BLOG: Why Do We Use Dew Point and Not Relative Humidity?

Meteorologists talk about the “dew point”. A LOT. This is especially true in recent years. Many people undoubtedly remember watching weathercasts on TV in which the presenter only talked about “relative humidity”. The problem with relative humidity?

It’s kinda useless.

Let me explain why. Imagine if you will 2 cups with water in them. The smaller cup is 100% full of water. The larger cup is only 1/2 full, BUT there is more water in that cup than in the smaller cup.


If we think of the atmosphere as our “cup”, the relative humidity is much higher in the situation on the right but there is actually MORE moisture in the air in the situation on the left. In February it can be 21 degrees with a relative humidity of 100% but that does not mean it is humid. In July, it can be 90 degrees with 32% relative humidity and still feel stifling because of the AMOUNT, or VOLUME of moisture in the atmosphere.

So that’s why, especially in the Summer, we rarely talk about relative humidity. It can be very deceiving. The dew point is not.

In our part of the country, dew point become a big deal when they get well into the 60s and 70s. In mid-Summer, dewpoints in the 50s are a nice treat.



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