How to Measure Dewpoint
By Joan Whetzel
Water can be found just about everywhere on Earth; in the oceans, lakes, and rivers; in plants; it even in the atmosphere as gas or vapor. In fact, about 4% of the Earth's air is composed of water. A slight change in that water vapor content produces clouds and can even drive storm systems, or it can simply raise the humidity enough to make us feel sticky. One measurement of the air's moisture content, mentioned sometimes in the weather reports, is the term dewpoint.
What Is Dewpoint?
The dewpoint temperature is the temperature at which the air has become saturated enough for dew to form. The air can't hold all that moisture, so the water vapor condenses into a liquid which comes to rest on the ground and any other flat surface. When the air temperature and the dewpoint temperature are the same, then the relative humidity equals 100 percent.
Dew Point vs. Relative Humidity
Relative humidity, as its name would suggest, is a relative measurement of the moisture content in the air, whereas dewpoint is an absolute measurement of the air's moisture content. In warm, humid conditions the dewpoint temperature may reach 75 to 80 degrees Fahrenheit. To say that the dewpoint temperature is absolute means that a dewpoint temperature of 75 degrees, for instance, means that at 75 degrees the air will always be saturated to the point where it produced dew, and it will always have a relative humidity of 100 percent.
Relative humidity is relative because, though the air temperature may be 75 degrees, the air may not be saturated. The relative humidity is defined as a ration between the moisture content in the air at a given temperature and the maximum amount of moisture that the air could hold at that temperature. It's usually expressed as a percentage. The percentage of moisture in the air indicates how much of a difference there is. With dewpoint temperature that is lower than the actual air temperature, the humidity will be relative, meaning it will be represented by a percentage that is less than 100 percent. The greater the difference gets between the air temperature and the dewpoint temperature, the lower the percentage of relative humidity turn out to be and the dryer the air becomes.
Dewpoint temperature measurements more clearly indicate the humidity or moisture levels during the warm summer months than the rest of the year. The higher the dewpoint temperature gets (75 degrees or higher), the more soupy the air becomes. During the winter months, when the dewpoint temperature falls below freezing (32 degrees Fahrenheit, 0 degrees Celsius) it is call the frost point because the colder temperatures turn the moisture into frost rather than dew.
What the Dewpoint Temperature and Humidity Levels Tell Us
Pilots look for the dewpoint temperature before flying. Dewpoint temperatures are a good indicator of the likelihood of foggy conditions or that there may be problems with icing when the weather is cold. On the other hand, if the humidity level reaches 100% on a summer afternoon, then it's an indication that it is either raining or a dense fog has moved in. Dewpoint temperatures are a good index for thunderstorm formation. If the dewpoint temperature is in the 40s, then thunderstorms are unlikely. With dewpoint temps in the 50s, a few thunderstorms may be possible. Dewpoint temps in the 60s mean that thunderstorm possibilities are fairly good. Thunderstorms are very likely, though, once the dewpoint temps reach the 70s.
Measuring the Dewpoint
Today's scientists and meteorologists use sophisticated equipment to measure all the variables of temperature, wind, humidity, precipitation amounts, and barometric pressure. But one good way to determine the dewpoint temperature is by using the dry-bulb and wet-bulb (psychrometer) method of temperature measurements. There are a easy steps involved.
1. Using an outdoor thermometer, measure the actual air temperature and record it on a sheet of paper.
2. Wet a cotton ball with water, then wrap it around the bulb of the thermometer, making sure the cotton ball covers the bulb completely.
3. Tie a string around the top of the thermometer, or mount it to a piece of cardboard, and swing the thermometer so that the air continuously moves around the wet bulb or blow a fan over the wet bulb of a permanently mounted thermometer.
4. After a few minutes of air movement around the wet bulb, read the temperature and write it down on the sheet of paper.
5. Determine the dewpoint temperature by subtracting the wet bulb reading from the dry bulb reading. Then compare it to either the Celsius or the Fahrenheit chart to determine the dewpoint temperature.
To find the relative humidity, use a temperature and relative humidity chart that makes use of dewpoint temperatures like the one shown here. Find the air temperature along the left side. Follow that row across until you find the dewpoint temperature. Follow that column up to the top of the chart to determine the relative humidity, or the percentage of moisture in the air. On this chart, the temperatures listed in the light gray area indicate a warning zone. Temperatures in the dark gray area are a danger zone, meaning that the temperature and humidity levels are high enough to produce a health risk, especially for young children, the elderly, ant those with medical conditions.
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Weather Questions. What Is the Dew Point Temperature?
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WKRG News. What Is Dew Point?
McColl, Nickolas. Lyndon State College. Temperature and Dew Point Tutorial.
Radio Shack. Measuring the Relative Humidity and Dew Point Temperature.