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Dew Point Vs. Relative Humidity

Dew Point Vs. Relative Humidity

Christina Talamo
Updated: July 29, 2020 02:28 PM
Created: July 29, 2020 02:16 PM

The dew point temperature is something we discuss quite often during our broadcasts when trying to represent how sticky or humid the air will feel, but questions always arise as to why we are discussing the dew point and not the relative humidity. Although the relative humidity has the word humidity right in the name, the relative humidity will not always be an accurate representation of just exactly how humid it will feel outside.

The dew point temperature is the temperature at which the air is saturated, or at which the air needs to be cooled to at a constant pressure in order to achieve 100% relative humidity. This temperature basically represents the amount of total moisture within the air.

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The relative humidity differs from the dew point because it is the percent of saturation within the air at a given temperature, and as temperatures change, so will your relative humidity. As temperatures increase and the air warms, the air will expand, and thus be able to hold more water vapor. To be exact, the ability of the air to hold water vapor doubles with almost every 11 degree increase in temperature.

Most people will assume that the higher the relative humidity percentage, the higher the “feels-like” temperature will be, and therefore the more humid it will feel outside. However, even when relative humidity is low, with high air temperatures and dew point temperatures, it can actually feel very humid. This occurs because of the rising dew point temperature, and the lower the dew point is, the drier the air will be, while the higher the dew point is, the more humid the air will be.

Let’s discuss this in an example. If it was 90 degrees outside today with a dew point of 70 degrees, we would expect it to feel very oppressive outside with a dew point temperature that high. However, with these two temperatures, the relative humidity would only be at about 52%! Now say we increase the air temperature to 95 degrees but keep the dew point temperature at 70 degrees. We would again expect it to feel just as oppressive outside if not more oppressive as the dew point has remained the same, but in this case the relative humidity has dropped down to about 45%! In this case, it would feel more humid on the day with the lower relative humidity percentage, proving that the relative humidity is not a reliable representation of how humid it will feel outside.

The dew point temperature measures how much total water is in the air, and does not vary based on temperature. When there is more moisture in the air, it becomes difficult to cool off your body by evaporating your own sweat, and therefore the dew point temperature, being a measure of total moisture in the air, is a much more reliable measure of humidity. When you want to know just how humid it is going to feel when you step outside, don’t worry about the relative humidity, and just focus on the dew point.


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