Answer to "dew point" question

DEW POINT CHANGE -- Jennifer Nichols of Kearny, Arizona asked "What causes the dew point to change?"

The question was originally posed by her father, a weather buff, but it was Jennifer who sought out the answer - COOL! -- 11/20/98

Dew point is a measure of the absolute amount of moisture in the air. The higher the dew point, the greater the amount of water vapor (invisible form of water) in the air.

Water vapor gets into the air through evaporation. This includes puddles disappearing; plants transpiring; clothes drying; even a pot of water boiling. These all act to increase the amount of water vapor in the air.

Water vapor can also be transported from places nearby and far away. Winds carry moisture-laden air from ocean areas toward land, where the air would otherwise be dry. The mountains in the western United States are snow covered (snowy precipitation) primarily because of this transport of water vapor. Similarly, Arizona's summer rainy season is directly linked to the transport of air with a high water vapor content from the Gulf of California and points south.

The dew point of air can be decreased in the opposite ways. Air conditioners or dehumifiers take moisture from the air; a chilled glass of water outside on a summer day in many places will become wet as the air next to it is cooled by contact with the cold glass; and don't forget the daily chilling of the atmosphere itself. This can create a dewy or frosty morning.

Again, transport of low dew point air can bring drier conditions to an area. When arctic or polar air masses (from high latitude continental locations) invade the United States in the winter, they are usually dry (i.e., with low dew points).

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