Weather 101: Dew Points vs. Relative Humidity

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Dew points compared to relative humidity can be confusing, but they actually are two different things. The dew point is measured as a temperature, while the relative humidity is expressed as a percentage.

DEW POINT: The Dew point is the temperature to which air must be cooled for saturation to occur at a constant pressure. When the air temperature and the dew point are at the same number, or around the same number, that is when fog or precipitation will form. It is when the water in the air can no longer stay in its gaseous state and goes into a liquid state. 

This is important for us to forecast weather, because if there is more moisture in the air, or if the dew point is equal to the air temperature, that can lead to clouds, rain, or snow. Dew point’s are also great for figuring out how comfortable we will be outside. 

When Dewpoints are really high, in the 60s, that is when the air can feel very sticky and uncomfortable.  This usually happens in the summer.  When Dewpoints are really low, in the single digits to the 20s, it means the air is very dry. When Dewpoints are this low, we tend to get chapped lips, or feel static shocks.

RELATIVE HUMIDITY: Relative humidity is the measure of the amount of water vapor in the air currently, compared to the amount that can exist at the current air temperature. This is expressed as a percentage, and also is why dew points are better to use to describe how the air makes us feel.

For example, a temperature of 20 degrees with a dew point of 20 degrees (dry air), will give you a relative humidity of 100%. This would lead you to believe it is sticky out, when it isn’t. If the air temperature was 85 degrees, and a dew point of 65 degrees (humid air), which would indicate it being a more muggy day compared to a 20 degree day, the relative humidity would only read as 51.2%. This would make it seem like it wasn’t very sticky outside when it would be.

Relative humidity isn’t used to know how to feel, but it is helpful for meteorologist to find out how saturated the air is. When the air is saturated, that means clouds can form and we can get precipitation.

LEARN ABOUT AIR PRESSURE AND HOW WE USE IT TO FORECAST WEATHER HERE –>

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