climatology.4.temp dist

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Distribution of Temperature

Sun is the ultimate source of heat. And the differential heat received from sun by

different regions on earth is the ultimate reason behind all climatic features. So

understanding the patterns of distribution of temperature in different seasons is

important for understanding various climatic features like wind systems, pressure

systems, precipitation etc..

Insolation

Earth intercepts only one in two billion parts of solar radiation. This intercepted

radiation is called Insolation.

Insolation Proportion of Solar energy received or intercepted by earth.

Some heat within the core and mantle is transfered to the surface and ocean bottoms

through volcanoes, springs and geysers. But this heat received at the surface form

interiors of the earth is negligible.

Earth receives Sun’s radiation (heat) in the form of short waves (ultraviolet radiation)

which are of electromagnetic nature. The earth absorbs short wave radiation during

daytime and reflects back the heat received into space as long-wave radiation (infrared

radiation) during night.

Ways of Transfer of Heat Energy

The heat energy from the solar radiation is received by the earth through three

mechanisms—


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1. Radiation Heat transfer from one body to another without actual contact or

movement. It is possible in relatively emptier space, for instance, from the sun to the

earth through space.

2. Conduction Heat transfer through matter by molecular activity. Heat transfer in

iron and other metals is by conduction. Generally, denser materials like water are good

conductors and a lighter medium like air is a bad conductor of heat.

3. Convection Transfer of heat energy by actual transfer of matter or substance from

one place to another. (heat transfer by convection cycles in atmosphere as well as

oceans)

Factors Affecting Distribution of Insolation

The Angle of Incidence or the Inclination of the Sun’s Rays

Duration of Sunshine

Heat recived depends on day or night; clear sky or overcast, summer or winter etc..

Transparency of Atmosphere


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Aerosols (smoke, sooth), dust, water vopour, clouds etc. effect transperancy.

If the wavelength (X) of the radiation is more than the radius of the obstructing particle

(such as a gas), then scattering of radiation takes place.

If the wavelength is less than the obstructing particle (such as a dust particle), then

total reflection takes place.

Absorption of solar radiation takes place if the obstructing particles happen to be water

vapour, ozone molecules, carbon dioxide molecules or clouds.

Most of the light received by earth is scattered light.

Land-Sea Differential


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Albedo of land is much grater tha albedo of oceans and water bodies. Especially snow

covered areas reflect upto 70%-90% of insolation.

Average penetration of sunlight is more in water—upto 20 metres, than in land—where

it is upto 1 metre only. Therefore, land cools or becomes hot more rapidly compared to

oceans. In oceans, continuous convection cycle helps in heat exchange between layers

keeping diurnal and annual temperature ranges low. (more while studying salinity and

temperature distribution of oceans)

The specific heat of water is 2.5 times higher than landmass, therefore water takes

longer to get heated up and to cool down.

Prevailing Winds

Winds transfer heat from one latitude to another. They also help in exchange of heat

between land and water bodies.

The oceanic winds have the capacity to take the moderating influence of the sea to

coastal areas—reflected in cool summers and mild winters. This effect is pronounced

only on the windward side (the side facing the ocean).

The leeward side or the interiors do not get the moderating effect of the sea, and

therefore experience extremes of temperature.

Aspects of Slope


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The direction of the slope and its angle control the amount of solar radiation received

locally. Slopes more exposed to the sun receive more solar radiation than those away

from the sun’s direct rays.

Slopes that receive direct Sun’s rays are dry due to loss of moisture through excess

evaporation. These slopes remain barren if irrigational facilities are absent. But slopes

with good irrigational facilities are good for agriculture due to abundant sunlight

available. They are occupied by dense human settlements.

Slopes that are devoid of direct sunlight are usually well forested.

Ocean Currents

Ocean currents influence the temperature of adjacent land areas considerably. (more

while studying ocean currents).

Altitude

With increase in height, pressure falls, the effect of green house gases decreases and

hence temperature decreases (applicable only to troposphere).

The normal lapse rate is roughly 1⁰ C for every 165 metres of ascent.


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Earth’s Distance form Sun

During its revolution around the sun, the earth is farthest from the sun (152 million

km on 4th July). This position of the earth is called aphelion.

On 3rd January, the earth is the nearest to the sun (147 million km). This position is

called perihelion.

