Relative Humidity vs Dewpoint

Posted: August 11, 2016, 12:06 pm by sgraham

This summer has had many days of hot and sticky weather. You may have checked on the humidity, only to find it was at a meager 50%. How could the humidity be so low, when it feels so high? The answer: dewpoint!

Dewpoint is the temperature at which water vapor in the air condenses into liquid water, such as in the form of dew, fog, or possibly rain. The dewpoint is always lower or equal to the air temperature, hence why dew or fog often occurs during the early morning hours, when the air temperatures are typically lowest and the dewpoint highest.

On the other hand, the relative humidity is the ratio between the current amount of water vapor in the air at a given temperature to the maximum amount of water vapor possible in the air at that temperature. The relative humidity is not an exact ratio between the current air temperature and dewpoint!

For example, a 95°F day may only have a relative humidity of 45%, but it still feels incredibly hot and humid. Instead of checking the humidity on such days, one should focus on the dewpoint and the resulting heat index to determine how sticky it will feel outside. The heat index utilizes temperature and dewpoint (or relative humidity) to determine how hot the body perceives the environment. Because the body cools down through the evaporation of sweat, moist air will not allow evaporation as readily as dry air. So, in turn, the body will feel hotter on days with higher dewpoint values. In general, a dewpoint of 60 - 63°F begins to “feel” more humid, and a dewpoint of 70°F or higher becomes rather oppressive on a summer day.

A heat index chart based on dewpoint and air temperature courtesy of NWS El Paso.
 

In the summer months, we have many days reach an air temperature of 90° with a dewpoint around 70°. This only yields a relative humidity of 52%. However, using a heat index calculator, these conditions “feel” like 96°F! So next time you plan to be outdoors on a “hot and humid” day, check the dewpoint and heat index rather than the relative humidity to get a sense of how hot it will feel. As always, remember to try to stay in cooler, shaded areas, as well as hydrated. Your body will surely thank you!

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Director of Forensic and Data Services / Lead Meteorologist
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