how does the difference in temp. between a dry bulb and wet bulb indicate relative humidity?

Answer 1

Dry bulb is the temp of air as measured contains some humidity. The ordinary temp read or reported on TV is dry bulb temp.for a given air pressure in Hg.

Wet bulb measures saturated air temp for that same particular air pressure in Hg. .


A plotted chart for these two exists and is known as a Psychrometric Chart wherein Dry bulb, wet bulb and lines of Relative humidity are indicated. Thus RH is read directly from this chart when both Dry bulb and wet bulb temperatures are taken for a specific air pressure.

From this chart, also, the amount of water in pounds, or grams can be gotten for that particular air.

Answer 2

Relative Humidity = Ps/Pd x 100 if expressed as a percentage, where,

Ps = partial pressure of water vapor in the air at the wet bulb temp. as taken from a steam table. (in Hg)
Pd = pressure of saturated steam in the air at the dry bulb temp. (in Hg)

If Dry Bulb Temp = 79 deg F then Pd = 0.9047 inches of Hg
If Wet Bulb Temp = 56 deg F then Ps = 0.4518 inches of Hg

So the RH = Ps/Pd = .4518/.9047 x 100 = 49.9%

Answer 3

In the Wikipedia article on Relative Humidity or Dew Point, there is a copy of the table, plus a rough, easy to use formula and a messier but more accurate formula.