AGRON 221 Introductory Agrometeorology and climate change (Practical manual)
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S. No. |
Exercise |
Signature |
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1 |
Study of Meteorological Observatories, Site
Selection and layout |
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2 |
Measurement of Bright Sunshine Hours, Total
Shortwave and Long wave Radiation |
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3 |
To study about wind direction and wind speed at
the time of observation. |
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4 |
Measurement, tabulation and analysis of rainfall |
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5 |
Measurement
of maximum and minimum air temperatures, tabulation, and variation analysis |
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6 |
Determination of relative humidity and vapour
pressure |
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7 |
Measurement of open pan
evaporation and evapotranspiration |
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8. |
Measurement of Atmospheric Pressure and Analysis
of Atmospheric Conditions |
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Exercise 1
Object: - Study of Meteorological Observatories, Site Selection and
layout
Ø Objective
We know
that quality and quantity of any crop production mainly depend on weather. Weather has a direct and indirect effect
on crop production. For investigating precisely and quantitatively the relationships between crop and weather, the detailed development observations of the crop and the regular observations of the
weather are essential. Recording of
weather phenomena is essential as relation between the climate of a region and
the kind of plant is obvious. Meteorological observatory is a place
where all the necessary instruments are exposed
for measuring weather
phenomenon.
Ø Types or classes of meteorological observatories
ü
Four types of weather stations are recognized depending on the
number of weather elements measured,
the frequency of measurement, status of the observer and location. These four types
of weather stations are as follows.
1.
Synoptic stations: These are stations managed
by full-time observer
who maintain continuous weather watch and make hourly
instrumental observations of the weather elements
on which information is required for the compilation of the synoptic charts or weather
maps used in weather forecasting.
2.
Agricultural stations: These are stations managed by part-time observer making at
least twice daily instrumental
observations of the major weather elements. Evaporation, grass minimum and soil temperatures and solar
radiation are also usually measured in view of their obvious
importance in agriculture.
3.
Climatological Stations: These are stations managed
by part-time observers
making only once or twice
daily instrumental observations of temperature, humidity, rainfall and wind.
4.
Rainfall stations: These are stations managed by
part-time observers who take daily reading of rainfall only.
Ø The surface
observatories are letter coded into six classes
1.
Class A Observatories: These are provided
with eye reading
instruments and self-
recording instruments. The observations are recorded every after
hour round the clock.
2.
Class B Observatories: Most of these are furnished with eye-reading instruments and self-recording instruments. Regular observations
are made at least twice a day.
3.
Class C Observatories: These have the same instruments or equipment that of described in Class B Observatories
but observations are recorded only once a day.
4.
Class D, Class E and Class F Observatories: These have a
smaller number of instruments or equipment’s or are non- instrumental.
Ø Selection of site for Agrometeorological Observatory:
Agrometeorological observatory is a place where all meteorological as well as biological observations are recorded
simultaneously.
ü
The site of the observatory should be located at the center of
the agricultural research farm and
the size of the plot may be of 60 X 40 m with its longer side running north- south direction.
ü
The site should be enclosed with barbed wire fencing and should
be easily accessible during rains.
ü
Water logging should be avoided.
ü
The site should be well exposed i.e. away from high buildings,
trees, main channels and drains etc.
A.
Location of instruments: Generally, every agrometeorological observatory is provided with
given below instruments.
ü Sunshine recorder
ü Anemometer
ü Wind vane
ü Stevenson screen
ü Double size Stevenson screen
ü Ordinary rain gauge
ü Self-recording rain gauge
ü Soil thermometer
ü Grass minimum thermometer
ü U.S.W.B. Class 'A' Pan
ü M.C.P. (micro climatic pole)
ü Dew gauge.
ü Soil moisture plot
All these instruments are fixed in observatory in a given sequence (see lay out).
B. Calculation of LMT: -
The local mean time corresponding to Indian Standard Time varies
from place to place depending upon
longitude of a place. L.M.T. at a
place is the mean time determined with reference to the sun. When sun
rises at any place it is
said to be 7° clock locally.
LMT = IST + 4 (L1- L2)
L1 = Longitude of Allahabad
L2 = Longitude
of the station
I.S.T.: The Indian Standard
Time is the L.M.T. of longitude 82 ½° East. It
is five and half-hours ahead of Greenwich Mean Time.
G.M.T.: The Greenwich
Mean Time is the Local Mean Time of Greenwich. Greenwich Mean Time (GMT) was established in 1884 at the International Meridian Conference, when it
was decided to place the Prime Meridian
at Greenwich, England. Greenwich Mean Time (GMT) is also known as Zulu Time
Ø Where is Greenwich, England ?
ü Longitude 0° 0' 0"
ü Latitude 51° 28' 38"N (North
of the Equator)
Ø Time of observations: In Agricultural meteorological observatory the observations are to be taken and recorded at 0700 hrs and
1400 hrs LMT (i.e. 0730 and 1430 hrs LMT of Navsari
location) except Pan evaporation
& rain gauge observations are to be taken at 0830 IST.
