WEATHER
OBSERVATIONS
Weather observations consists of two types of information namely'
1) Instrument acquired information: This is in most cases accurate.
2) Visually estimated information, eg cloud cover and visibility.
Meteorological observations are required for a variety of purposes which can be divided
into two broad categories such as :
• synoptic observations
• climatological observations
Components of Meteorological/ Information in a meteorological observation include:
• Temperature (dry bulb and dew point)
• Humidity( calculated or read from instrument)
• wind speed and direction
• radiation, sunshine hours
• cloud cover and clouds
• soil temperature
• present or existing weather
• past weather
• pressure and pressure tendencies
• visibility
• evaporation
Weather information observed above is coded according to the WMO
standards which all meteorological services the world over use. The coding
is the synoptic code.
Radiation measurement:
a) Def: Radiation is the emission of energy by a body in the form of
electromagnetic waves
b) Solar radiation consists of a spectrum of
i) X ray, gamma ray and UV . These comprise about 9% of solar
energy
ii) Invisible rays making 50 % of solar energy
iii) Visible comprising 41 % of solar energy
c) Solar radiation I short wave and is often referred to as insolation.
Radiation emitted by the earth is long wave and is called terrestial
Radation
d) Solar Constant is the solar energy received per m per sec on a
surface perp to the sun’s beam at the mean earth –sun distance in
the absence of an atmosphere value about 1370 wm.
e) Radiant Flux rate of transfer of energy by electromagnetic radiation
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(JS = wat). Radiation flux of the sun is =3.9 * 10 wats.
f) Global radiation this refers to receipt of solar radiation regardless of
the source
g) Diffuse radiation refers to radiation reradiated or reflected by the
atmosphere and any matter held in the atmosphere such as haze.
h) Direct radiation: Energy received directly from the sun.
Factors affecting the distribution and amount of solar radiation
i) Distance from the sun
ii) Atmospheric obscurity, clouds,dust particles etc
iii Sun's angle: angle at which the sun strike he earth winter the angle is
high and the radiation is low and low angle in summer gives high
radiance
iv) Sun's pot:
Radiation Instruments(WMO Instrument Manual)
Diserable properties of meteorological instruments:
Radiation instruments:
a) Sunshine recorder/ campbell stroke: Measures sunshine hours which are then
converted to radiation taking into cinsideration location of the place, average
duration of bright sunshine. A glass sphere focuses light on a card which gets
charred without burning. This card has chemical which prevents it from burning.
The brass bowl is oriented in special angles depending on latitude and time of the
year.
Connection between solar radiation and sunshine:
Q/Q(R) = a + b Cos Φ(n/N), N = no of hrs of daylight, n = no of sunshine hrs
received, φ = station latitude, b = constant usually taken as 0.52 for Zimbabwe, a =
constant usually taken as 0.29 for Zimbabwe, Qq = amount of solar radiation received
at the ground.
From the cards (i) direct conversion to solar radiation can be
Done.
ii) time of when it was cloudy can be
determined
iii) continuos trace of radiation is made on the
card
b) Pyranometers/solarimeters: These measure short wave radiation especially
global radiation. Solarimeters receive sun's rays and develop a thermocouple and
electrical current proportional to the intensity of solar radiation received. This
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current is then measured to determine the radiation. These equipment are
calibrated to world standards.
c) Moll pyranometers: more accurate than the Gunn-Bellan but expensive
d) Pyrheliometers : i) Angstrome pyheliometer
ii) Linke-Fuessner pyheliometer
iii) Cavity pyheliometer (radiometer)
iv) Wet pyranometer
v)
Eppley pyranometer
Reliability of Radiation Measurement Instruments:
Gunn-Bellan:
i)Water may freeze in the cylinder
ii) Obstacles can establish themselves on the exposure
iii)Heat can be lost from the blackened cylindrical surface
iv) Cold junctions of Moll pyranometeres may not be at constant temperature
v) For the sunshine recorder, change of position can affect the accuracy of the
instrument
NB: The simple pyranometers are less accurate and the sophisticated ones are very
expensive
Limitations of available radiation data:
1) Lack of extensive observation network
2) Use of different sensors with different operating characteristics fron one place to
another
3) Great variability of data over short distances due to clouds and turbidity
conditions.
4) Decay of the spectral characteristics of the plastics or glass used in many radiation
sensors and their possible change with time or exposure..
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Measurement of air temperature and soil temperature:
1) Temperature : Heat level of a substance and is the condition which determines
the flow of heat from one substance to the other. The temperature of a substance
will vary according to the heat it is receiving or it has received from a source.
