Atmospheric Moisture

Atmospheric Moisture - Atmospheric Moisture:1 Atmospheric...

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Atmospheric Moisture:1 Atmospheric Moisture Evaporation & Condensation Humidity Cooling the Air Clouds Precipitation Stability Convectional Precipitation Orographic Precipitation Evaporation & Condensation We’ve mentioned evaporation and condensation before, but let’s dig a little deeper on the topics. Imagine water in a lake. The water molecules in the liquid are bouncing around, as molecules do. Those at the surface are bouncing and some of them bounce enough to escape the water and go into the air. This is evaporation . At the same time, molecules of water vapor are in air and some of them bounce and hit the liquid water surface and join up with the liquid. This is condensation . If there isn’t much water vapor in the air, then there will be more evaporation than condensation. At some point, though, there is no room for more water vapor in the air and then we say that the air is saturated. At saturation , the amount of evaporation is the same as condensation. In terms of systems, saturation is an equilibrium situation where inputs to the air (evaporation) equal outputs (condensation). The same concepts apply to sublimation , where ice turns directly to vapor and vice versa. Evaporation: evap > cond. Condensation: evap < cond liquid liquid Humidity The water vapor content of the air is called humidity . Humidity is described in several ways. Absolute humidity is the density of the water vapor, or the mass of water in a cubic meter of air. When air heats up it expands, so the absolute humidity decreases as temperature rises. (The same amount of water is in a larger volume, so density decreases.) Relative humidity relates the amount of
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Atmospheric Moisture:2 water vapor in the air to the maximum amount that can be in the air at a particular temperature: RH = (amt. in air/max. amt.) x 100; multiplying it by 100 makes it a percentage. Since the maximum amount increases with temperature and since the maximum amount is in the denominator, there is an inverse relationship between relative humidity and temperature. If we follow relative humidity over a typical day, we find that it usually is highest at dawn, when the temperature is at its minimum and then decreases until sometime in the mid afternoon, when the temperature is highest. Then it rises until dawn again. The absolute humidity may stay the same, but the relative humidity varies with the temperature. The dew point is another measure of the water content of the air. For a given amount of water vapor in the air, there is a temperature at which the relative humidity will be 100%. This temperature is the dew point. This is a bit confusing–just remember that dew point is a temperature, but it really is a measure of the amount of water vapor in the air. As the dew point approaches the air temperature, the relative humidity increases. When the two are the same, the air is saturated.
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