It will feel super humid today. Here’s what the dew point has to do with that.

This fried dough cart sits outside of the Park Street T stop on June 30.
–Ryan Breslin /

When you want to check the upcoming weather, you probably look to your smart phone’s five-day forecast. However, there is one aspect of the forecast that’s not there—and that will help you better prepare for the days ahead. This aspect, called the dew point, will help you understand why air ends up feeling so humid at times.

First, we need to review a few properties of air itself: Air primarily consists of nitrogen, oxygen, carbon dioxide, and water vapor. All of those air parts make up what is floating around us all the time. The piece that goes up and down the most dramatically from day to day is the water vapor. Water vapor fits into the spaces between the other gases.

The warmer the air, the more space there is between the other gases, and the more water vapor can fit in there.

The molecules of cold air are packed tighter together and can't
The molecules of cold air are packed tighter together and can’t “hold” as much water vapor. —

Relative humidity


The term “relative humidity” tells you little about how much actual moisture is in the air; it only tells you how close the air is to being full of water vapor.  A relative humidity of 100 percent means the air is full of the potential moisture. A relative humidity of 50 percent means the air is half full of potential moisture. The issue is, if the air is cold, the potential for total moisture is small.  So when you have 100 percent relative humidity in cold air, the air is full of moisture, but the air molecules are packed tighter and there’s not much space in there for water vapor. Hot air with a relative humidity of 50 percent has half the potential moisture, but because the air can hold so much water vapor, there’s a lot present. Think about the difference between an 8-ounce glass of water that’s full and a swimming pool that’s half full. There’s a lot more water in the 50-percent full container. This is why relative humidity is rather meaningless when it comes to how you feel.

Relative humidity does tell us how close the air is to being saturated. You can assume that if its over 80 degrees and the relative humidity is over 50 percent, there must be a lot of moisture in the air.

Dew point


Because relative humidity isn’t a good descriptor for how you will feel, years ago, meteorologists starting using the dew point to better explain how much water is actually in the air. The chart below shows how much water vapor is in the air at various dew points.

There's a lot more water vapor in warm air
There’s a lot more water vapor in warm air. —Dave Epstein

Notice how much more water vapor is present as the dew point rises. If the dew point is 68F, you have 15 grams of water vapor in a kilogram of air. That’s enough water vapor for us to feel it—we would say it’s a humid day. It might actually be better if meteorologists started using the grams of water vapor as an actual variable on TV. I think people would quickly get the concept of more water versus less.

If you like numbers, use the current temperature and dew point along with the chart above and the equation below to see for yourself how relative humidity, dew point, and temperature are all related.

How to calculate relative humidity
How to calculate relative humidity. —NOAA

How high can the dew point get?

I consider a high dew point in the upper 60s or above. In Boston there’s never been an 80-degree dew point in August, but we’ve gotten close. Providence and Hartford have reached that milestone. This month, we did see one of the top 10 most humid air masses on record:

August 2016 brought one of the most humid air masses on record to the region
August 2016 brought one of the most humid air masses on record to the region. —NOAA-Taunton, MA

Today and tomorrow will bring high dew points and lots of moisture in the air.

The chart below along with the forecasted dew point will help you know when the sticky air is arriving and when the drier air will replace it. Throughout today and tomorrow morning, you’ll feel quite uncomfortable, especially during the afternoon today when the moisture and the heat combine.

The higher the dew point the more uncomfortable yo will feel.
The higher the dew point, the more uncomfortable you will feel. —Dave Epstein

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