|MadSci Network: Earth Sciences|
In a technical sense, it is not true that warmer air "holds" more water vapor than cold air. Actually, it is the temperature of the water vapor itself that governs the amount of water vapor that may be held in the atmosphere. The warmer the water vapor, the greater its maximum vapor pressure. Vapor pressure is the portion of atmospheric air pressure attributable to water vapor. The greater the maximum (saturation) vapor pressure is the greater the capacity of the mixture of air and vapor to hold water vapor. Since the amount of water vapor in the air is quite small compared to the rest of the gases in the atmosphere, the temperature of the water vapor is governed by the temperature of the rest of the air in which it resides. This leads to the somewhat inaccurate but very convenient notion that warmer air holds more water vapor. In order to explain this to 4th graders, we won't differentiate between the notion of vapor pressure versus "air capacity." It is probably sufficient to say that the air is like a sponge. When air temperature increases, that sponge grows a little and the air can hold more water vapor. When air temperature decreases, the sponge shrinks and the air can hold less vapor. Keep in mind though the reality of the situation: If air temperature increases, water vapor temperatures does too. This results in a higher saturation (or maximum) vapor pressure. If there isn't enough vapor in the air to meet the maximum, evaporation occurs as the atmosphere strives to reach balance. If air temperature decreases, the saturation vapor pressure decreases as well. If there is more vapor present than this maximum value can support, the condensation occurs as the atmosphere strives to reach balance.
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