It’s hot. Really hot. Not only is the temperature rising, but so is the humidity. That makes it feel a lot worse. With high temperatures and high humidity, we start talking about the heat index. But what does that all really mean? What is humidity and what is dew point? Let’s dive in:

Dew point is defined by the National Weather Service as “a measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation.” So, in short, it’s a measure of the amount of moisture in the atmosphere.

Relative Humidity is defined by the National Weather Service as “…the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated… the relative humidity is a function of both moisture content and temperature. As such, relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present.”

Let’s simplify:

In the atmosphere we have temperature. We also have a dew point temperature. Temperature tells us how hot or cold the air is, while dew point temperature tells us the amount of moisture in the atmosphere. These two combined can give us the relative humidity. (See equation below)

humidity equation

The closer the temperature is to the dew point temperature, the higher the humidity is.

Example 1:

Temperature = 70°

Dew point = 69°

Humidity = 97%

Example 2:

Temperature = 70°

Dew point = 50°

Humidity = 49%

Now that being said, you can have high humidity in winter months as well…

Example 3:

Temperature = 30°

Dew point = 26°

Humidity = 85%

But when the temperature is that low, we really don’t worry about humidity much. Unless we’re trying to forecast rain or snow. But we as humans don’t feel the humidity at that point. It’s in the spring and summer months that it feels oppressive.

When the dew point is 60° or below its pretty pleasant. Anything in the 60-degree range and it’s getting muggy. Beyond 70° dew points and it’s oppressive.

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