out the small burrowing mammals attacking my cabbage patch. I think that means that it's uncomfortable for all species.

Temperatures at 15Z (11 AM EDT) are already in the low 80's and dewpoints in the and high 60's in Vermont.

The situation is serious.

reads: The map at left shows three large distinct air masses clashing over the northeastern U.S.
between a warm front to the east and a cold front to the west.

By


RH:	55.8 %

80 F is obviously hot.

Cool, humid Sunday

2008-06-01T16:38:00.000-04:00

Another great day for the indoors. Yesterday's low pressure system (see below) has moved northeastward and is now situated over Atlantic Canada. If you remember that the flow is counterclockwise and inward around the low, you'll see that this brings a flow of maritime polar air from the North Atlantic ocean across Quebec and into New England (see red arrows in figure at left). This keeps things cool, cloudy and humid, with a few showers.

Rainy Saturday I (warm front, and low pressure system

2008-05-31T13:14:00.000-04:00

A rainy Saturday. A good day to stay inside and work on your weather studies.

The map at left for 15Z (11 AM) shows us why. We have a low pressure system currently situated just north of the Great Lakes and moving in from the west. A warm front extends eastward and across New England. This is bringing in steady rain of moderate intensity extending over a broad area from New York State to Atlantic Canada. This type of rain is called stratiform precipitation because it falls from a low-level, flat layer of stratus cloud. Rain like this usually lasts about a day because it is so widespread. It takes time for a system this large to move through an area.

As studies in Chapter 1, the wind circulation around this low (red arrows) is counterclockwise and inward. East of the low (i.e. New England and eastern Canada), this circulation causes southerly winds (FROM the south) that bring in a moist tropical air mass from the south behind the warm front. You'll notice that even though it is cloudy outside, it is still pretty warm.

You can find more maps like this at the NWS-HPC website.

Temperature Inversions

2008-05-29T16:03:00.000-04:00

In Chapter 2, we learn that the temperature in the troposphere generally decreases with height at a rate of about 6.5 degrees C/km. These are average conditions, and in fact the temperature profile of the atmosphere varies considerably. In the case of cool, polar airmasses like the one currently over Vermont, there are small layers of the troposphere where the temperature actually increases with height. These are called temperature inversions.
The temperature profile chart at
right (the red line measures temperature) is taken this morning at 12Z (8AM) over Maniwaki, Quebec, and represents the sounding station closest to Vermont from the the North. You will note here that the temperature between 925 and 850 mb (about 2500 to 4500 ft) actually rises from about 6 to 10 degrees C (about 43 to 50 degrees F). On days like this, you'll find that the temperature is actually warmer at the top of a mountain than at its foot!

Charts like this that plot temperature against heigh are called STUVE diagrams. You can accesscurrent charts like this one at the LSC-MET webpage for upper-air data.

High pressure system over North America II

2008-05-28T13:38:00.000-04:00

The image at left provides some additional information about the high pressure system discussed below, adding station plots and radar echoes to the analysis. Wind barbs confirm the circulation of wind around the high pressure center over the Great Lakes. In particular, the wind barb over Portland, Maine confirms a northwesterly flow (i.e. wind coming FROM the northwest) over New England. Remember that wind barbs point in the direction the wind is coming from. They point INTO the wind, whereas the arrows in the post below point in the opposite direction (WITH the wind).

It should be no surprise that it is so cool and clear today in Vermont. It's common knowledge that northerly winds bring in cool, dry air (i.e. continental polar air) from Canada.

The image above is updated hourly the Unisys website. It too can be accessed on the course website.

High pressure system over North America

2008-05-28T12:43:00.000-04:00

The image at left represents a depiction of pressure contours, fronts, high and low pressure systems, and IR satellite imagery from the National Weather Service. It is valid at 8AM (12 UTC) this morning. The continent is dominated by a massive high pressure system that extends into New England. Note the large area of clear skies associated with this system. A band of cloud associated with a long cold front that extends from the Labrador sea to the southeastern U.S. and is moving out to sea. Arrows indicate the direction of winds circulating around the high pressure system. Note how over New England, northwesterly winds (winds blowing FROM the northwest) push the band of cloud out to sea.

Note that this image is updated and archived every six hours at the NWS/HPC Surface analysis archive and can be accessed on the course weatherpage.

Welcome to ProfWW's weatherblog for Spring 2008

2008-01-21T16:27:00.000-05:00

You have managed to get to ProfWW's weatherblog for Lyndon State College's online course in Elementary Meteorology Online. Please watch this space for weather discussion and other discussion topics.