Ø Table: 1. Latitude,
Longitude and Altitude of different stations
of Gujarat
|
Location |
Latitude N |
Longitude E |
Altitude (m) |
|
SK Nagar |
240 19‟ |
720 19‟ |
154.5 |
|
Anand |
220 35‟ |
720 58‟ |
45.0 |
|
Surat |
200 00‟ |
720 52‟ |
11.3 |
|
Navsari |
200 57‟ |
720 54‟ |
10.0 |
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Rajkot |
220 18‟ |
700 47‟ |
138.0 |
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Junagadh |
210 30‟ |
700 30‟ |
61.0 |
|
Jamnagar |
220 27‟ |
700 02‟ |
20.0 |
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Amreli |
21o 18' |
71o 12' |
130.0 |
|
Bhavnagar |
21o 46' |
72o 08' |
30.0 |
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Porbandar |
21o 38' |
69o 36' |
7.0 |
|
Surendra Nagar |
22o 43' |
71o 38' |
74.0 |
Exercise 2
Object: -Measurement of Bright Sunshine Hours, Total Shortwave and
Long wave Radiation
Objective:
1. To study
about bright sunshine and solar radiation
2. To compute global radiation
3. To compute
net short-wave radiation, net outgoing
radiation and net radiation
The energy
that travels in the form of electromagnetic waves through space is called as radiant energy. This energy
received from the sun is called as solar radiation or insolation. The sun emits almost a constant amount of
solar radiation (1.94 cal/ cm2/ min) continuously. It is called solar constant.
Solar radiation is one of the most important factors
in photosynthesis and transpiration
of crops. But since its measurement involves advanced and costly instruments, indirect estimation of the same from
sunshine hours data is useful. Hence, the measurement of bright sunshine hour is important.
Ø Sunshine recorder
Parts of Sunshine recorder:
F Campbell-Stokes sunshine recorder
F Sunshine cards
F Sunshine plastic scale
Ø Procedure:
F Select the appropriate card as per the season.
F Insert the card in the appropriate groove
of the recorder after sunset.
F Remove the burnt card in the
evening after sunset and mark the
date of observation on the reverse
of the card.
F Tabulate the duration
of sunshine recorded
during each hour of the day.
F Calculate the bright
sunshine duration using the special
plastic scale.
F Estimate the total
amount of clouds
in the sky in Oktas.
Ø Measurement of Sunshine:
F The sunshine is measured by means of Campbell-Stokes sunshine
recorder.
F This consist of a glass sphere
of 10 cm diameter, mounted
concentrically in a section of spherical bowl, the diameter of which is such that the
sunrays are focused sharply on a card held in the grooves cut into the bowl.
F Three overlapping pairs of grooves are provided in the bowl to
take cards suitable for different
seasons of the year.
F Long curved cards are used in summer, short curved cards in winter and straight cards in equinoxes.
F The time indicated by a correctly adjusted sunshine recorder is the true solar
time or local apparent time.
Ø Sunshine cards:
1. Three types of
cards namely, the short curved card (13th October to 28th
February), the long curved card (13th
April to 31st August) and straight card for other seasons (during
equinoxes) are used in grooves.
2. These cards are
subdivided into hourly intervals.
3. While inserting the
new cards its 12 hour line should
be adjusted to coincide with noon line on the bowl.
4. As the sun moves across the sky, its focused image
burns a trace on the card so that by measuring the trace for the whole day the duration
of sunshine during
the day can be accurately recorded.
Ø Sunshine scale:
1. A sunshine scale measures the burn hour. It is made of celluloid.
2. A special plastic scale is provided in which the
subdivisions of the hour are marked.
3. There are 10 parts
in scale each part consists of 0.1
hour (6 minutes).
4. The parallel
sunshine scale is used for straight card and trapezoidal scale is used
for long and short curved cards.
5. The duration of
sunshine can be obtained correct to 0.1 of an hour. The hours marked in the sunshine card refer to local mean time
(LMT) of the station.
6. The sunshine is
measured in the units of bright sunshine hours per day.
Ø Installation:
The sunshine recorder is installed on a masonry pillar of 5‟ (1.52 m) or 10‟ (3.04 m). There should not be any obstruction having
an elevation of 3o above
the horizon.
Ø Precautions:
1. Avoid excessive
vigour in polishing the glass sphere.
Avoid cleaning the glass bowl with any cloth.
2. Remove any deposits of dew, frost, snow or bird droppings immediately.
3. If the trace us not parallel
to the central line of the card, carry out leveling and other adjustments of the recorder.
4. Use appropriate cards for the season.
v Radiation instruments:
Pyrheliometer: To measure direct
solar beam on a plane surface at normal incidence.
Pyranometer: The instrument used to measure total incoming radiation (total
short wave radiation) is called pyranometer. Principle
of working is temperature differentials between two surfaces (black & white) is directly proportional
to difference in solar radiation incident upon them.
Albedometer: It is used to measure reflectivity of short wave radiation is called albedometer. (Principle: as same as pyranometer).
Net radiometer: Net radiometer is measure net radiation. It has two
pyranometers, the sensors of which
are exposed to earth and sky. The sensor exposed to sky measures the incoming
radiation and the other facing
towards earth‟s surface measures
outgoing radiation. The sensor is shielded with plastic domes,
which are transparent to both short and long wave radiation.
Quantum sensor: It measures the photo synthetically active radiation (visible
radiation). This instrument
is most useful because it measures portion of solar radiation, which is essential
for photosynthesis.
Spectro-radiometer: This instruments measures solar radiation in narrow wave bands. This has been developed by ISRO in the
wave band width between 400 and 1010 nano micron.
Luxmeter: For measurement of intensity of radiation.
v
Units of measurement:
Solar radiation is expressed as watt per square meter.
In meteorology, it is measure
in cal/ cm2 / m and another
unit is Langley / min
1 watt = 1 jule / s 1 cal/ cm2/ min = 697.93
watt/ m2
v Precautions:
ü Pyranometers should
be kept horizontal while in use.
ü Observation should
be repeated to check for accuracy.
ü Any shading
on sensors should
be avoided.
