Expert: Dave Dombek - 8/2/2006

Question
Good explanation, but it's not clear why the human body feels uncomfortable when the dew point is high. Could you talk about that?

Thanks,

Ric Gerace
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Followup To

Question -
How do 1) dew point, 2) heat index and 3) Real Feel Temperature differ from one another?...

Here's a link that uses Real Feel Temperature
http://weather.boston.com


Answer -
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.
The heat index is simply a combination of temperature and dew point, and how it feels. When the dew point is relatively low...say in the low 60s or lower, then the heat index is basically the temperature. However, as it gets more humid (higher dew point), then it feels hotter than the actual temperature. There really is something to the expression..."it's not the heat but the humidity". When the temperature is quite hot, and the dew points are very high, the heat index is extremely high. Sometimes the heat index can go into the dangerous category of 110 degrees or more.
The Real Feel Temperature is something we developed in-house here at AccuWeather. It takes into consideration other factors besides just temperature and dew point. It factors in the time of day, the sun angle, and amount of cloud cover, the winds or lack thereof, etc. For example, a temperature of 90 and a dew point of 70 will feel worse at noon or 1 pm with the sun high in the sky than it will at 5 pm or 9 am. Also, a temperature of 90 and a dew point of 70 will feel worse when it is dead calm than if there is a 15 mph wind blowing. The same thing goes for 90/70 with bright sunshine vs. 50% cloud cover. For all of these examples, the heat index will be identical...no difference
whatsoever, since it is based solely on temperature and dew point. The Real Feel Temperature, however, will be different for each case.   
I hope I answered your question. If so, please close out the question and give me a rating. Thanks!

Sincerely,

Dave Dombek
Expert Senior Meteorologist
AccuWeather.com  

Answer
Ric,
The higher the dew point is, the slower water (perspiration) evaporates from our skin. That is why we feel so uncomfortable, when that sweat just won't evaporate to cool us  off, since evaporation is a cooling process. With a temperature of 90 and a dew point of 70, it is much tougher to evaporate the sweat from our skin than when it is 90 with a significantly lower dew point, say 55. In the latter case, we will feel much more comfortable, even though we are still losing just as much water from our bodies. That is why it is very important to stay hydrated during hot weather, especially when it is not that humid. Our bodies have an internal "thermostat" that only responds to temperature. In other words, we will sweat just as much when it is 90 and humid as we will when it is 90 and dry. However, when it is 90 and not very humid, we don't realize that we are losing so much water, because the water is evaporating so quickly off of our skin.
I hope this answered your question.

Sincerely,

Dave Dombek
Expert Senior Meteorologist
AccuWeather.com