Therefore, the annual insolation received by the earth on 3rd January is slightly more

than the amount received on 4th July.

However, the effect of this variation in the solar output is masked by other factors like

the distribution of land and sea and the atmospheric circulation.

Hence, this variation in the solar output does not have great effect on daily weather

changes on the surface of the earth.

Latitudinal Heat Balance

The amount of insolation received varies from latitude to latitude.

Regions within the equator and 40° N and S latitudes receive abundant sunlight and

hence more heat will be gained than lost. Hence they are energy surplus regions.

Regions beyond 40° N and S latitudes lose more heat than that gained from sunlight.

Hence they are energy deficit regions (This is because of slant sunlight and hight albedo

of polar regions).

Going by this logic, the tropics should have been getting progressively hotter and the

poles getting progressively cooler. And the planet would have been inhospitable except

for few regions near mid latitudes. But, in reality, this does not happen.

The atmosphere (planetary winds) and the oceans (ocean currents) transfer excess heat

from the tropics (energy surplus region) towards the poles (energy deficit regions)

making up for heat loss at higher latitudes.

And most of the heat transfer takes place across the mid-latitudes (30° to 50°)[more

while studing jet streams and cyclones], and hence much of the stormy weather is

associated with this region.

Thus, the transfer of surplus energy from the lower latitudes to the deficit energy zone

of the higher latitudes, maintains an overall balance over the earth’s surface.


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Heat Budget

The earth receives a certain amount of Insolation (short waves or ultraviolet radiation)

and gives back heat into space by terrestrial radiation (infrared or longwave radiation).

Through this give and take, or the heat budget, the earth maintains a constant

temperature.


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The Mean Annual Temperatue Distribution Over Earth


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Isotherm: An imaginary line joining places having equal temperatures.

The horizontal or latitudinal distribution of temperature is shown with the help of a

map with isotherms.

Effects of altitude is not considered while drawing an isotherm. All the temperatures

are reduced to sea levels.

General characteristics of isotherms.

1. Generally follow the parallels. Isotherms have close correspondence with the latitude

parallels mainly because the same amount of insolation is received by all the points

located on the same latitude.

2. Sudden bends at ocean - continent boundaries. (due to differential heating of land

and water, temperatures above the oceans and landmasses vary even on the same

latitude) (we have seen how land sea differential effects temperature distribution)

3. Narrow spacing between isotherms indicate rapid change in temperature (high

thermal gradient).

4. Wide spacing between isotherms indicate small or slow change in temperatures (low

thermal gradient).

General Trend

The highest temperatures occur over tropics and sub-tropics (high insolation). The

lowest temperatures occur in polar and subpolar regions. in continents due to the

effect of continentality.

Diurnal and annual range of temperatures are highest in the interiors of continents due

to the effect of continentiality (in continental interiors these will no moderating

effect of oceans).

Diurnal and annual range of temperatures are least in oceans.

Low temperature gradients are observed over tropics and high temperature gradients

over middle and higher latitudes.

Temperature gradients are usually low over the eastern margins of continents. (This

is because of warm ocean currents)

Temperature gradients are usually high over the western margins of continents.

(This is because of cold ocean currents)


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The isotherms are irregular over the northern hemisphere due to an enhanced land-sea

contrast. Because of predominance of land over water in the north, the northern

hemisphere is warmer. The thermal equator lies generally to the north of geographical

equator.

While passing through an area with warm ocean currents, the isotherms show a

poleward shift. (Nort Atlantic Drift and Gulf Stream combined with westerlies in

Northern Atlantic; Kurishino Current and North Pacific current combined with

westerlies in Northern Pacific) (we will see about ocean currents in detail later.)

Mountains also affect the horizontal distribution of temperature. For instance, the

Rockies and the Andes stop the oceanic influence from going inwards into North and

South America.

Distribution of Temperature

The global distribution of temperature can well be understood by studying the

temperature distribution in January and July.

The temperature distribution is generally shown on the map with the help of isotherms.

The Isotherms are lines joining places having equal temperature.

In general the effect of the latitude on temperature is well pronounced on the map, as

the isotherms are generally parallel to the latitude. The deviation from this general trend

is more pronounced in January than in July, especially in the northern hemisphere.

In the northern hemisphere the land surface area is much larger than in the southern

hemisphere. Hence, the effects of land mass and the ocean currents are well

pronounced.