Rate of change of temp depends on the specific heat (define) of that substance.
Temperature variation from day to night is called the diurnal variation. The max is
about early afternoon (2p.m) and the min about sun rise around 5-6a.m.
Temperature is measure at 1.25 to 2m and when the wind is light the variation is
large and with strong winds turbulent mixing reduces the variation. Clouds also
reduce the variation due to reduction of outgoing terrestrial radiation.
2) Measurement of temperature are required for the following :
• air near the ground surface
• soil at various depth
• surface level of the sea and lakes
• upper air temperature
Units Of temperature:
These are (a) Kelvin ( K), Celcius ( C), Fahrenheit ( F). Mercury, alcohol and spirits
are in all simple thermometers
F = 9/5*C + 32,
C = 5/9(F - 32).
K = 273.15 + C. Kelvin scale is the absolute scale
Types of thermometers:
1) The Dry bulb thermometer: Very simple and easy to use . Read upper meniscus
of the mercury.
2) The Wet Bulb:This is the dry bulb covered by a wet piece of cloth( muslin).
Water from the constant level bottle via capillary. The wet and dry bulb together
can measure Relative humidity using the Wet bulb dip (difference between the
wet bulb and the dry bulb). When the wet bulb dip is low humidity is high and
when it is high then the humidity is low and the air is dry. When the air is dry
evaporation is encouraged and there is lots of cooling due to this process. On the
other hand when the air is moist(humid), there is less evaporation and less cooling
3) Maximum thermometer: Uses mercury and measures the maximum temperature
from 0800 a.m today and 0800 the following morning. The maximum
temperature normally occurs around 1300 to 1500. Mercury rises as temp rises to
reach the max and stay there due to a constriction in the tube near the bulb. The
thermometer is hung horizontally in the screen and has to be reset every morning.
After resetting the max temp must read the same as the dry bulb thermometer.
The major fault is retracting, which occurs when the max thermometer reads 1/2
a degree© lower or more than the previous day's reading or lower than the highest
dry bulb temperature.
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4) Minimum thermometer: measures the lowest temp reached from 0800 to 0800
the following day. It uses spirit (alcohol) instead of mercury. The minimum
temperature occurs about sunrise but not always. Inside the glass tube there is a
small pointer called the index which moves only when the temperature is going
down. When the temperature has reached its lowest value the index stops moving
and remains at the lowest temp. Reading must be taken at end of the index
furthest from the bulb ie right-hand end. After resetting it should read the same as
the dry bulb thermometer.
Faults: are the development of bubbles in the bore.
5) Casella Bi-metallic strip Thermograph: These include the thermistors,
Platinum resistance thermometer(PRT).
Errors common to all liquid in glass thermometers:
1) error of parallax: due to refraction
2) elastic error : due to glass type which may be expanding or contracting too
slowly
3) adhesion error: which is associated with spirit thermometers, the swpirit wets the
glass and when the temperature falls rapidly, some liquid will remain attached to
the glass causing errors ( low reading).
4) Breaking up of liquid column: Drops of liquid often form on the upper part of
the stem due to evaporation and condensation. In this case the thermometer
should be heated gently.
5) Slow changes in the liquid charateristics(decomposition): the organic matter
will tend to polymerise with age when exposed to light and the volume of the
liquid will be reduced, this can be speeded up by the presence of impurities such
as dyes used to colour the liquid to make it more visible.
Question 1:Discuss the purpose and how the Stevenson Screen protects the
instruments it houses.
Principles of design of Thermometer:
a) Liquid in glass: mercury and alcohol. . Alcohol is used for temperatures lower than 36 C
b) Liquid -in-metal:
c) Thermocouples
d) Bimetallic Thermometers
e) Platinum resistance thermometers
f) Thermistors(see handout on temperature)
Measurement of Soil temperature:
a) A bent -stem liquid(Hg) with a bulb at different depths usually 5, 10, 15, 20 and 30
cm depth and thermometers are placed at these various heights.
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b) Or holes are dug in the ground of 5, 10, 15, 20 and 30 cm depth and the normal dry
bulb thermometer are inserted into these holes.
c) Ground minimum temperature is measured at 0 cm ie at the surface
Factors which affect the response time of a thermometer are:
• thermal capacity of the actual sensor
• conductivity of the material used
• speed of heat transfer between the thermometer and the air
Measurement of Pressure
Pressure = Force per unit area
Force = mass * acceleration = M A
Pressure = Mg/A, M = mass, g = gravitational constant 9.8 m/s
Units are pascal(Pa), 100 times =hectopascal (hPa) , 1hPa = 100Pa =1mb
1Pa = 1wm = 10 mb.
