Home » Science & Nature » Meteorology » Laying Some Groundwork-2: Humidity - Page 2

Laying Some Groundwork-2: Humidity - Page 2

© Keith C. Heidorn

unchanged.

Similarly in winter, the outdoor relative humidity may be 63 percent but when outdoor air permeates our warm homes and offices, the relative humidity level may drop to 35 percent or lower. In this example, the absolute humidity is quite low in the outdoor air, but the saturation value for cold air is also low, and thus, the outdoor air is more humid, relatively speaking.

The key term in the relative humidity definition is saturation. Saturation is defined as the condition when the partial pressure of water vapour in the atmosphere is at its maximum level for the existing ambient temperature and pressure. At saturation, equilibrium exists between water vapour and liquid water, and there is no net evaporation or condensation.

Given the temperature of a volume of air and its pressure, we can determine the saturation value in any form required. You will often hear saturation called the condition where the air contains "all the water vapour that it can hold." This, however, is not technically correct (for a good discussion of this, see Professor Alistair Fraser's comments at http://www.ems.psu.edu/~fraser/Bad/BadClouds.html).

We can saturate a parcel of air by adding more water vapour to it (through evaporation or mixing with another parcel of more humid air), or by cooling the parcel down to its saturation temperature. Both processes are at work continually in the atmosphere, but the latter is more familiar to us as it forms fog or dew (or frost if cold enough).

The saturation temperature of the ambient air is commonly called the dew point temperature or simply the dewpoint. Its value is solely dependent on the absolute humidity of the air. You might hear a weathercaster or meteorologist discuss the dewpoint of a particular air mass. Since dewpoint, like the absolute humidity, varies little within an air mass, it is a good indicator measurement of the air mass type. Dew point temperature is a regularly reported element in weather observations while absolute humidity is not.

When the ambient air temperature equals the dewpoint, the Relative humidity is 100 percent and the air is saturated. Should the air parcel temperature fall lower, water vapour will begin to condense into very small liquid droplets to form clouds or, at the surface, fog.

If the temperature of a surface element such as vegetation or rooftops or car exteriors falls below the dewpoint while most of the surrounding air remains above it, dew forms through the condensation of water vapour onto that surface. And, should the dewpoint temperature be below freezing -- now called the frost point temperature or frost point -- ice crystals will deposit on the surface. We call this frost.

The copyright of the article Laying Some Groundwork-2: Humidity - Page 2 in Meteorology is owned by . Permission to republish Laying Some Groundwork-2: Humidity - Page 2 in print or online must be granted by the author in writing.