At 18:27 6/08/98 +0100, Paul Murphy wrote:
>This question is NOT related to sundials so I apologise in advance.
>
>I have made a wet and dry bulb hygrometer to measure realtive air humidity.
>Unfortunately the only table to convert temperature differences into
>relative humidity is in one of my children's books. Perhaps some diallist
>would know where I could find a more advanced one on the 'net.
>
Hi Paul,
Below are some formula for calculating Relative Humidity (RH) from dry bulb
(Td) and wet bulb (Tw) temperatures. All temperatures are in degrees
Celsius. These formula are an approximation for non-ventilated
thermometers. They should be suitable for most circumstances. You can use
them to compute your own psychometric tables. Dew point (Dp) is another
useful parameter (a measure of absolute humidity) often calculated and
quote by Weather Offices. You can "understand" it by thinking of it as the
temperature at which fog will form if you were able to reduce the dry bulb
temperature at the same time. Therefore the Dp is always less than or equal
to the Td, and when equal, the RH=100% and you would be in near fog or dew
forming conditions.
RH = 100 * E/Es
E = vapour pressure (hPa)
Es = saturation vapor pressure (hPa)
E = exp (1.8096 + {17.2694*Tw}/{237.3 + Tw}) - 0.0007866*P*(Td -
Tw)*(1+Tw/610)
Es = exp (1.8096 + {17.2694*Td}/{237.3 + Td})
The P used in the equation is the station ambient pressure (hPa) which is
usually standardised to station altitude as in the following table.
Pressure Station Altitude
(hPa) (Metres)
998.3 0 to 250
969.0 251 to 500
940.4 501 to 750
912.5 751 to 1000
885.2 1001 to 1250
858.6 1251 to 1500
832.7 1501 to 1750
807.4 1751 to 2000
Calculating Dew Point Temperature
Dew point (Dp) is normally calculated using an iterative process. This can
be a lengthy process so the following equation, while not as accurate, is
suitable for many applications, (within +/- 0.2 C at normal operating
temperatures, and +/-0.8 for large wet bulb depressions and very low dew
points (-40 C).
After calculating the vapour pressure at the wet bulb temperature (E)
calculate the dew point temperature (Dp) using:
E = exp (1.8096 + {17.2694*Tw}/{237.3 + Tw}) - 0.0007866*P*(Td -
Tw)*(1+Tw/610)
Dp = (237.3*ln(E) - 429.41808)/ (19.079025 - ln(E))
Hope you find them useful - and I hope I haven't made a mistake.
Cheers
Hank de Wit
Regional Computer Manager
South Australia
[EMAIL PROTECTED]
ph: 08 8366 2674
http://www.sa.bom.gov.au/~hdewit (Internal access only)
