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Understanding Wet Bulb Temperature and Dewpoint Temperature

There are three kinds of air temperatures that are commonly taken and used and all three are important for defining the weather around us. The three kinds of temperatures are dry bulb, wet bulb, and dew point. The three are similar, but they measure different things because they come at the temperature in different directions.

To understand the difference between these, it should be noted that our weather is made up of five components: Temperature, humidity, air pressure, wind velocity, and wind direction. 

Dry bulb temperature

Dry bulb temperature is the sort of measurement that most people are acquainted with. This is generally registered by a standard bulb thermometer that many people have at their homes. Unlike the other two temperature measurements, the dry bulb method doesn’t measure more than the temperature at a given time.

Wet bulb temperature

To take a wet bulb temperature, damp cloth is wrapped around the bulb of the thermometer. As water in the cloth evaporates, it lowers the temperature. This is the idea behind evaporative coolers. This is important because the more moisture the air contains, the less evaporation occurs. The amount of moisture the air can contain is a function of the temperature of the air. Cold air can’t contain as much moisture as warm air. The key here is that if the air is totally saturated with moisture, which doesn’t happen often except in fog, the humidity is 100%. When the humidity is 100%, it can’t absorb more moisture without first losing moisture, so no water evaporates and the wet bulb temperature is the same as the dry bulb temperature.

Dewpoint temperature

The dewpoint temperature is the temperature at which moisture in the air condenses. Again, cooler air can’t contain as much moisture as warmer air, so the dew point is when the temperature drops to the point that dew or frost forms. This is tied in with the amount of humidity is in the air. This means that if the air is dry, the dew point will be much lower than if the air is humid. If temperatures are above the dew point, there is no dew formation. If the temperature is below the dew point, there is dew or frost. It should be mentioned that frost occurs when the dew point is below freezing and the moisture condenses out of the air in a frozen state. Thus, frost and dew are the same things. They are just at different temperatures.

Direct comparison

While it may seem that wet bulb temperature and dew point temperature are the same things, they are not. An easy way to put this would simply be that wet bulb temperature is a measurement of evaporation in relation to temperature. Dew point temperature is a measurement of condensation in relation to temperature. Evaporation and condensation are nearly opposites, so wet bulb and dew point are quite different.

Both wet bulb temperature and dew point temperature are important to understanding the current weather conditions, though the two measurements are not the same. Despite the differences, both deal with the same two aspects of weather and both can change due to changes in a third aspect of weather; air pressure. There is no need to go into that part, though, since the air pressure effects wet bulb temperature and dew point temperature equally.


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Written by Rex Trulove

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