The secret behind humidity is NOT relative humidity

YOUNGSTOWN, Ohio ( WKBN ) – There might not be a more misunderstood phrase in meteorology than “relative humidity.”

Every summer when humidity returns, this phrase echoes across the United States: “It is humid because the relative humidity is high.” That statement is incorrect.

It’s not because of relative humidity, but due to the dewpoint temperature — the temperature air must reach for water vapor to condense into a liquid. The dewpoint temperature is a much better indicator of how humid it feels outside compared to relative humidity.

When the dewpoint is 65°F or higher, it begins to feel humid.

Then why does relative humidity continue to get the blame for humid days? It’s because the emphasis is placed on the word “humidity” when it would be best to focus on the word “relative.”

The variable relative humidity is a measure of humidity relative to the temperature of the air. (Hint, hint!)

Relative humidity measures how close the atmosphere is to a point of saturation. Put more simply, relative humidity measures how close a location is to having a cloud or precipitation. If the relative humidity is 100%, then according to meteorological law, there must be a cloud or precipitation present (unless the sensor is faulty).

Another way to explain relative humidity is to take a look at wintry weather. Have you ever looked at the relative humidity during a snowstorm? Let’s say it is snowing and the temperature is 25 degrees. Guess what the relative humidity is. The answer is 100%.

What if it’s snowing at minus 10 degrees? The relative humidity is 100%. If it is snowing at minus 40 degrees, the answer would be the same. Yet, it does not feel humid when it’s snowing.

That is not to say that relative humidity is not a valuable variable. On the contrary, the variable can help determine the location of clouds and/or precipitation. It is just not the best variable to use to describe how humid it feels to humans .

Now you know the secret. Do not blame relative humidity for those hot and humid days. Blame the dewpoint.

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