Temperature differs from one part of the world to the other. Since Insolation is the basic source of energy for the atmosphere, the distribution of insolation would determine the temperature of the earth. Thus latitude, altitude, distance from sea, features of the surface, nature of the landscape are some important factors that affect the distribution of temperature.

Since, the insolation is highest at equator; temperature should be highest at the equator and lowest near the poles, however actually it is not. Highest temperature on earth is recorded at a few degrees north of equator. Altitude is the second major control of temperature of a place. The temperature depends upon albedo of the surface also.

One major factor affecting the distribution of the temperature of Earth is distribution of Land and Oceans. Since there is more land in Northern Hemisphere and more waters in Southern hemisphere and there is a big difference between the specific heat of land and water; the loss of heat from the continents is bigger than the oceans. The continents get heated faster and get cooled faster in comparison to the Oceans. This is the reason that the temperatures of the Oceans are moderate while that of continents is extreme. The moderating effect on temperature of the land due to proximity of the seas is called Maritime influence. The increasing effect on temperature of the land at interior of the continents is called Continental Influence.

Three Broad Temperature Zones

The earth can be generally divided into three broad temperature zones viz. Torrid Zone, Temperate Zone and Frigid zone.

Torrid Zone

Torrid Zone is the tropical region. The temperature remains high. Sun is directly overhead at least once during the year. In the Northern Hemisphere, the overhead Sun moves north from the equator until it reaches 23.5 °North (Tropic of Cancer) for the June solstice after which it moves back south to the equator. The year is consequently divided nearly into four equal parts by the two times at which the sun crosses the equator (Equinoxes) and those two at which it attains greatest declinations (Solstices). The Torrid Zone forms the hottest region of the world with two annual seasons namely a dry and a wet season. This zone includes most of Africa, southern Asia, Indonesia, New Guinea, northern Australia, southern Mexico, Central America and northern South America.

Temperate Zones

Temperate zones are the mid latitudinal areas, where the temperature is moderate. There are two temperate areas viz. North and South. In the two Temperate Zones, consisting of the tepid latitudes, the Sun is never directly overhead, and the climate is mild, generally ranging from warm to cool. The four annual seasons, Spring, Summer, Autumn and Winter occur in these areas. The North Temperate Zone includes Great Britain, Europe, northern Asia, North America and northern Mexico. The South Temperate Zone includes southern Australia, New Zealand, southern South America and South Africa.

Frigid Zones

The two Frigid Zones, or polar regions, experience the midnight sun and the polar night for part of the year – the cliff of the zone experiences one day at the solstice when the Sun doesn’t rise or set for 24 hours, while in the centre of the zone (the pole), the day is literally one year long, with six months of daylight and six months of night. Please note that the Frigid Zones are not the coldest parts of the earth, and are covered with ice and snow. The coldest temperature on earth has been recorded a few degrees below the 90°N.

Patterns of Global Isotherms

The global distribution of temperature can be represented with the help of isotherms. Isotherms are the lines that join the places with the identical temperatures. Please note that isotherms are drawn after correcting the temperature of a place to the sea level so that the differences due to altitude can be minimized. The Isotherms on the earth run parallel to the latitudes.

Due to the difference between the specific heat between water and land, at any latitude, the temperature over the landmass is higher in summer and lower in winter in comparison to the seas. Here we discuss about the global isotherms drawn in the month of January and July. As shown in the picture, Isotherms for the month of July bend towards Northward while moving from Sea to Land.

For the Month of January, the isotherms bend towards south while moving from sea to land. The only thing you have to note about Isotherms is that water in the South Atlantic and Pacific is absorbing greater amounts of energy during January and the land is rapidly heating and reradiating energy. Please also note that due to difference in the specific heat, both highest and lowest temperatures are observed in the interiors of the continents.

Vertical Distribution of Temperature

The vertical distribution of temperature on earth is also unequal. As we studied above in detail that in troposphere, the temperature falls uniformly with height as per the Environmental Lapse Rate. The normal value of this Lapse Rate is 6.4°C per kilometers. When a parcel of air rises upwards and cools this is known as adiabatic cooling. This adiabatic cooling is the result of the expansion of air as it is lifted upwards. When the air descends, it gets warmed and this is called adiabatic warming.

