Agrometeorological Instruments in our Observatory
Open Pan Evaporimeter
❖ Evaporation is measured by means of
evaporimeters. The loss of water by evaporation
is measured at regular intervals.
❖ The most common pan evaporimeters used for
the measurement of evaporation is USWB
(United States Weather Bureau) Class A pan
evaporimeter.
❖ Consists of a cylindrical reservoir of 120.7 cm in
diameter and 25 cm height. The reservoir is placed
above a wooden frame (120 cm x 120 cm) covered
with mesh wire to avoid water loss by birds.
❖ Water loss by evaporation in a day is measured by
adding water with help of a measuring cylinder up
to the tip of the point of stilling well. It is measured
at 8.30 am IST daily
Rain Gauge
❖ Natural Syphon (Float) rain gauges is most
commonly used for the measurement of rainfall.
❖ Rainfall collected by funnel connected to circular
collecting area of 203 mm in diameter.
❖ The rainwater moves through the funnel into a float
chamber. The float rises with the rise of water level
in the chamber which leads to lifting of the pen
point, which records the cumulative rain over time
on graph paper wrapped around a rotating drum.
❖ When the float reaches maximum level, the siphon
arrangement empties the float chamber.
❖ The readings are taken at 8.30 am IST ever
morning.
Anemometer
❖ A simple 3 cup anemometer was invented in
1846 by Dr. John Thomas Romney Robinson
for measuring wind speed.
❖ The 3 metal hemispherical cups are mounted
on arms which rotates on a vertical axis.
❖ The vertical axis is connected to a
mechanical counter called cyclometer in
which the number of rotations are counted
and converted into speed in km/hr.
❖ The height from the center of the
anemometer cups is 10 ft. above the ground
level
Soil tharmometer
❖ Soil thermometers are mercury-in-glass
thermometers. The bulb of this thermometer
is bent at an angle of 120°.
❖ The range of the thermometer is -20 to + 60
°C. The thermometers are placed in a plot of
size 180 cm x 120 cm in the observatory.
❖ Soil temperature is measured at four
different heights below the surface ( 5, 10, 15
and 25 cm).
❖ The thermometers are placed 45 cm apart at
an inclined depth of 5.8, 11.6 and 23.2 cm to
ensure a vertical depth of 5, 10, 15 and 20 cm
respectively.
Wind Van
❖ Wind vane is used to indicate the direction of
wind. It is installed over a concrete pillar of
10 ft height from the ground.
❖ One side of the brass arm there is an
arrowhead and on another side there are two
flat vanes forming an acute angle (about 20°).
❖ The arrowhead indicated the direction of
wind.
❖ The direction can be measured in two ways:
by direction (North , South, West, East) or
by degree (90°, 180°, 270° and 360°).
Sunshine Recorder
❖ The Sunshine recorder was designed by John Francis
Campbell in 1899 and is used for measuring sunshine
duration in a particular day. It is installed on a pillar of 10 ft
height from the ground.
❖ Campbell-Stokes sunshine recorder concentrates sunlight
through the glass sphere (convex lens) onto a recording
card placed at its focal point in the groove. The length of the
burn trace left on the card represents the sunshine duration
in a day.
❖ Three different recording cards are used depending on the
season.
❖ During winter season ( 13th Oct – 28th Feb) short curve card is
used and is placed in upper groove.
❖ During 13th April – 31st August long curve card is used in the
lower groove.
❖ During equinoxes (3rd Sept – 14th Oct and 1st Mar – 11th Apr)
straight card is used.
Stevenson screen
❖ The Screen was designed by Thomas Stevenson in 1866. It is a specialized shelter
to accommodate the four thermometers (maximum, minimum, dry bulb and wet
bulb thermometers) for recording of air temperature.
❖ Stevenson’s screen is a rectangular wooden box of dimensions: Length 56 cm,
width 30 cm and height 40 cm with a double-layered roof and louvered sides.
❖ The screen is painted white and is
mounted on four wooden supports. The
support of the screen is placed at a
height of 4 ft. (1.22 m) above the ground.
❖ The screen is set up with its door facing
north side (opening downward) so that
minimum sunlight would enter while the
observer is reading the instruments
Dew Gauge
❖ The amount of dew deposition is measured by using Duvdevani dew wooden
blocks (Duvdevani, 1947), also known as Drosometer.
❖ The amount of dew deposition
is recorded at four height
levels ( 5, 25, 50, and 100 cm )
above the ground surface in
the early morning (before
sunrise) before evaporation of
accumulated dew.
❖ The dew deposited on wooden
blocks at four different heights
( 5, 25, 50, and 100 cm ) is
compared with standardized
photographs as per IMD, Pune.