Polar and subtropical jets

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

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.

Steering flow

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.