Dew Point

How It Relates to Heat Index, Relative Humidity, and Frost Point

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The air at any given temperature is capable of holding a certain amount of water vapor. When that maximum amount of water vapor is reached, that is referred to as saturation. This is also known as 100 percent relative humidity. When this is achieved, the temperature of the air has reached the dew point temperature. It is also called the condensation temperature. The dew point temperature can never be higher than the air temperature.

Said another way, the dew point temperature is the temperature at which the air must become cooled to in order to become completely saturated with water vapor. If the air is cooled to the dew point temperature, it will become saturated, and condensation will begin to form. This could be in the form of clouds, dew, fog, mist, frost, rain, or snow.

Condensation: Dew and Fog

The dew point temperature is what causes dew to form on the grass in the morning. The morning, just before sunrise, is the lowest air temperature of the day, so it is the time when the dew point temperature is most likely to be reached. The moisture evaporating into the air from the soil saturates the air around the grass. When the temperature of the grass' surface hits the dew point, moisture comes out of the air and condenses on the grass.

High in the sky where the air cools to the dew point, evaporated moisture becomes clouds. At ground level, it's fog when a layer of mist forms at a point just off the ground surface, and it's the same process. Evaporated water in the air reaches the dew point at that low elevation, and condensation occurs.

Humidity & Heat Index

Humidity is a measurement of how saturated the air is with water vapor. It is a ratio between what the air has in it and how much it can hold, expressed as a percentage. You can use dew point temperatures to help determine how humid the air is. A dew point temperature close to the actual temperature means that the air is quite full of water vapor and thus very humid. If the dew point is significantly lower than the air temperature, the air is dry and can still hold much additional water vapor.

Generally, a dew point at or lower than 55 is comfortable but more than 65 feels oppressive. When you have a high temperature and a high humidity level or dew point, you have a higher heat index as well. For example, it may only be 90 degrees Fahrenheit, but it actually feels like 96 because of the high humidity.

The Dew Point vs. the Frost Point

The warmer the air, the more water vapor it can hold. The dew point on a warm and humid day can be fairly high, in the 70s Fahrenheit or in the 20s Celsius. On a dry and cool day, the dew point can be quite low, approaching freezing. If the dew point is below freezing (32 degrees Fahrenheit or 0 degrees Celsius), we instead use the term frost point.