Diurnal variation of pressure( Diagram):
NB: The trend or variation is important in observations because a rise or a fall
might not be a true rise or fall.
There are two types of pressure measuring equipment namely:
• mercury barometer
• aneroid barometer barographs
Mercury barometer comprises of a graduated cylinder with an attached thermometer.
Types of barometers are: 1) Kew barometer - only one setting on the vernier scale is
required.
2) Fortin barometer.
Corrections to be made on pressure reading are:
• Index correction - instrument error due to incorrect positioning of the barometer or
inaccuracy in scale or bore of the tube.
• Gravity correction; this caters for variations of the gravity with altitude and
latitude
• temperature correction: temperature affects the expansion and contraction of
mercury so this correction caters for this.
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Principles of Barometers:
P = ρgh, or ρgz = hydrostatic Equation
dp/dz = ρg, p = atmospheric pressure, ρ = density of the liquid which varies with
temperature. Ρ for mercury is 13.5961 * 10kgm, T is temp in C, g gravitational
constant, h is height of the liquid column.
The two types of mercury barometers use the manometer principle where the weight
of the atmosphere is supported by the weight of the mercury column or any liquid.
Diagram for the manometer
h = height = p( pressure being exerted by the atmosphere).
Errors in mercury barometers are:
• dirty mercury or tube: affects the expansion characteristics of the mercury.
• temperature gradients in the room.
• vacuum defects: the mercury may vaporise giving mercury vapour in the vacuum
hence resulting in errors.
• wind effects: There are two types of pressure measurement namely
j) dynamic pressure; which is pressure for moving air usually on an aircraft
k) static pressure: this refers to pressure of still air. When we talk about pressure we
will be referring to static pressure. When there is wind there will be an error due
to the departure from static to dynamic pressure.
Barographs:These give a continuous record of pressure. There is a collapseble
partially vacuumed metal attached to the pens and has a clock on which a chart is
attached for the pen to write/mark. There are time marks at 0800 when the chart is
changed and also at 0900.
Errors or limitations of the Barographs are:
• stepping: These are sudden pressure changes.
• clock stoppges.
• changes in the zero setting
• chart has to changed
Advantages are:
• very precise
• give a continuos record of pressure
• portable and trustworthy
There has to be check reading every Saturday.
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Define QFE, QNH, QFF. Other pressure instruments are i) altimeters and ii)
precision aneroid barometers
Wind speed and direction measurement:
Wind velocity is a vector that embraces both magnitude and direction.
Wind speed is a scalar quantity as it has a magnitude only.
Units of are i) ms ii) kmh iii) knots(kt).
1 knot = 1 nautical mile per hour
1knots =kt = 0.515ms = 1.853 kmh
Wind Direction is direction from which the wind is blowing from measured clock
wise from 360/0 .
Exposure of the Equipment:
• open terrain at least 10m from the ground
• at least 10H from the nearest obstruction where H = height of the obstruction
Wind velocity has variations which are not constant and this property is called
gustiness/turbulence. For most purposes , the velocity is taken as the mean wind
speed for period of 10 minutes.
Gustiness factor (G) is given by
G = (Vmax -Vmin)/Vmean, Vmax =max wind speed recorded for the ten minutes,
Vmin = min wind speed recorded for the ten minutes, Vmean = average wind speed
for ten minutes.
Measurement of wind speed is facilitated by the wind vane. The response is about +
1%. The response to sudden winds is more intricate than in other instruments because
of mechanical inertia.
Limitations are:
• large response time due to friction from the pivot.
• sensitivity is low for light winds.
• for most recording vanes , errors can arise from the translation of the wind direction
from the vane to the scale.
Wind speed: is measured by a cup anemometer which consists of 3 or 4 cups mounted
symmetrically about thew vertical axis that rotates in one direction. The anemometer
essentially measures length of the wind that passes over that point, usually for 6- 10
minutes, So that V =lenth /time, units ms.
Limitations in the cup anemometer are:
• It has been found out that the cup wheel accelerates more quickly with an increase
in wind but decelerates in falling winds, hence it reads more when winds are
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increasing and less when winds are decreasing. The result will be a mean wind which
is higher than the true wind.
to use
5)
i)
j)
k)
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