Mechanisms of Tropospheric Heating

1. absorption of infrared radiation being emitted by the Earth (temporary "trapping"). Two gases in the troposphere, carbon dioxide and water vapor, do most of the absorbing. The process goes by the name of "The Greenhouse Effect".
2. conduction (transfer of energy by direct molecular collisions). Atoms and molecules in the surface of the warm Earth (ground or water) are vibrating with speeds directly related to temperature. When slower moving molecules in the atmosphere bump into these vibrating molecules on the surface, they bounce back faster than when they came in, and their greater speeds indicate a temperature increase has resulted (heat energy is said to have been "transferred"). Heat and temperature are not synonymous: heat is the energy that is transferred between two objects which are at different temperatures (and heat always flows naturally from a higher temperature object to a lower temperature object). The amount of heat energy that an object contains is dependent on the object's temperature AND on the amount of substance involved. Conduction is not very effective where air is concerned (air is said to be a good thermal insulator), so a layer of air only about six inches thick typically gets heated by conduction near a warm object at the Earth's surface. You can see evidence for this by the effect known as a "heat mirage", which makes the pavement on a hot sunny day look shiny and wet when seen from a distance (the density changes caused by conduction in a thin layer of air above the road make that layer of air act like a mirror).
3. convection of atmospheric gases once they have been heated first by conduction. Convection is the transfer of heat by the buoyant movement of a gas or liquid, whose density decreases as a result of heating. Air, when heated non-uniformly, will not immediately mix with the cooler surrounding air. Instead, the heated air will expand because the faster-moving molecules are able to push slower ones out of the way. The expansion is accompanied by a density decrease (same mass occupies a bigger volume), and the parcel of less-dense air then rises because it tends to float above the surrounding more-dense air. On its way up, this warm air can come into contact with more molecules than if it simply stayed on the ground-- thus conduction of heat is enhanced. Once the rising air has cooled, its density increases again causing it to sink back down and gather more heat to start the process over (this is called a "convection cycle"). Convection can occur over short distances (local movement of the air over a hot parking lot, for example, boosting a hawk to high altitudes riding the "thermals"), or it can be a large scale phenomenon (the carrying of excess heat energy through the troposphere over huge distances from the equator to the poles, which is known as "global atmospheric convection").
4. The transfer of latent heat energy from surface to atmosphere via the evaporation and condensation (phase transformation) of water. This is discussed in the next page.

You will find a summary of the main points of the entire atmospheric science unit (besides the electromagnetic spectrum) at this Web page: Weather Facts. See also this related Web site on the phases of water.

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