Dear Dr. Panda,

we share Dr. Hoogenboom's experience with
the Vaisala probe sold by Campbell. It seems
to be a reliable instrument.

However, since you were asking for a none-automatic
(manual) method you might use simple
thermometers, which are standard equipments of manual
weather stations. You will need two thermometers. Use
a wick to permanently moisten the measuring tip of
one thermometer using destilled water placed in any
type of cup. Place the other thermometer next to the
wet-bulb thermometer and shield the whole apparatus
from direct solar radiation using a naturally ventilated
box or hut (It is important that the sun doesn't heat up
the equipment, which would produce erroneous readings).
The procedures to calculate relative humidity
are described in standard textbooks like
Cambell G.S. and Norman J.M. (1998) An
Introduction to Environmental Physics.
Springer, New-York, Berlin.
You may use the following procedure :

1) Perform a simultaneous reading of both temperatures
   (dry and wet-bulb)

2) Determine saturation vapor pressure ES (kPa) from
   the "dry" thermometer (i.e. dry bulb temperature Ta
   in degrees Celsius):

   ES=f(Ta)=0.1 exp(1.894646+0.061788*Ta)

3) Determine saturation vapor pressure ES (kPa) from
   the "wet" thermometer (i.e. wet bulb temperature Tw):

   ES=f(Tw)=0.1 exp(1.894646+0.061788*Tw)

4) Estimate actual vapor pressure EA (kPa) using the psychrometer
   equation :

   EA=ES-gamma*pa*(Ta-Tw)

   where gamma is the psychrometer constant (6.66 10^-4 C^-1)
   and pa is the atmospheric pressure in kPa which is

   pa=101.3 exp(-A/8200)

   where A is the altitude in meters. Please note that
   gamma is slightly temperature dependant, which probably
   doesn't matter (0.01%/°C).

5) Calculate humidity from corresponding
   sets of dry and wet bulb thermometers:

   hr=EA/ES

The problem of the manual method (which you asked for)
is, that you need to "watch" the thermometers
to find the maximum and minimum humidities
(which should be at pre-dawn and just after
solar noon in most cases... but not always !
For example, rains and advective conditions are
two exemptions from this rule).

A more convenient option would be to
use a mechanical, clock-driven hair-hygrometer,
which is  also standard meteorological equipment.
Most of them operate on a weekly basis.
You will find an example at :
http://www.thiesclima.com/feuchte/hygrograph_e.htm

If there is a nearby meteorological service, ask
them for finding an instrument manufacturer or
distributer close to your place. If there isn't
any I would ask at local airports, which must
operate meteorological instrumentation.

Wish you success.

Kind regards,
Matthias



--
Matthias Langensiepen, PhD
Hannover, Germany

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