Seasonal Distribution of Temperature — January


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During January, it is winter in the northern hemisphere and summer in the southern

hemisphere.

The western margins of continents are warmer than their eastern counterparts, since

the Westerlies are able to carry high temperature into the landmasses.

The temperature gradient is close to the eastern margins of continents. For instance,

along the east coast of Asia and North America, the temperature changes at the rate of

1.5°C per degree latitude, while along the western margins it changes at the rate of 0.5

C per degree latitude. The isotherms exhibit a more regular behaviour in the southern

hemisphere.

Northern Hemisphere

The isotherms deviate to the north over the ocean and to the south over the continent.

This can be seen on the North Atlantic Ocean. The presence of warm ocean currents,

Gulf Stream and North Atlantic drift, make the Northern Atlantic Ocean warmer and

the isotherms show a poleward shift indicating that the oceans are warmer and are

able to carry high temperatures polewards.

An equatorward bend of the isotherms over the northern continents shows that the

landmasses are overcooled and that polar cold winds are able to penetrate southwards,

even in the interiors. It is much pronounced in the Siberian plain.

Lowest temperatures are recorded over northern Siberia and Greenland.

Southern Hemisphere

The effect of the ocean is well pronounced in the southern hemisphere. Here the

isotherms are more or less parallel to the latitudes and the variation in temperature is

more gradual than in the northern hemisphere. The isotherm of 20° C, 10° C, and 0° C

runs parallel to 35° S, 45° S and 60° S latitudes respectively.

The high temperature belt runs in the southern hemisphere, somewhere along 30°S

latitude.

The thermal equator lies to the south of geographical equator (because the

Intertropical Convergence Zone or ITCZ has shifted southwards with the apparent

southward movement of the sun).


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Seasonal Distribution of Temperature— July


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During July, it is summer in the northern hemisphere and winter in the southern

hemisphere. The isothermal behaviour is the opposite of what it is in January.

In July the isotherms generally run parallel to the latitudes. The equatorial oceans

record warmer temperature, more than 27°C. Over the land more than 30°C is noticed

in the subtropical continental region of Asia, along the 30° N latitude. Along the 40° N

runs the isotherm of 10° C and along the 40° S the temperature is 10° C.

Northern Hemisphere

The highest range of temperature is more than 60° C over the north-eastern part of

Eurasian continent. This is due to continentality. The least range of temperature, 3°C,

is found between 20° S and 15° N.

Over the northern continents, a poleward bend of the isotherms indicates that the

landmasses are overheated and the hot tropical winds are able to go far into the

northern interiors.

The isotherms over the northern oceans show an equatorward shift indicating that the

oceans are cooler and are able to carry the moderating effect into tropical interiors. The

lowest temperatures are experienced over Greenland.

The highest temperature belt runs through northern Africa, west Asia, north-west India

arid southeastern USA. The temperature gradient is irregular and follows a zig-zag

path over the northern hemisphere.

Southern Hemisphere

The gradient becomes regular over the southern hemisphere but shows a slight bend

towards the equator at the edges of continents. Thermal equator now lies to the north

of the geographical equator.

Questions from NCERT

The atmosphere is mainly heated by the: (a) Short wave solar radiation (c) Long wave

terrestrial radiation (b) Reflected solar radiation (d) Scattered solar radiation

The main reason that the earth experiences highest temperatures in the subtropics in

the northern hemisphere rather than at the equator is : (a) Subtropical areas tend to


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have less cloud cover than equatorial areas. (b) Subtropical areas have longer day hours

in the summer than the equatorial. (c) Subtropical areas have an enhanced “green

house effect” compared to equatorial areas. (d) Subtropical areas are nearer to the

oceanic areas than the equatorial locations.

30 words

How does the unequal distribution of heat over the planet earth in space and time cause

variations in weather and climate?

What are the factors that control temperature distribution on the surface of the earth?

In India, why is the day temperature maximum in May and why not after the summer

solstice?

Why is the annual range of temperature high in the Siberian plains

150 words

How do the latitude and the tilt in the axis of rotation of the earth affect the amount of

radiation received at the earth’s surface?

Discuss the processes through which the earth-atmosphere system maintains heat

balance.

Compare the global distribution of temperature in January over the northern and the

southern hemisphere of the earth.

climatology.4.temp dist

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