First Alert Weather In-Depth: Battle between dew point and relative humidity. Which is better?

ROCHESTER, N.Y. – This time of the year it is especially important that we effectively communicate the amount of moisture in the atmosphere on any given day. There are different ways of doing this, but we often use dew point or relative humidity. However, we need to be clear that these two measurements are vastly different and honestly, there can be a little misleading as to the exact meaning of the two meteorological terms.

I may be able to clarify this when showing an illustration of the dew point and relative humidity for the next 24 hours. The dew point (the temperature in the air would have to be cooled to reach saturation) over this period remains in the comfortable range with readings expected to be in the 50s. However, for the same period, relative humidity drops dramatically through the afternoon and then it goes way up close to 90 percent through the overnight hours. In this example, is the amount of moisture in the atmosphere changing overnight? Not at all. Relative humidity is best used when comparing the rate of evaporation. An example is taking a droplet of water and running it through a parcel of air that has a temperature of 40 degrees. At that temperature, that parcel is about 70 percent full of moisture (70 percent relative humidity). But if we now run that droplet through a warmer parcel at a temperature of up to 60 degrees that parcel is bigger and has a much greater capacity to hold moisture. In this case, relative humidity may decrease to just 35 percent, but it still has the same amount of moisture. The rate of evaporation is now increasing with the warmer temperature. As a result, the warmer temperature means the air can hold more moisture.

Relative humidity might be important if you have a muddy backyard and you are trying to dry it out. Or if you are a gardener and your soil is too moist. But a more precise measurement of weather that meets our comfort level, dew point is the clear winner – especially for this time of the year.