What is the Dew point?
QUESTION: Hi, I understand relative humidity, but what the heck is dew point. Please explain, my kids and I are constantly debating this. -Best regards ANSWER: Excellent question! Dew point and relative humidity are somewhat misunderstood, yet both are very important to our weather! Even though you said you understand relative humidity, let me review it all so everyone’s on the same page. First off, you need to understand that relative humidity means just that its the amount of humidity RELATIVE to how much moisture the air can hold! Yup, the air can only hold a certain amount of moisture before it cant hold any more. When it cant hold any more as invisible water vapor, that moisture becomes visible in the form of fog, steam, and clouds. Still with me? OK, Now lets add another wrinkle, air can hold MORE moisture when it is warm than when it is cold. How does this relate to RELATIVE HUMIDITY and DEWPOINT? Hang on, were getting there. Since air can only hold a certain amount of humidity,
According to our Glossary, the dew point is “the temperature to which a sample of air must be cooled, while the mixing ratio and barometric pressure remain constant, in order to attain saturation by water vapor. When this temperature is below O°C, it is sometimes called the frost point.” In simpler terms, it is a measure of how moist the air is. Moist air is more unstable, which is why you hear meteorologists talking about the Dew point when they talk about thunderstorms. Dew points in the 50’s and higher can significantly contribute to thunderstorm development, and Dew points in the 70’s indicate extremely unstable air. Since it not expressed as a percentage related to temperature, it won’t change day to night like the Relative Humidity (see above).
The dew point is the temperature at which a given air mass will be fully saturated with water and unable to hold any more, which means that dew will start to form. When the dew point is below 32 degrees Fahrenheit (0 degrees Celsius), it is known as the “frost point,” referencing the fact that frost will form rather than dew because the temperature is so low. This weather data is important to a number of people, such as pilots, who use information about dew points to predict potential aviation hazards. Dew point measurements assume that the barometric pressure will remain constant. If the level of pressure changes, this can bring this point up or down. This number can also be altered by changes in the air mass, such as a collision with another mass of air, but it can be a useful yardstick. As a general rule, clouds and fog will start to form when the total saturation point is reached. People often define the dew point as the temperature to which an air mass would need to cool in order
The dew point temperature is the temperature to which the air must be cooled to reach saturation (assuming air pressure remains the same). The dew point is a direct measure of the amount of moisture present in the air, and directly affects how you feel… or in other words… measures the amount of humidity in the air. Remember, the temperature never drops below its dew point, but can drop to it. Generally, we start to feel some discomfort when the dew point gets to or just above 60 degrees »ww2010.atmos.uiuc.edu/(Gh)/guide···ewp.
A. For any particular sample of air, which is cooled at constant pressure, there will be a temperature below which water vapour condenses to form liquid water drops, assuming sufficient hygroscopic nuclei present. That temperature is known as the Dew Point and is a measure of the Absolute Humidity (see Q/A 2A.17). 2A.
