What's the Point?...It's the Dew Point!

In the summer, the buzzword is often humidity. You can feel it in the air and on the really humid days, it can really drag you down. It causes you to do things more slowly so you don`t over heat. But is the relative humidity that you often hear reported on the radio and TV really the best way to tell how humid it really feels?

Well, if you analyze the word and the definition of the term itself, you can see that it isn`t. Relative humidity tells the percentage of moisture in the air relative to the air temperature. This is done by comparing the dew point with the air temperature, which means that it is really the dew point that tells you how much moisture is the air and thus how humid it really is outside.

The dew point is the temperature at which water would condense, if the air temperature dropped. When you see dew on the ground, it means the air temperature dropped to the dew point, which typically happens on a clear night with light winds.

Meteorologists use the dew point to measure how much water vapor is present in the atmosphere. For example, when the dew point is 70 degrees the atmosphere has more than twice as much water vapor than when the dew point is 50 degrees.

Let`s assume the dew point will not change much throughout the day, as often is the case. In the mornings, when temperatures are usually at their lowest, the air temperature and the dew point will be close together. This is when the relative humidity is highest. When the temperature reaches its high point in the afternoon, the dew point and temperature will have the greatest difference, and the relative humidity will be at its low point.

On a warm, humid day, you would say that it feels humid and uncomfortable all day, even when the relative humidity is 90% in the morning and only 35% in the afternoon. You are right too, and this is because the dew point has not changed. The dew point is the only true measure of how humid it will feel to a person.

Knowing the dew point each day and how it might change throughout the day and week will give you a great idea as to how comfortable the day might be and how your comfort level might change in the future. Use the adjacent table to help determine how the dew point will make you feel each day.

WeatherBug is a great source for monitoring the dew point. Make sure to check the LIVE dew point on your live weather screen each day. So, this summer as you swelter in the heat and more importantly, the humidity, make it a point to check the dew point!

(Image Note: There are two ranges of how it feels because the West generally have drier air (lower dew points) so it just feels hotter at lower dew points in the West than the East. Of course, if an Easterner was in the West, they would still use the "East of Rockies" table. Also, since Hawaii is in the tropics, residents of the Islands should use "East of the Rockies".)