ü While tacking
observation, plastic dome should be in fully inflated state.
ü The instrument is usually held 50- 100 cm above the surface
over which observation is to be recorded.
ü Plastic dome should be clean and free from wrinkles and deposits of foreign matter.
v Shortwave and long wave radiation estimation
ü Global radiation is
defined as the total of direct and indirect radiations
(diffused, scattered radiation).
ü The amount of radiation at the top of the atmosphere (Ra) is dependent on latitude and the time of year (Table Value).
ü Radiation while
passing through atmosphere is scattered
and absorbed by the atmosphere constituents and clouds.
ü Hence, the amount
of radiation reaching to earth surface is identified as solar radiation or global radiation
(Ra).
ü This is largely
dependent on cloud cover.
ü This radiation when
falls on soil, crop or water surface
are reflected back and lost to the atmosphere. This reflection depends on the
nature of surface.
Exercise
- 3
Objective: To study about wind
direction and wind speed at the time of observation.
Air expands when heated
and gets compressed when cooled. This results in variations in the atmospheric
pressure. The differences in atmospheric pressure cause the movement of air
from high pressure to low pressure, setting the air in motion. Atmospheric
pressure also determines when the air will rise or sink. Air in horizontal
motion is known as wind. The wind redistributes the heat and moisture across
latitudes, thereby, maintaining a constant temperature for the planet as a
whole. The vertical rising of moist air forms clouds and bring precipitation.
Ø Name of instrument:
1.
Cup counter anemometer
2.
Wind vane
v Wind Vane
The common instrument to determine wind direction is wind vane.
This instrument indicates the direction from which wind blows. It is a balanced
lever that turns freely about a vertical axis. One end off the lever exposes a
broad surface to the wind, while the other end of which is in the form of an
arrow head points to the direction from which the wind blows. This narrow end
is the form of an arrow head. Under this movable system, there are eight fixed
rigid bars that are set to the eight cardinal directions i.e. North,
North-East, South-East, South, South West, West and North-West.
Ø Installation: - It is installed over a wooden plank, is fixed on a wooden post.
The height between the pointer and ground level is exactly 10 feet (3.05m). The
north indicator should be se to true north and not to the magnetic north. The
axis of the wind vane should be exactly vertical
Wind Vane
Ø Units: - there are two ways of expressing wind direction
By sides: - In
sixteen points of a compass as N, NNE, NE, ENE, E, ESE, SE, SSE, S, SSW, SW,
WSW, W, WNW, NW, NNW.
By degrees (from north, measured in clockwise as 360, 20, 50, 70, 90, 110,
140, 160, 180, 200, 230, 250, 270, 290, 320, 340)
Wind Direction
v Cup Counter Anemometer
Wind speed is measured by cup counter anemometer. This
instrument consists of three or four hemispherical cups fixed at the end of
metal arms from a central point. The cup wheel is pivoted at the center to a
vertical spindle passing through a brass tube attached to the anemometer box.
The cups are set in motion due to the pressure difference occurring between two
faces of the cup. The vertical spindle (about which the cups rotate) is
connected to a mechanical counter through a gear system from which the number
of rotations made by the cups can be counted. The counter is directly
calibrated in kilometers.
Cup Counter Anemometer
Ø Installation: - In an Agro-meteorological
observatory, wind instruments are installed at an open site. It is installed on
pillars or wooden posts so that the height of the center of anemometer cup or
the arrow head should be 10‟ (3.05 m) above the ground. The minimum exposure criterion for
the wind instruments is that any obstruction should be away by at least 10
times the height of the obstruction.
Ø Units of measurement: - The wind speed is directly measured in terms of km/ hr.
However, the wind speed in synoptic charts is given in knots. The conversion of
different units:
1 knot = 1 Nautical mile = 1.15 mile/ h = 1.85 km/ h = 0.51 m /
s.
1 mph = 0.8684 knot = 1.609 kmph = 0.447 m/s.
1m/s = 1.94 knots = 3.6 kmph = 2.24 mph.
Ø Maintenance:
1.
The screw cap of the wind
vane should be lubricated by clock oil every fortnight.
2.
Anemometer should be
lubricated every week.
3.
Every six monthly the
bearing should be washed and lubricated thoroughly.
Ø Procedure:
1.
Watch the wind vane for
few minutes and identify the direction.
2.
Read the direction to
which the arrowhead points, nearest to the sixteen prints of the compass.
3.
Note the initial and final
reading of the anemometer after 3 minutes. Subtract the initial reading and
multiply by 20 to get the instantaneous wind speed at the time of observations
in kilometer per hour.
4.
Subtract the anemometer
reading at 7:00 hrs. LMT of the observation date and divide the difference by
24 to get the main daily wind speed for the observation dates.
Ø Observations & calculation of wind speed:
-
Instantaneous wind speed: - Note down two readings from anemometer at an interval of three
minutes. Multiply the difference by 20 to get wind speed at the time of
observation in km/h.
Mean daily wind speed: - Subtract the Anemometer reading at 0700 LMT of the
previous day from that at 0700 LMT of the observation day and divide the
difference by 24 to get the mean daily wind speed for the observation day.
v Barograph
It is used for recording continuously the atmospheric pressure.
It consists of several metallic chambers, one on top of other. The combined
motion of these is communicated to a lever, terminating in a pen. The pen
writes a record of the pressure upon a graph paper wound around a drum while
the drum is being rotated by clock within it. Exercise: Take the reading of
wind vane and Cup counter Anemometer. Calculate a wind speed at the time of
observation and mean daily wind speed.
