GC Leong: Summary of Introduction of Weather | Additional Study Material for UPSC PDF Download

Weather
Pertains to the condition of atmosphere at any place at a specific time or for a short period of time (Generally for hours or days)

Climate

• Average weather conditions of a specified area for a considerable time i.e. 30 - 35 years.
• Generally, climate of temperate latitudes is far more variable than that of tropics.
• The climate of British Isle is so changeable that many people have commented that ‘Britain has no climate, only weather’.
• Conversely, the climate of Egypt is so static that it makes a good deal of sense when people say that ‘Egypt has no weather, only climate’.
• Death rates are normally high in tropical countries & low in deserts, because germs are not transmitted readily in regions of high temperature & low humidity.
• It is because of the variable water content in the atmosphere that we have such great contrasts in weather & climate over different parts of the world.
• If we were to live in a dry atmosphere, absolutely without water, there would be no weather & not much climate.

Elements of Weather & Climate

1. Rainfall

• Measured by Rain gauge
• An inch of rainfall means the amount of water that would cover the ground to a depth of 1 inch, provided none evaporated, percolated or drained off.
• Daily record of rainfall will be added at the end of the month to find the total rainfall for that month.
• Total for each month is again added at the end of the year to find the annual rainfall.
• The mean annual rainfall is obtained from the averages of annual rainfall taken over a long period of say 30 - 35 years.
• For plotting in maps, places having the same mean annual rainfall are joined by a line called an isohyets.

2. Pressure

• Measured by barometer, invented by Galileo & Torricelli
• Air is made up of number of mixed gases & has weight, therefore exerts pressure on the earth’s surface which varies from place to place & from time to time.
• As shown in diagram, variation in the atmospheric pressure on the mercury surface is balanced by a column of mercury in glass tube.
• Any liquid can be used for this purpose, but mercury has been chosen because it is the heaviest liquid known.
• If ordinary water was used, corresponding column for normal atmospheric pressure would be 34 feet
• On map, places of equal pressure are joined by the lines called isobars
• Pressure reading at different places varies with a number of factors viz. Altitude, Gravitational forces at different latitudes & Temperature; due to sensitivity of mercury to it.
• A mercury barometer that dips in liquid mercury is inconvenient for outdoor measurements.
• Hence a more probable but less accurate type known as aneroid barometer is used.
• In airplanes, a modified type of aneroid barometer called altimeter is used & for continuous record of pressure changes a self-recording barogram is used.

3. Temperature

• Temperature is measured by thermometer, a narrow glass tube filled with mercury or alcohol.
• A temperature taken in open daylight is very high, as it measures the direct insolation of the sun, better described as temperature in the sun, used for measuring temperature for agricultural purposes.
• But the temperature that we are accustomed to in climatic graphs is shade temperature, i.e. temperature of the air.
• Hence, precautions must be taken to exclude the intensity of sun’s radiant heat, which is done by placing thermometers in a standard meteorological shelter known as Stevenson screen.
• Lines joining places of equal temperature in the map are termed as isotherms.

4. Humidity

• Defined as amount of water vapour present in air
• At any specific temp., amount of water vapour that can be held by air has a definite limit known as saturation point.
• Air at saturation point is known as saturated air.
• Temperature at which saturation occurs is known as Dew point.
• Capacity of air to absorb water vapour increases with increase in temperature.

5. Absolute Humidity

• Expressed in gm / m³.
• Weight of actual amount of water vapour present in unit volume of air.

6. Specific Humidity

• Expressed gm / kg.
• Weight of water vapour present in per unit weight of air.
• Not affected by change in pressure or temperature

7. Relative Humidity

• Expressed in %age.
• Ratio of water vapour in air at a particular temp, to total amount of water vapour required to saturate the same air at same temperature.
• Relative humidity increases with increase in water vapour in air & decreases with increase in temp.
• Relative humidity is measured by Hygrometer.

8. Winds

• The instrument widely used for measuring wind direction is a wind vane or weather clock.
• The speed of the wind is generally measured by anemometer.

Sunshine

• In meteorological station, sunshine duration is recorded by a sun dial
• On maps, places with equal sunshine duration are joined by isohels

Clouds

• When air rises, it is cooled by expansion & after dew point has been reached, cooling leads to condensation of water vapour in atmosphere.
• Tiny droplets of water vapour which are too small to fall as rain or snow will be suspended in the air & float as clouds.
• The amount of cloud cover in the sky is expressed in eighths or Oktas for e.g. 4/8 is half covered & 8/8 is complete overcast
• On maps, places with an equal degree of cloudiness are joined by lines known as isonephs.

Classification of Clouds

A. High Clouds

• Mainly Cirrus
• Feather Form
• 20-40000 feet above ground 

(i) Cirrus

• Fibrous
• Appears like wisps
• Often called mares tails
• Indicates fair weather
• Gives brilliant sunset

(iii) Cirrocumulus

• Appears like white globular masses

(iii) Cirrostratus

• Thin white sheet or veil
• Sky looks milky
• Sun/moon shines through it with characteristic halo

B. Medium Clouds

• 7-20000 feet

(iv) Alto Cumulus

• Wooly
• Bumpy clouds
• Arranged in layers
• Appears like waves in sky
• Indicate fine weather

(v) Alto stratus

• Denser
• Greyish clouds
• Watery look

C. Low Clouds
(vi) Strata Cumulus

• Rough
• Bumpy cloud
• Great contrast between the bright and shaded parts

(vii) Stratus

• Very low cloud
• Grey and thick
• Appear like a low ceiling and highland fog
• Dull weather with a light drizzle
• Reduces the visibility of aircraft
• Danger clouds 

(viii) Nimbo Stratus

• Dark
• Dull Cloud
• Clearly Layered
• Also known as rain cloud
• Bring continuous rain/snow or sleet

D. Clouds with great vertical extent

• 2-30000 feet
• Cumulus/heap clouds
• No definite height

(ix) Cumulus

• Vertical cloud
• Rounded top and horizontal base
• Typical of humid tropical region
• Fair weather cloud

(x) Cumulonimbus

• An overgrown cumulus cloud
• 2000 feet-30000 feet
• Black and white globular masses
• Take a fantastic range of shapes
• Frequently seen in tropical afternoons
• Referred in thunder cloud
• Brings convectional rain
• Accompanied by lighting and thunder

Other Elements

1. Haze

• The term is usually used connection with the reduction of visibility in regions of low humidity.
• This is caused by smoke & dust particles in industrial areas; or by unequal refraction of light in air of different densities in the lower atmosphere

2. Mist

• Condensation in water vapour in air causes small droplets of water
• Forming clouds at ground level
• Reduces visibility
• Occurs in wet air

3. Fog

• Water condensing on dust
• Occurs in the lower strata of the atmosphere