Dew point and humidity?

Question

Ok so i live in tampa florida i am looking at the weather for may 2008. humidity above 65% is when it gets uncomfortable. is that right? so what is the difference between dew point and humidity? and what dew point is uncomfortable?

Answer 1

For that uncomfortable feeling, it's not so much the relative humidity as it is the dewpoint.  At a dewpoint of 65 F about half the people will feel uncomfortable, above 70 almost everyone will. Once it gets much higher than that there is a real risk of heat exhaustion or heat stroke if the air temperature gets into the high 80's (or higher).  On the other hand, you could have a very high relative humidity (90+%) but if the temperature is in the low 70's or less it's not really that big a deal.  Interestingly, a lot of people talk about 90 degrees/90% humidity, but that almost never happens in the US--it requires a dewpoint of 86 F or higher and dewpoints above 80 are extremely rare.  They do happen regularly in the Persian Gulf and Red Sea, though.

Answer 2

Dew point is the temperature at which the air can not hold anymore liquid and precipitation will begin. Air becomes saturated with moisture. Warmer air can hold more moisture. If damp air is cooled to a point of saturation and cooled more, the liquid will begin precipitating out. Humidity is generally given as percent humidity, or percent of the moisture that it is holdding compared with what it could hold at that temperature.

Answer 3

Generally speaking, relative humidities above 65% - 70% are progressively more uncomfortable than a drier environment.  The reason for that is that humid air reduces the rate of sweat evaporation from the skin surface.  Evaporation has a cooling effect on the skin.  For a demonstration of this, dab a little rubbing alcohol on your skin and blow on it.  Your skill will feel cooler at that location.  The alcohol evaporates more quickly than sweat, thus resulting in more cooling or heat removal.

There are various ways to calculate the relative humidity (RH).  The easiest way without special instruments is to convert the air temperature and the dew point temperature to their respective vapor pressure values.  To make a long story short, use this handy little Java app.:

http://www.csgnetwork.com/vaporpressurec...

Although the app does the work for you, you'll see that the RH is given as a percentage being the ratio of the dew point vapor pressure over the air temperature vapor pressure then multiplied by 100.

For example, the current values reported at Tampa at this time (5:30 p.m. April 29, 2008, EST) is:

Air temperature = 79° F.

Dew point           = 51°F.

RH = 37.72%

Though warm, you're probably fairly comfortable without that muggy, damp feeling on your skin.  Don't worry, that'll change in a couple of months.  :o)