Weather Blog: The differences between dew point and humidity

By Diane Kacmarik, Meteorologist
Last Updated: Saturday, April 26, 2014, 9:26 PM EDT

Have you ever wondered what the difference between humidity and dew point is?

Simply stated, the dew point is what should concern you if you have health issues involving breathing difficulty.

The reason humidity is not the primary concern is because it changes every hour of the day. Every morning of the year, humidity will be around 87 percent to 100 percent (raining or fog will give 100 percent). By afternoon, as the temperatures climb, the humidity drops to around 50 percent.

These humidity readings will be very similar every single day of the year, winter and summer. 

Here is why it changes so much in one day:

The humidity is dependent on the temperature, which changes every hour. Humidity is a combination of the temperature and the dew point. The dew point will tell you the amount of moisture in the air. It does not fluctuate in a day that much unless a front comes through.

For example, say in January, the temperature is 45 degrees and the dew point is 44.  Humidity = 96 percent.

Then in the summer, the temperature is 75 and the dew point is 74. Humidity = 96 percent.

The summer combination of 96 percent humidity would be more detrimental to health issues than the same winter combination of 96 percent.

I would encourage you to look more at the dew points if you are at risk when spending time outside. 

Dew points above 70 degrees are hazardous to people with breathing difficulty. 

We often get dew points like that in July through September, and of course the combination of a dew point of 75 and a temperature of 95 would make the afternoons on days like that most dangerous.