Metr 201 (502) Quiz # 2 Solution

September 22, 2000

1) Anchorage, Alaska is located at latitude 60^o N and longitude 150^o W. How many hours of sunlight will Anchorage receive today?

Since today happens to be the Autumnal equinox, every location (including Anchorage) on Earth will receive 12 hrs of sunlight.

2) Suppose there is a volume of air that is at 100% relative humidity. If this volume rises in the atmosphere (and therefore its total pressure decreases), how will its relative humidity change? How about its specific humidity. (You can assume that the temperature of the air stays the same).

Relative humidity is basically the ratio between the amount of water in the air and the maximum amount of water the air could hold. The amount of water in the air (i.e., the water vapor pressure) will decrease along with the amount of all the other types of gases in the air as the air pressure decreases. The maximum amount of water the air can hold (i.e., the saturation vapor pressure) just depends on temperature. So, since the amount of water vapor will decrease, while the amount of water the air could hold remains the same (temperature stays the same), we would expect the relative humidity to decrease.

Specific humidity is defined as the mass of water vapor in the air divided by the total mass of the air itself. As the air pressure decreases, we would expect the amount of water to decrease by the same amount as the air itself, so specific humidity would not change.

3) Explain why dew often forms even when fog does not. In other words, why does water condense on the surface and not in the atmosphere?

At night, the ground often cools off more than the air since it emits radiant energy more efficiently. Sometimes the cool surface cools the lowest few meters of the atmosphere below its dew point, which results in radiation fog. However, sometimes the surface is only able to cool a very thin layer of air below its dew point. This could be because the temperature of the surface is only slightly less than the dew point of the air, or because the air is extremely calm and so the temperature changes very rapidly just above the surface. When this happens, any condensation will occur either on the surface, or so close that it will deposit on the surface very rapidly. The result is dew without fog.