Tropical Storm Bill

Tropical storm development
Tropical storm development shows how important atmosphere-ocean interactions are in climate and weather. Tropical storms introduce and transport enormous amounts of heat and moisture into the atmosphere.

The video below shows the formation of Tropical Storm Bill in June 15 through June 17, 2015  as it makes it way westward across the the Gulf of Mexico in the easterly trade winds (remember: easterly trade winds flow FROM the East carrying weather systems westward).  Note that this is not a hurricane. A hurricane would be rotating more quickly and have an "eye" in the middle as it intensifies. These systems can cause severe flooding because of the tremendous amount of water that evaporates from the warm ocean surface of ther Gulf of Mexico  in the atmosphere




Current satellite imagery
You will be able to see how this tropical system dies out as it makes it way northward and its moisture and cloud gets advected and dispersed (that is to say transported blown) eastward in the mid-latitude westerlies. A little guide to the imagery is provided below. Note that blue indicates cold cloud tops (the colder the higher) while red generally represents surface temperatures.   You should be able to see the cloud and precipitation due to the moisture of this system if you are in Southern New England.

Link to satellite imagery.

A greenhouse gas

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:

• Univesity of Wisconsin - Satellite images
• Water vapor satellite images
• Hands-on activity
  

Water vapor equilibrium

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

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.