Exercise – 4
Object: - Measurement, tabulation and analysis of rainfall
Precipitation is water
in liquid or solid forms, falling to the earth. Common precipitation forms are
rain, drizzle, snow, sleet and hail. Fog, dew and frost are not, generally,
considered to be precipitation forms.
Measurement of Rainfall
All forms of precipitation are measured as vertical depth of
water on a level surface, if the entire precipitation remained where it fell. The
total amount of precipitation falling on earth’s surface in a given period of
time is expressed as the depth to which it would accumulate on horizontal
projection of earth’s surface if there were no losses by evaporation or runoff
and if vertical sides can be used as a gauge for measuring rainfall. The
refined receptacles for measuring rainfall are called rain gauges. Non recording
rain gauges (ordinary rain gauges) and self-recording rain gauges (automatic
recording rain gauges) are the two commonly use instruments for measuring
rainfall.
Ordinary rain gauge
The
non-recording rain gauge, as the name indicates, does not record the rain, but
only collect the rainfall. The collected rain can be measured as indicated
below:
However, there is no need for using any equation
since the collected rain is measured by means of a graduated cylinder to
directly represent the amount of rainfall volume receives in cm of water depth.
Simon
type ordinary rain gauge used to be widely used in India till 1969. Since then,
the IMD has adopted another model called standard gauge at all the
observatories in the country.
The 20 cm
capacity rain gauge with 200 cm2 collector and 4 liters bottle is widely used
and is sufficient to measure 24 hours rainfall at most of Indian observatories.
The 40 and 100 cm capacity rain gauges are used at few places with high
rainfall. The rain gauge consists of a funnel of diameter127 mm. Rim of rain
gauge is 30 cm above the ground level and 25.4 cm above the cemented platform.
The collector and base are locked to each other by means of two complementary
locking rings fixed inside the collector at its lower end and the base at its
top end.
Ø These rain gauges just collect the rainwater but do not record
the quantity of rainfall
Ø Most extensively used non recording rain gauge in India –
standard gauge.
Ø Circular collecting area of 12.7
cm diameter connected to a funnel. Rim of the collector is set in a horizontal
plane at a height of 30 cm above the GL.
Ø Funnel discharges the rainfall catch into a
receiving vessel. The funnel and collecting vessel (bottle) are housed in a
metallic container.
Ø Water collected in the bottle is measured using a
suitably graduated measuring cylinder supplied with the rain gauge with 0.1 mm
accuracy.
Ø Rainfall is measured in mm or cm of water depth.
Measurement
Ø Remove the funnel by rotating it.
Ø Take out receiver containing rainwater and measure
it with measuring cylinder up to the first decimal.
Ø If water is more, measure again by and count the
full cylinders.
Ø Check the value of non-recording rain gauge with
self-recording one.
Rainfall collected in the bottle will
be measured daily at 0830 IST and 1400 hours LMT, if rain occurs. The process
of measurement at 0830 AM for the past 24 hours is most common throughout the
country. However, at times of heavy rainfall, two or three intermediate
readings may be taken and their sum reported as rainfall for the past 24 hours.
Rainfall exceeding 2.5 cm in a day is called a rainy day.
For the measurements,
collected rainwater is transferred to rainwater measuring cylinder graduated in
depth units at 0.2 mm interval. Rainfall less than 0.1 mm can be measured by
personal judgment and recorded as traces (tr). Measuring cylinder is calibrated
in depth units by taking the ratio of diameter of rain gauge funnel and
measuring cylinder. In the absence of rainfall measuring cylinder, the
following procedure is adopted for reporting rainfall in mm.
Rainwater collected in receiver is transferred to any measuring
cylinder graduated in volume units (cc or ml). volume of the rainwater and
diameter of the rain gauge are measured. Collected area of cross section is 
If the area of rainfall collector is 200 cm and rainwater collected is 100 cm, then the rainfall will be
Self- Recording Rain Gauge
This type of rain gauges,
with mechanical arrangement for recording rainfall on a graph paper, can give
us automatic record of rainfall without any bottle reading. Various models have
been designed with different gauges such as:
Ø
Tipping bucket type.
Ø
Weighing type.
Ø
Floating type.
Float type gauge, provided
with a self-starting siphoning arrangement, is most widely used in India. In
India, it is popularly called as natural siphon recording rain gauge. Rainwater
entering the gauge at the top of the cover is lead via funnel to the receiver
consisting of a float. As the water level rises in the receiver, the float
rises and the pen records on the chart, wrapped round a clockwise rotating
drum, the amount of water in the receiver at any instant. The rotating drum
completes one revolution in 24 hours or some times in 7 days and the chart will
have to be replaced accordingly. Siphoning occurs automatically when the pen
reaches top of the chart, after siphoning pen arm comes down to zero.
As the rain continues,
the pen rises again from the zero line of the chart. Rain gauge should be
installed on a level ground and fixed on a masonry foundation of 60 × 60 × 60
cm sunk into ground. Base of the gauge is cemented into the foundation so that
the rim of the gauge is exactly 75 cm above the case of self-recording rain
gauge.
þ
Recording rain gauges
give a permanent automatic record of rainfall. It has mechanical arrangement by
which the total among of rainfall, since the start of record, gets automatically
recorded on a graph paper. It produces a plot of cumulative rainfall vs time
(mass curve of rainfall). These rain gauges are also called integrating rain
gauges since they record cumulative rainfall.
þ
In addition to the total
amount of rainfall at a station, it gives the times of onset and cessation of
rains (thereby gives the duration of rainfall events). Slope of the plot gives
the intensity of rainfall for any given time period.
