Expert: Dave Dombek - 3/8/2006

Question
Dave,
I was using a data logger to track the temp/RH/Dewpoint of a shipment of material from south carolina to massachusetts. At one point the RH went below the Dew Point. Is this possible? What does it mean? How can that happen? I can send you the graph if you give me your e-mail address. Thanks, Jim

Answer
Jim,
Here is an explanation of the relationship between dew point and the relative humidity. Hopefully it will answer your question...
Dew point is simply the saturation point of the air, or the temperature to which the air has to be cooled in order to get condensation. It is really a true measure of how much water vapor is in the air...the higher the dew point, the more water there is in the air, and the lower the dew point, the drier the air mass is. In the summer, dew point is an excellent tool to use in determining how comfortable, or uncomfortable, it feels outside. Here is a general summertime guideline for dew points...
Below 50:  very dry
50-59:  comfortable
60-64:  moderately  humid
65-69:  humid
70-79:  oppressive
80 and above: steam bath!
Now you may be wondering...how does dew point relate to relative humidity? Well, the relative humidity is dependent on the temperature and the dew point. The closer together the dew point and temperature are, the higher the relative humidity is. The farther apart they are, the lower the relative humidity is. Here's an example of what I mean, and also an illustration why the term relative humidity does tell you anything about how comfortable or uncomfortable it is outside.
Let's say we've got a very humid air mass in place, and the dew point is 70. It's daybreak, and it's very murky outside...hazy, and perhaps even foggy. The grass, car tops, etc. are all covered with heavy dew. Let's say the temperature at that time is 70 degrees. With a temperature of 70, and a dew point of 70, the air is saturated, and the RELATIVE HUMIDITY at that time is 100%. The sun comes up, burns off the fog and evaporates the dew. There is no front coming through that day...no air mass change whatsoever. So, the air mass will have basically the same amount of water vapor in it all day long, i.e., the dew point will stay close to 70. Now with the sun out, the air heats up, and the temperature rises. Let's say the high temperature that day, around 4pm, is 90 degrees. At that time, the hottest part of the day, the RELATIVE HUMIDITY is only 50%!!! That doesn't tell the average person anything about how it feels outside!!!
The dew point is a good tool to use all year long. In the winter time, the lower the dew point, the drier the air mass is, and the more likely precipitation will fall as snow, even if the temperature initially is above freezing. That's because the air temperature will cool as the precipitation initially evaporates.

Sincerely,

Dave Dombek
AccuWeather.com