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Dew Point and Humidity

Dew Point and humidity are two very commonly confused weather variables. Both of these are related to moisture in the air and often used for similar purposes. So what are dew point and humidity?

The dew point is simply the temperature where the air cannot hold any more moisture than it currently has. This relates to the fact the air can hold more water when it’s warm. So, when the air has more moisture, it’s dew point is higher, and vice versa when there’s less moisture. So, what happens if the temperature drops below the dewpoint? The fact is, the temperature can never fall below the dew point! If the temperature does fall below the dewpoint, then the dewpoint will fall with the temperature. When this happens, moisture is forced out of the air. This is what causes clouds, fog, and precipitation.

Humidity, on the other hand, is the measure of how “full” the air is with moisture. When the humidity is low, that means that the air can soak up much more water than it currently has. Conversely, when the humidity is high, it’s almost full. Humidity is measured as a percent. Relating back to the dew point, when the humidity is at 100%, the temperature and dew point have the same value.

So generally speaking, dewpoint is a measure of how much moisture is in the air, and humidity measures how saturated the air is with moisture. So on a hot day, it may feel muggy even though the humidity is relatively low. To find out how muggy it feels out , you would look at the dew point. According to the National Weather Service, a dew point at or below 55°F is considered comfortable, while a dewpoint from 55 to 65°F feels muggy. When the dew point is at or above 65°F, the air is very uncomfortable. In Indiana, this only an issue in the summer, as the temperature and dew point is much lower during the winter.

Graph created using Davis WeatherLink program

To further explain how dew point, temperature, and humidity are related, the graph above shows 3 days of dewpoint (dark green), temperature (red), and humidity (blue). As you can see, the dew point didn’t change much over the 3 days, but the temperature and humidity did. That’s because the actual amount of moisture in the air stayed close to the same, but the temperature fluctuated. The fluctuating temperatures is what caused the humidity to fluctuate. As you can see, the humidity is high when the temperature is close to the dew point, and it’s low when they’re far apart. As expected, the temperatures generally drop at night, nearing the dew point, which causes the humidity to rise. As shown by the graph, the temperature and dew point were almost the same value at about 8 a.m. on the third day, there was probably fog and/or dew that morning.

So dew point is the amount of moisture in the air and humidity is how much moisture the air can hold. These values are very important in analyzing precipitation, fog, and other factors relating to moisture.

Sources:

Dew Point vs Humidity (weather.gov)

It’s not the heat, it’s the dew point – Portland Press Herald

Cover image source:

Fog in the valley wallpapers and images – wallpapers, pictures, photos (zastavki.com)

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