The slides are about Weather station and various instruments used in it.

1. CH.2.4 WEATHER

2. MEASURING THE WEATHER
• THE WEATHER STATION
-Place where elements of weather ex rainfall,
humidity, air pressure, wind direction & velocity,
sunshine and cloud cover
-measured and recorded
-open piece of land

3. • INSTRUMENTS
-Stevenson screen
-Rain gauge
-Barometer
-Wind vane
-Anemometer
-Sunshine recorder
• Sited away from buildings, fences, trees and
other obstacles

4. STEVENSON SCREEN

5. • STEVENSON SCREEN
-wooden box on four legs at about 120 cm
-screen built so that shade temp. of air can be
measured
-side of box, slatted to allow free entry of air
-roof: double boarded, prevent sun’s heat
-outside painted white, reflect sun’s energy
-screen is usually placed on grass covered
-surface, reducing radiation of heat from ground
-Instruments kept inside are max.-min. thermometer
and wet-dry bulb thermometer (hygrometer)

6. MAXIMUM AND MINIMUM THERMOMETERS

7. SIX’s THERMOMETER

8. • MEASURING TEMPERATURE
-Variations in temp. due to differences in insolation,
amount of energy from sun
-using Thermometer
-Continuous temp. reading: Thermograph
1. Maximum Thermometer: temp. rises, mercury expands &
pushes index along the tube. temp. falls, mercury
contracts but index stays where it was pushed to by
mercury.Max. temp at point where index is. Index is then
drawn back to mercury by magnet for next reading.
2. Minimum Thermometer:Temp. falls, alcohol contracts &
its meniscus pulls index along tube. Temp rises, alcohol
expands. Read same way as max thermometer
3. Six’s Thermometer:measure max & min temp at same
time

9. • READING OF MAX & MIN THERMOMETERS
-to work out:
-Mean Daily temp
-Mean monthly temp
-Mean annual temp
-Diurnal temp range
-Mean Monthly temp range
-Mean annual temp range

10. RAIN GAUGE AND ITS PARTS

11. • MEASURING RAINFALL
-by “RAIN GAUGE”: consists of
-Cylinder containing collecting can
containing glass/plastic jar, funnel
-placed in open space, only raindrops enter the funnel,
no runoff water
-gauge sunk into ground, so that top of funnel is 30 cm
above to prevent sun’s heat from evaporation and no rain
splashes up from ground into the funnel
-rainwater falling over funnel collects in jar, which is
emptied every 24 hrs and measured in glass, graduated in mm.
-Isohyte: a line on map that joins areas of equal rainfall
-check rain gauge everyday, at same time, to avoid false
reading because of dew effect.

12. WET AND DRY BULB THERMOMETER

13. • MEASURING RELATIVE HUMIDITY
:by Wet and Dry Bulb Thermometer
-also known as Hygrometer
-Dry-bulb is a glass thermometer, records
actual air temp.
-Wet-bulb is similar thermometer, but with
bulb enclosed in a muslin bag dipped into water,
measures temp, unless RH close to 100%, is
lower than dry-bulb temp

14. BAROMETER

15. • MEASURING AIR PRESSURE, WIND SPEED & DIRECTION
1. AIR PRESSURE:by Barometer
-air has weight, exerts pressure on Earth’s surface.
-at sea level, 1.03 kg/cm3
-varies with temp and altitude
-measure in millibars
a. Mercury Barometer: hollow tube, with mercury without
air.When pressure of Hg in tube balances the pressure of air on exposed
Hg, Hg in tube stop rising. Hg column rises as pressure , falls as pressure

b. Aneroid Barometer: vacuum chamber in form of small metal
cylinder. Inside a small metal spring.Spring contracts and expands with
change in atm. pressure. Changes magnified with a series of levers and
conveyed to a pointer which moves a calibrated scale.
-Barograph: is a tracing from an aneroid barometer which records
continuosly for 1 week, recorded by flexible arm which traces an ink line
on a rotating paper covered drum, divided by vertical lines at 2 hr
intervals.
-Isobars:Atm. Pressure recorded at numerous weather stn at sea
level, plotted on map. Same pressure lines drawn on map called Isobars.

16. 2. WIND DIRECTION: by WIND VANE
-consists of horizontal rotating arm pivoted
on a vertical shaft.
-rotating arm has a tail and pointer.
-as wind blows, arm swings until pointer faces
wind.
-directions NEWS marked on arms which are
rigidly fixed to shaft.

18. WIND ROSE

19. 3. MEASURING WIND SPEED: by ANEMOMETER
-consists of 3 or 4 metal cups fixed to metal arms
that rotate freely on a vertical shaft.
-where there is wind, cups rotate.
-no. of rotations, recorded on a meter to give speed in
km/hr.
-Wind rose: wind direction for a specific place can be
shown on Wind rose, made up of a circle from which
rectangles radiate.
directions of rectangles represents points of the
compass.
lengths of rectangles, by no. of days/times wind
blows from that direction.
centre of the rose, no wind.

20. SUNSHINE RECORDER

21. • MEASURING SUNSHINE: by “Sunshine Recorder”
-no. of hrs & minutes of sunshine received at a place,
measured and recorded by sunshine recorder.
-Glass sphere partly surrounded by a metal frame.
-a strip of special card, divided into hrs & min. is
placed below the sphere.
-when sun shines, sphere focuses sun’s rays on the
card. As sun moves, rays burn a trace on the card.
-at the end of the day, card is removed and replaced.
-the length of the trace represents the amount of
sunshine that the location received.

22. ASSESSMENT QUESTIONS:
1. Describe and explain the main characteristics
of a Stevenson screen.
2. What information does a Six’s thermometer
show?
3. Why are weather readings taken at the same
time each day?
4. Where is the best place to locate a rain
gauge? Briefly explain why?
5. How are wind speed and wind direction
measured?

23. TEN DIFFERENT TYPES OF CLOUDS

24. • RECORDING THE WEATHER
-CLOUDS: Ten main types of clouds into 3 catergories
1. High Clouds: usually of ice crystals, 5500- 14000 m.
a. Cirrus- white filaments
b. Cirrocumulus-small rippled elements
c. Cirrostratus- a transparent sheet, often with a halo
2. Medium Clouds: usually of water droplets or mixture
of water droplets and ice crystals, 2000- 7000 m.
a. Altocumulus- layered, rippled generally with white
with some shading
b. Altostratus- a thin layer, grey, allow sun to appear as if
through ground glass

25. 3. Low Clouds: usually water droplets, although
cumulonimbus clouds include ice crystals, base
below 2000m
a. Stratocumulus- layered, series of rounded rolls,
white with some shading
b. Stratus- layered,uniform base, grey
c. Nimbostratus- thick layer with low base, dark &
rain or snow may fall from it
d. Cumulus- individual cells, vertical rolls or towers
with a flat base
e. Cumulonimbus- large cauliflower-shaped
towers, often with ‘anvil tops’, & sometimes
giving thunderstorms or showers of rain/snow

26. • Oktas (eights): Cloud Cover measure in oktas
(eights)
– Visual assessment of how much of the sky covered
by cloud