Atmospheric Moisture

Dew point temperature is the temperature at which condensation takes place and is used as a measure of moisture content. The dew point temperature depends on the amount of moisture in the air, the more moisture in the air, the higher the dew point temperature. It gets its name "dew point" because dew will form on surfaces when the air reaches saturation.

Have you ever noticed that even though it's 100% relative humidity out, it feels a lot drier during the winter than the summer?  To see why, we have to examine relative humidity. Relative humidity is the ratio of the amount of water vapor in the air to its saturation point. Often relative humidity is defined as the amount of water vapor in the air to "how much it can hold" at a given temperature. The notion of a holding capacity is dispelled when one considers what saturation really means. Regardless, what we do know is that the saturation level of the air with respect to water vapor depends on the air's temperature. We know that as air temperature increases, the ability for the air to keep water in its vapor state is easier. That is, as the air temperature increases it can keep more water in the vapor state.  So why does saturated cold air feel drier than warm air at saturation? Let's look at an example. 

Continental polar air (cP) has an average temperature of 5^o C (41^o F). Its saturation mixing ratio is 6 g/kg. So continental air at 100% relative humidity is

RH = 6 g/kg (in the air) / 6 g/kg (saturation)

The average temperature of maritime tropical air (mT) is 22^o C (71.6^o F) with a saturation mixing ratio of 16 g/kg. So maritime tropical air at 100% relative humidity is

RH =16 g/kg  (in the air) / 16 g/kg (saturation)

Therefore, polar air is drier at 100% relative humidity because it has much less moisture in it at saturation than the warmer maritime tropical air. 

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