þ They can provide continuous record for a number of
days. They are very useful in hilly and far off areas. In other areas, they are
installed along with a non-recording rain gauge.
Exercise - 5
Object:
- Measurement of maximum and minimum air temperatures, tabulation, and
variation analysis
Atmospheric/air temperature is
measured by means of thermometers housed in a special wooden box called Stevenson
screen fixed at about 1.22 m above ground level. It is a wooden rectangular
box of length 56 cm, width 30 cm and height 40 cm with a double roof having
lower sides (big Stevenson screen 56 × 30 × 40 cm to house self-recording
thermograph). The screen es painted white and mounted on four wooden supports
with the bottom of the screen at 1.22 m above ground level. The screen is set
up with its door-facing north (opening downward) to minimize the sunlight
observations.
Stevenson screen protects the
thermometers from direct heating from ground and nearby objects and from losing
heat by radiation during night. It allows free air circulation besides protecting
the thermometers from rain.
Maximum and minimum thermometers
are placed in horizontal position on the upper and lower sides end rest at an
angle of 
to horizontal plane. Dry and wet bulb
thermometers are kept vertical in the wooden box on the left and right sides,
respectively.
ü Meteorological
thermometers are:
· 
Maximum thermometer
· Minimum
thermometer in Stevenson screen
· Dry
bulb thermometer
· Wet
bulb thermometer
v Maximum
temperature
Maximum temperature has
historically been measured with a mercury-in-glass thermometer which has a
constriction in the nick of the thermometer tube. Principle of working is that
all liquids expand or contract with increase or decrease in temperature. As the
air temperature rises mercury is forced past the constriction. However, as the
temperature fall the constriction prevents the mercury from returning to the
thermometer. The height of mercury in the remains at that reached at the
hottest time of day. The thermometer is reset by gentle shaking. The range of
maximum thermometer graduation is from - 
C to 
C, Maximum temperature, generally, occurs in
the world between 1400 and 1600 hours.
v Precautions
1.
It should be placed in a ventilated shelter.
2.
Parallax error should be avoided while taking
observation.
3.
It should be reset after making the observation at
8.30 am
4.
It should be placed in horizontal position in the Stevenson
screen.
5.
It should be reset by shaking the thermometer till
mercury moves back through the constriction and it attains the temperature of
the surrounding air.
v Minimum
temperature
Alcohol-in-glass thermometers
containing a moveable index are used to manually record minimum temperatures.
Working principle is that all liquids expand or contract with increase or
decrease in temperature. In the minimum thermometer alcohol is used, as its
freezing point is – 39˚ C. when the temperature falls, the liquid and index move
down the column, but when the temperature raises the index remains in the
lowest temperature reached since the last reset – which is achieved by tilting
the thermometer, bulb end upwards. Lowest temperature of the day, generally,
occurs just before sunrise or clear day and after sun rise on cloudy day. The
range of minimum thermometer graduation is from – 40˚C to 50˚C
The highest temperature over the
24 hours prior to observation (0700) is recorded as the maximum temperature for
the previous day. The lowest temperature for the 24 hours prior to (0700) is
recorded as the minimum temperature for the day on which the observation was
made. Thermometer is read to the nearest 0.1 degree Celsius.
v Recording
and setting of Air Temperature thermometers
þ Open
the door of Stevenson screen.
þ Note
the reading of maximum and minimum thermometers in sequence to the accuracy of
0.1˚C and verify for the correctness of the observation.
þ While
taking reading of maximum thermometer, watch carefully first the mercury column
and shining line and record the point.
þ To
avoid parallax error, your eye and the mercury level should be exactly in one
line.
þ While
recording the minimum thermometer, the set reading of the earlier day is first
seen in the pocket register and then the end of the glass index farthest from
the bulb is read.
Reading
of the maximum thermometer should be at least as high as or higher than any of
the dry bulb temperature readings taken since the previous setting. Reading of
the minimum thermometer should be as lower than any dry bulb reading taken at
or since the previous setting.
Ø Setting
of maximum thermometers:
After noting the reading, firmly
the grip the thermometer and swing vigorously up and down in a semicircle. No
jerks should be given. After setting, the reading be equal to that of dry bulb
thermometer.
Ø Setting
of minimum thermometers:
After afternoon reading, it
should be set. Thermometer should be gently titled keeping bulb with the
meniscus. The end of the index farthest from the bulb then indicates the
minimum temperature of the air at that moment.
v Dry
bulb temperature
Mercury-in-glass thermometer
called dry bulb thermometer is used for measuring dry bulb temperature (air
temperature range is from -35˚ to +55˚C. Least count is 0.5˚C However, observation
can be recorded up to 0.1 this temperature is used for calculating humidity,
vapour pressure and dew point.
v Wet
bulb temperature
Temperature on cool air (Wet
bulb temperature) is measured with wet bulb thermometer. It is similar to dry
bulb thermometer, but the bulb of the thermometer acts as evaporating surface.
The bulb of the thermometer is continuously kept moist by muslin cloth covering
the bulb. Four strands of cotton thread placed in a small container with
distilled water keeps the muslin cloth covered bulb continuously wet.
Temperature readings of both dry
and wet bulb thermometers will be same under saturated conditions. However, when
the air becomes dry, the difference would increase. The difference is known as
wet bulb depression. Which is used to know dew point, vapour pressure and
humidity.
v Self-recording
of temperature
Thermometer placed in big Stevenson
screen, is an automatic self-recording instrument which mark the prevailing
temperature continuously on a graph paper wounded round a drum. The drum makes
one revolution in a day, marking temperature changes. The graph paper is
changed every day at 0700 hours LMT.