Inversion of the Temperature

In the mountain valleys, the temperature of the air is found increasing with increasing altitude. Thus there is an inversion of the temperature. This is because during the night, the quick radiation from the upper exposed slopes of the mountains causes the surface and air over it to cool rapidly. This cooler air is denser and gets drained by the valley slopes and displaces the warmer air toward up. So, when we go up in a valley, the temperature seems to getting increased. This phenomenon is also called drainage inversion.

Mean Thermal Equator

Thermal equator is a global isotherm having the highest mean annual temperature at each longitude around the globe. Thermal equator does not coincide with the geographical equator. The highest absolute temperatures are recorded in the Tropics but the highest mean annual temperatures are recorded at equator. But because local temperatures are sensitive to the geography of a region, and mountain ranges and ocean currents ensure that smooth temperature gradients (such as might be found if the Earth were uniform in composition and devoid of surface irregularities) are impossible, the location of the thermal equator is not identical to that of the geographic Equator.

Further, we know that the Earth reaches perihelion (the minimum distance from the Sun in its orbit) in early January and is at aphelion (maximum distance) in early July. During winter season of the respective hemispheres, the angle of incidence of the sun’s rays is low in tropics. The average annual temperature of the tropical regions is therefore lower than the observed near the equator, as the change in the angle of incidence is minimum at equator.

The thermal equator shifts towards north and south with north south shift in the position of vertical rays of the sun. However, annual average position of the Thermal equator is 5° N latitude. The reason is that highest mean annual temperature shifts towards northwards during the summer solstice to a much greater extent than it does towards south at the time of winter solstice.

Daily variation of Temperature

Sun is at the highest point at noon but the highest temperature does not occur at 1200 hours because the atmosphere does not get the heat directly from the Sun. It receives heat from the earth’s surface slowly and that is why maximum temperature is generally attained by 1400 hours (2.00p.m.). The daily minimum temperature at a place does not occur at about 0400 hours (4.00 p.m.) in the morning because radiation of heat continues upto the sun rise.

Here are some notable observations on daily temperature ranges:

• Daily temperature range is low in clouded areas because the clouds obstruct the receipt and loss of insolation.
• The sky is clear in hot desert’s areas. Insolation is received without obstruction in the day and lost without obstruction in the night. This causes high temperature range in deserts.
• Ice or snow absorbs less and reflects the insolation more. Hence, the daily temperature range is low is snow bound areas.
• The air is thin in areas of high altitude. There is great loss of insolation in the night. There is no obstruction in the receipt of insolation in the day. Such places have a high temperature range.
• There is a higher temperature range in than interior areas of continents than at seas because the sea heats and cools slowly but the land heats and cools rapidly.
• Warm and cool winds also disturb the temperature range.

Annual temperature range

On Equator

The duration of the day or night is the same in equatorial countries. The sun’s rays are vertical all through the year. Hence, there is no worthwhile difference between the summer and winter seasons. This is the reason that the lowest annual temperature range is found in equatorial areas.

On Poles

Towards the poles, the duration of the day and the inclination of the sun rays go on increasing. It causes a lot of difference between the temperatures of the two seasons. Hence, towards the poles, the annual temperature range goes on increasing.

Near Oceans

Near the seas and oceans, the equalizing effect of sea water makes the winter less cold and the summer less hot. This reduces the annual range of temperature near the seas. The equalizing effect of the sea water cannot reach land areas, away from the seas. The countries  like Mongolia  and Tibet which are situated  far into the interior  of the continent  have a high annual range of temperature. The ocean currents near the coasts also affected the temperature range. Due to the warm gulf stream, the winter of western Europe is less cold than what it Europe is less cold than  what it should have been without the gulf stream. This reduces the annual temperature range.

The shifting attitude of ocean currents has a lot of effect on the annual temperature range. For example, the weather and seasons have to undergo greater changes on the eastern coasts of Indian and Australia due to the shifting of ocean currents. It increases the annual temperature range on these coasts as compared to that on the opposite side coasts.

Impact of Winds

The prevailing winds also have a greater effect on the annual temperature range. Winds from the land blow in Arabian countries and therefore increase the annual range of temperature. Winds from the oceans and seas blow into Western Europe and reduce the annual temperature range. The variation in the annual temperature range in west and east European countries is due to land and sea winds. The effect of winds from the ocean has a far smaller effect in Eastern Europe than in Western Europe. It is why the annual temperature range is higher in eastern than in Western Europe.