What is dew point?

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Jeff Peterson asked for me to explain dew point and how it is measured.

Dew point is the term that meteorologists use to describe the amount of moisture in the air. The National Weather Service defines dew point as the temperature to which air must be cooled to in order to reach saturation, assuming air pressure and moisture content are constant. Basically, the dew point is an accurate measurement of the moisture content in the air.

Relative humidity and dew point both give us an idea of the amount of moisture in the atmosphere, but dew point is a true measurement of the atmospheric moisture.

The difference is that relative humidity is the amount of atmospheric moisture present, relative to the amount that would be present if the air were saturated. That's why humidity is maxed out in the morning. Not because it's more humid, but because the temperature has cool closer to the dew point.

One way to measure the dew point is a hygrometer. Which uses mirror and light to measure how much moisture collects on a mirror. Think of a mirror in your bathroom after a hot shower. The moisture collects on the mirror because of the high moisture content of the air.

A psychrometer is an another instrument used to determine the dew point. A psychrometer has two thermometers, one of which has a wet wick around the bulb and is called the wet-bulb thermometer.

We've talked in the past about how evaporation is a cooling process. So, evaporation of water off the wick removes heat from the thermometer. The temperature of the wet-bulb thermometer drops according to the rate of evaporation, which depends on the dew point.
The thermometers have to be ventilated by either whirling the instrument around or using a fan. After a few minutes, the temperature of the wet-bulb stabilizes at a particular temperature.
The temperature difference between the two thermometers is converted into a dew point temperature using a chart.

Okay that's how it's measured but what does the dew point mean to you? What you need to know is how it feels to you.

A dew point of 70 or above is considered oppressive. At 65 most would say it’s uncomfortable. At 60 it’s sticky. 55 is considered pleasant. 50 and below is considered invigorating.