Bimetallic thermograph consists
of a sensitive element of two strips of different metals (brass and iron), welded
together along the flat surfaces and bent into as arc. One end of the arc is
fixed to the base of the instrument and the other connected to bend arm, which
traces the changes on graph paper. Changes in temperature cause the two metals
to expand or contract differentially so that they bend or unbend accordingly.
Exercise - 6
Object: - Determination of relative humidity and
vapour pressure
v Determination of relative humidity
Moisture
(water vapour) content in the air can be measured in a number of ways. Most commonly
used reference to water vapour is relative humidity (RH), which is the ratio of
the amount of water vapour actually in the air as percentage of that contained
in the same volume of saturated air at the same temperature.
That
is, RH is the ratio of actual vapour pressure (AVP) to the saturation vapour
pressure (SVP)
RH% = 
×100 or
=
×100
(e is AVP and 
is SVP)
When air contains all the water vapour it can hold
at a given temperature, the air is said to be saturated and the pressure
exerted by the vapour is called situated vapour pressure. Water vapour actually
contained in the at a given temperature is called actual water vapour and the
air is said to be unsaturated. Pressure exerted by water vapour under
unsaturated conditions is called actual vapour pressure.
If a kg of air at constant pressure could hold 12 g
of water vapour at a certain temperature, but contains only 9 g that
temperature, it has a relative humidity of 75 percent.
Direct measurement of actual amount of water vapour
in the air is not feasible for ordinary observations. Following are the
instruments used for indirect measurement:
þ
Stationary psychrometer
þ
Whirling psychrometer.
þ
Hair hygrograph.
þ
Assmann psychrometer.
þ
Hair hygrometer.
v Stationary or simple psychrometer
Stationary
or simple psychrometer consists of dry and wet bulb thermometers, housed
vertically in the Stevenson screen. In other words, a set of dry and wet bulb
thermometers is known as psychrometer. Using the hygrometric table and dew
point temperature, relative humidity can be obtained.
v Assmann psychrometer
In
Assmann psychrometer, dry and wet bulb thermometers with cylindrical bulbs are
fixed vertically. The bulbs are protected from radiation by two coaxial tubes.
It is designed to measure temperature and relative humidity, both in the open
and in the crop canopy. The aspiration is provided by means of a clock work
fan, by which air is drawn at a speed higher than 10 feet per second.
v Whirling psychrometer
Temperature
and relative humidity of the air in open and in the crop canopy at different
height can be measured with whirling psychrometer. the dry and wet bulb
thermometers are attached horizontally to a rectangular wooden frame and it can
be rotated with handle. Four rotations per second should be given for obtaining
desired wind speed around 5 meters per second. With dry and wet bulb
temperature, due point temperature vapour pressure and relative humidity at different
heights can be obtained.
v Hair hygrometer
It is used to obtain humidity at the time of observation by using human
hair. The amount of water vapour air can hold depends on the temperature of the
hair. When the relative humidity is 100 percent, the air is saturated (holding
as much water vapour as it possibly can at that temperature). Hot air can hold
more water vapour than cold air, because cold air is denser and cannot hold as
much water vapour.
Humidity causes curly hair to
go frizzy or straight hair to go limp because it changes hair length by 3
percent from dry (0% RH) to very humid (100%RH) conditions. Whether the hair is
dark or light, straight or curly, this ratio is stays the same. That’s why we
can make relatively accurate measurements of air humidity using human hair. The
tool is called the hair hygrometer.
v Hair Hygrograph
Continuous record of humidity can be obtained with hair hygrograph. The
property of human hair to increase the length with increase in humidity and
decrease with decrease in humidity is used in the instrument. It consists is
used in the instrument. It consists of a bundle of de-oiled human hair tied at
both ends and kept tight in the middle by means of a hook attached to one arm
of the lever and second arm is associated with pen arrangement which can make
marking on graph paper attached on the clock driven revolving drum. Variations
in length of human hair cause displacement of the hook, which is communicated
by second arm of the lever to record changes on graph paper. The circular drum
makes one rotation in 24 hours or once in a week. The instrument is placed in a
Stevenson’s screen in the observatory.
The sensitive (activating) elements of hygrograph and thermograph are
combined in the same case with pens tracing their respective records on the
same chart. This combination is called hygrothermograph.
v Recording of Relative humidity
Hygrometric tables and saturation
vapour pressure tables are necessary for calculating the relative humidity as
indicated already.
Hygrometric table is used to find out
dew point temperature and relative humidity corresponding to dry and wet bulb temperature.
The saturation vapour pressure table is used to find out saturation vapour
pressure (mm hg) values corresponding to dry-bulb, wet-bulb and dew point
temperature values. If the altitude of a place of observation is less than 457
m,1000 mb and for higher stations, 900 mb hygrometric tables are to be used.
Dew point temperature and relative
humidity corresponding to specified values of dry and wet bulb temperatures are
given in the hygrometric tables at an interval of 0.C.
while using the tables, interpolation to nearest 0.1
has to be done wherever necessary. The RH and
vapour pressure is recorded and reported as indicated in table
Recording and reporting relative humidity and
vapour pressure.
|
Std
wk |
date |
Time
ist |
Dry
bulb |
Wet
bulb 0
C |
Depression 0 C |
RH |
Vp Mm
Hg |
|
|
1 2 3 |
M A M A M A |
|
|
|
|
|
v Vapour pressure
ü
Water vapour is a gas and
its pressure contributes to the total atmospheric pressure. The amount of water
in the air is related directly to the partial pressure exerted by the water
vapour in the air and is therefore a direct measure of the air water content.
