Tag Archives: dewpoint

What is the dewpoint temperature?

Posted on 19 July 2010

What is the dewpoint temperature?

dew on evergreens

Dew on evergreens, by Michael Theberge at NOAA

Think of a morning when you walked on a grass lawn or through a field. Did your shoes get wet? If yes, that is because the grass was wet with dew. The dewpoint temperature is the temperature to which the air must be cooled, at constant pressure, for dew to form. As the grass and other objects near the ground cool to the dewpoint, some of the water vapor in the atmosphere condenses into liquid water on the objects.

The dew point is reported by TV meteorologists and weather sites on the web because it is a great indicator of the air’s moisture content. As the dew point approaches air temperature, the air holds more water vapor. On a warm, humid summer day, the dewpoint may enter the upper seventies, but it rarely reaches 80 degrees. On cold winter days, dew point is often in the single digits.

Usually dewpoint, air temperature and relative humidity are reported together, and these weather variables are indeed related. When air temperature equals dew point temperature, the relative humidity is 100 percent, and fog forms.

Relative humidity is a relative measure of how humid the air is, the dew point temperature is an absolute measure of how humid the air. So while the dewpoint temperature is reported as temperature, it really informs us about humidity!

Steven A. Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at UW-Madison, are guests on the Larry Meiller‘s WHA-AM radio show the last Monday of each month at 11:45 a.m.

Where does the dew on blades of grass come from?

Posted on 9 March 2010

Where does the dew on blades of grass come from?

As fall begins, we have lots of mornings with dew on the grass. To explain this, we start with the fact that air contains water in the gas phase, called water vapor. Dew is made of liquid water that has condensed from that water vapor.

water drops cover blade of grass

Dew occurs when objects cool, such as when drops of liquid form on the outside of an ice-cold drink glass. Dew forms when the object, such as the glass, cools down to the dew point temperature.

Water molecules in the air continually bombard surfaces, like blades of grass. Some of the molecules stick, forming a very thin film of water. This film may not last long, as the water is also evaporating. The evaporation rate depends on the water temperature, which equals the temperature on the grass.

So condensation depends on the state of the atmosphere (primarily temperature and moisture), while evaporation depends on the temperature of the object. If the object gets cold enough, and the air contains enough moisture, condensation exceeds evaporation, and the film grows into dew drops.

Each night the weather report includes the temperature and the dew point temperature. If the two temperatures are close, dew is likely during the night. Because wind and cloudy skies keep the ground from cooling, dew also tends to form on calm nights with clear skies.

Steven A. Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at UW-Madison, are guests on the Larry Meiller‘s WHA-AM radio show the last Monday of each month at 11:45 a.m.

What makes it feel “muggy”?

Posted on 16 February 2010

What makes it feel “muggy”?

See animation of NOAA’s Midwest Water Vapor Loop.

Summer days are referred to as “muggy” when they are very warm and humid. The word “muggy” is believed to come from the Middle English “mugen” which means drizzle. It may also arise from the fact that on such a day, a cold mug becomes covered with a film of liquid water when exposed to humid air.

Air’s humidity is directly related to the amount of water vapor it holds, which can vary considerably. During winter, the water vapor content is very low, as cold air has a quite limited capacity to hold water vapor. Warm air, however, can hold considerable amounts of water vapor. In fact, on July 13, 1995, each breath of air in Madison was about five percent water vapor!

High water-vapor content contributes to respiratory distress in some people and so extreme warmth and humidity can be a serious health risk. High water vapor content is also a key fuel for severe thunderstorms.

Where does the water vapor come from? Southern Wisconsin has two major sources; the Gulf of Mexico and Iowa! As strange that sounds, maturing corn plants are a huge source of water vapor to the atmosphere. In fact, so much water vapor is evapotranspirated (released by the plants during respiration) from Iowa corn in July that the entire state acts like a shallow sea!

Steven A. Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at UW-Madison, are guests on the Larry Meiller‘s WHA-AM radio show the last Monday of each month at 11:45 a.m.