CH.2.4 WEATHER

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

• INSTRUMENTS-Stevenson screen-Rain gauge-Barometer-Wind vane-Anemometer-Sunshine recorder

• Sited away from buildings, fences, trees and other obstacles

STEVENSON SCREEN

• 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)

MAXIMUM AND MINIMUM THERMOMETERS

SIX’s THERMOMETER

• 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

• 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

RAIN GAUGE AND ITS PARTS

• 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.

WET AND DRY BULB THERMOMETER

• 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

BAROMETER

• MEASURING AIR PRESSURE, WIND SPEED & DIRECTION1. 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 millibarsa. 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.

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.

WIND ROSE

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.

SUNSHINE RECORDER

• 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.

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?

TEN DIFFERENT TYPES OF CLOUDS

• 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 filamentsb. Cirrocumulus-small rippled elementsc. Cirrostratus- a transparent sheet, often with a halo2. 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 shadingb. Altostratus- a thin layer, grey, allow sun to appear as if

through ground glass

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, greyc. Nimbostratus- thick layer with low base, dark & rain or

snow may fall from itd. Cumulus- individual cells, vertical rolls or towers with a

flat basee. Cumulonimbus- large cauliflower-shaped towers, often

with ‘anvil tops’, & sometimes giving thunderstorms or showers of rain/snow

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

by cloud