ü
In standard SI units,
pressure is no longer expressed in centimeter of water, millimeter of mercury,
bars, atmosphere etc., but in pascals (pa). conversion factors between various
units and pa have been given. As a pascal refers to a relatively small force
(1newton) applied on a relatively large surface (1 
),
multiples of the basic unit are often used. Here, vapour pressure is expressed
in kilopascals (kPa = 1000 Pa)
ü
When air is enclosed
above an evaporating water surface, equilibrium is reached between the water
molecules escaping and returning to the water reservoir. At that moment, the
corresponding pressure is called the saturation vapour pressure (SVP,).
The number of water molecules that can be stored in the air depends on the
temperature. The higher the air temperature, the higher the storage capacity,
the higher its saturation vapour pressure ().
ü
Actual
vapour pressure (AVP,
)
is the vapour pressure exerted by the water in the air. When the air is not
saturated, the actual vapour pressure will be lower than the saturation vapour
pressure. The difference between the saturation and actual vapour pressure is
called the vapour pressure deficit or saturation deficit and is an accurate
indicator of the actual evaporative capacity of the air.
ü
Vapour pressure deficit (
-
)
is the difference between the saturation ()
and actual vapour pressure ()
for a given time period. For time periods such as a week, ten days or a the is
computed using the 
and 
averaged
over the time period and similarly the is computed using average measurments over the
period. Using mean air temperature and not and 
results
in a lower estimate of ,thus
in a lower vapour pressure deficit and hence an under estimation of the ET.,
Exercise - 7
Object:
- Measurement of open pan evaporation and evapotranspiration
v Evaporation
Evaporation
is the process whereby liquid water is converted to water vapour (vaporization)
and removed from the evaporating surface (vapour removal). Water evaporates
from a variety of surfaces, such as lakes, rivers, pavements, soils and wet
vegetation. Energy is required to change the state of water from liquid to
vapour. Direct solar radiation and, to a lesser extent, the ambient temperature
of the air provide this energy
v Transpiration
Transpiration consists
of the vapirisation of liquid contained in plant tissues and the vapour removal
to atmosphere. Crops predominately lose their water through stomata all water
taken up is lost by transpiration and only a tiny fraction is used within the
plant Transpiration, like direct evaporation, depends on the energy supply,
vapour pressure gradient and wind.
Evapotranspiration (ET)
Evaporation and transpiration occur
simultaneously and there is no easy way of distinguishing between the two
processes. Apart from the water availability in the topsoil, the evaporation
from a cropped soil is mainly determined by the fraction if the solar radiation
reaching the soil surface. This fraction decreases over the growing period as
the crop develops and the crop canopy shades evaporation, but once the crop is
well developed and completely covers the soil, transpiration becomes the main
process.
Units
The evapotranspiration rate is normally expressed in
millimeters (mm) per unit time. The rate expresses the amount of water lost
from a cropped surface in units of water depth. The time can be an hour,
decade, month or even an entire growing period or year.
As one
hectare had a surface of 10000 and 1mm is equal to 0.001 m, a loss of 1 mm of
wate corresponds tio a loss if 10 
of water per hectare. In other words, 1 mm da
is equivalent to 10
h
da
Water
depths can also be expressed in terms of energy received per unit area. The energy
refers or required to vaporise free water. This energy, known as the latent
heat if vaporisation (λ), is a function of the water temperature. For example,
at 20 C is about 2.45 k
. In other words, 2.45
MJ 
is able to vaporise 0.001 m ir 1 mm of water,
and therefore 1 mm of water is equivalent to 2.45 MJ .The evapotranspiration
rate expressed in units of MJ da is represented by ET, the latent heat flux.
v Measurement
of open pan Evaporation
The United States weather bureau
class A open pan evaporimeter or simply US-WB class A pan is most commonly used
in observatories of India.
The class a pan consists of a
pan made up of galvanized iron. The pan is 120.7 cm in diameter, 25.4 cm deep
and its bottom is raised 15 cm above the ground surface. It is supported by an
open wooden platform. Depth of water is to be kept in a fixed range such that
the water surface is at least 5 cm and never more than 7.5 cm below top of the
pan. The pan coefficient is, generally, taken as 0.7 or 0.75.
Ø Measurement
Ø Rate
of evaporation is measured by measuring the height of water level at fixed
times with a fixed-point gauge. A stilling well of size 10 cm diameter and 30
cm height is placed in the tank to isolate small portion of the water surface
in the tank to avoid disturbance by waves if any due to wind.
In the point gauge. The pointed
brass is fixed vertically at the center of the cylindrical stilling well. Tip
of the rod is located at 6 to 7 cm below rim of the pan. Three small holes are
located at the bottom of the well to permit flow of water in and out of the
well. At each observation, water level is brought to the same fixed point by
adding water with a graduated measuring cylinder.
Cross-sectional area of the
cylinder is exactly 1/100 of the area of evaporation pan. The scale from 0 to
20 cm is engraved inside it along the height and the graduation runs from top
to bottom ascending order. One full cylinder of water raises 2 mm height on the
pan. Evaporation can be measured accurately up to 0.1 mm.
þ When
there is no rainfall, evaporation rate (mm) will be equal to water added (mm)
to the evaporation pan (E = Number of measuring cylinders of water added to the
tank × 2).
þ On
a rainy day if water added is 3 mm and rainfall is 5 mm, evaporation will be 8
mm (rainfall 3 mm + added 5 mm).
þ If
the rainfall is heavy, water must be removed from the tank with measuring
cylinder. Difference between the actual rainfall of the previous day and water
removed from the tank gives the evaporation rate. If rainfall is 20 mm and
water removed is 10 mm then the rate of evaporation will be 10 mm (20 – 10 mm).
þ If
there is very heavy rainfall, the tank overflows and evaporation cannot be
obtained the message overflown or excess rainfall is written in the weather
report.
EXERCISE - 8
Measurement of Atmospheric Pressure and Analysis of Atmospheric
Conditions
Objective: To study about atmospheric conditions.
Technically, pressure is defined as the force per unit area. But
the pressure exerted by the
atmosphere on the earth’s surface is called
atmospheric pressure. It is defined as the pressure
exerted by a column of air with a cross sectional area of a given unit
extending from the earth’s surface to the upper most boundary of the atmosphere. The
standard sea level pressure is given
as 1013 mb or 76 cm or 29.92” at a temperature of 15˚C and 45˚C north latitude. Atmospheric pressure does not
have direct influence on crop growth. It is however, an important weather
parameter in weather forecasting.
v Instruments:
ü Fortin’s barometer
ü Kew pattern barometer
ü Aneroid barometer
ü Barograph
The standard instruments for measuring atmospheric pressure are
aneroid barometer and barograph.
v Fortin’s barometer
This
barometer is standard and accurate instrument for measuring pressure. It
consists of a small cistern vessel
containing mercury with a flexible leather bag and a screw at its bottom. The mercury level can be raised or
lowered with the help of the screw. In the cistern vessel, a glass tube filled with mercury is kept inverted. In
this vessel there is a pouted ivory pointer.
from the lower tip of this pointer, the zero of the scale starts and therefore
while taking reading, the mercury
level in cistern vessel must touch the lower tip. There are two scales on two sides of the tube, one in centimeters
and the other in inches. Vernier caliperare also
attached for accurate reading. To take pressure reading the height of mercury
column is measured on main scale and then Vernier scale
is read.
Atmospheric pressure
= MSR + VSR X Vernier
constant
The metal scale and the mercury expand
differently at different temperatures. They are,
therefore transformed to one common temperature which is zero degree centigrade
or 273˚ K. The gravitational pull changes according
to latitude. Hence, the gravitational correction is applied
and all the readings are transformed into one common
latitude i.e. 45˚ N. All the readings are transformed to
sea level height. Thus, three corrections such as temperature, gravity and latitude are applied.
v Kew pattern
barometer
This is also similar to Fortin’s barometer where
the cistern vessel is fixed and has no adjusting
screw. The divisions are made unequal in order to allow rise or fall of mercury column
in the cistern. In this barometer initial adjustment of cistern
is not required.
v Aneroid barometer
This
barometer does not contain any liquid. It consists of a evacuated box with a corrugated sheet of metal lid held in
position by means of a spring to avoid collapse of the top and bottom. This box is called as siphon
cell and is sensitive to change in pressure. When the pressure increases the cell is compressed and when it decreases
the cell is expanded. These variations
are magnified with the help of levers and are communicated through chain and pulley to the pointer, which moves on
graduated scale. This pointer gives direct pressure reading. This is not
an accurate instrument.
v Barograph
This
instrument is used for automatic and continuous record of atmospheric pressure; it is a special type of aneroid barometer
having recording system. It consists of several vacuum boxes similar to aneroid barometer placed one above
another. The combined motion of these
vacuum boxes becomes appreciable and is then communicated to a level system.
The changes are marked on a chart
paper fixed on the clock driven rotating drum. The chart is calibrated in cm or inches on one axis and hrs/days
of week on another axis.
Thus, a continuous record of atmospheric pressure is obtained. Before use the instrument must be standardized with the help of Fortin‟s barometer. This instrument does not give
correct pressure readings. However, it is helpful in recording the
barometric tendencies.
Ø Use of barometer
It is used for approximate forecasting, to measure atmospheric pressure and to measure height of a given station above mean sea level.
Ø Weather and pressure
ü Falling barometer indicates rain or storm (bad weather).
ü Rising barometer indicates fair weather
(clear and stable).
ü Steady barometer indicates steady or settled weather.
ü A continually rising pressure indicates
fine and settled
weather and a steadily falling
pressure indicates occurrence of unsettled and cloudy weather.
Ø Units of pressure
The pressure is measured in following units.
1 atmospheric pressure
= 29.92” = 76
cms = 760 mm
= 1013 mill bar
= 101.32 kilopascal (Kpa)
= 14.7 lbs/inch2
= 1.014 X 106 dynes /cm2
=1 bar
ü Height of mercury
column in inches or cms or mm
ü Bar is force equal to
106 dynes /cm2. This
is big unit and is therefore divided into smaller units 1 bar
= 1000 mb
ü In standard international unit of pressure is Pascal 1 Pascal
= force of 1 Newton/ sq.m.
Ø Calculation Based on Air Density
And Atmospheric Pressure
Ex.-1: Calculate the standard sea level air density, if the
standard sea level
pressure is 1013
hPa and temperature is 15.00C
Absolute temperature = C+273
= 15.0+273.0
= 288 0K
Air
density = P/RT
Where P =Atmospheric
pressure at msl R = Gas
constant = 2.87
T = Absolute temperature
Air density (kg/m3 ) = 1013/2.87*288
= 1013/826.56
= 1.225
Ex.-2: Calculate
the mean sea level
pressure, if the air density
and air temperature are 1.165 kg/m3
and 30.00C respectively.
Absolute temperature = C+273
= 30.0+273.0
= 303 0K
Air density (kg/m3 ) = 1013/2.87
1.165
= P/2.87
= 1013
hPa
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