Temperature indicates the relative degree of heat of a substance.
Heat is the energy which make things or objects hot, while temperature measures the intensity of heat.
Although quite distinct from each other, yet heat and temperature are closely related because gain or loss of heat is necessary to raise or lower the temperature.
The celsius scale, named after the swedish astronomer Anders Celsius, is accepted internationally by Scientists for reporting air temperature.
The historical temperature records of several English-speaking countries include values on the Fahrenheit scale, Fahrenheit temperatures may be converted to their celsius equivalents by the formula C = (F – 32) * 5 / 9.
Moreover, difference in temperature determines the direction of flow of heat. Distribution of temperature varies both horizontally and vertically.
- The horizontal distribution of temperature
- The vertical distribution of temperature
- Inversion of Temperature
Horizontal Distribution of Temperature
Distribution of temperature across the latitudes over the surface of the earth is called its horizontal distribution. On maps, the horizontal distribution of temperature is commonly shown by “Isotherms”, lines connecting points that have equal temperatures.
An isotherm is made of two words ‘iso’ and ‘therm’, ‘Iso’ means equal and ‘therm’ means” temperature. If you study an isotherm map you will find that the distribution of temperature is uneven. The factors responsible for the uneven distribution of temperature are as follows:
- (i) Latitude
- (ii) Land and Sea Contrast
- (iii) Relief and Altitude
- (iv) Ocean Currents
- (v) Winds
- (vi) Vegetation Cover
- (vii) Nature of the soil
- (viii) Slope and Aspect
The angle of incidence (insolation concept) goes on decreasing from equator towards poles. Higher the angle of incidence, higher is the temperature. Lower angle of incidence leads to the lowering of temperature.
It is because of this that higher temperatures are found in tropical regions and they generally decrease at a considerable rate towards the poles. Temperature is below freezing point near the poles almost throughout the year.
(ii) Land and Sea Contrast
Land and sea contrast affects temperature to a great extent. Land gets heated more rapidly and to a greater degree than water during sunshine. It also cools down more rapidly than water during night. Hence, temperature is relatively higher on land during day time and it is higher in water during night.
In the same way there are seasonal contrasts in temperature. During summer the air above land has higher temperature than the oceans. But the air above oceans gets higher temperature than landmasses in winter.
Not withstanding the great contrast between land and water surfaces, there are differences in the rate of heating of different land surfaces.
- A snow covered land as in polar areas warms very slowly because of the large amount of reflection of solar energy.
- A vegetation covered land does not get excessively heated because a great amount of insolation is used in evaporating water from the plants.
(iii) Relief and Altitude
Relief features such as mountains, plateaus and plains control the temperature by way of modifying its distribution.
Mountains act as barriers against the movement of winds. The Himalayan ranges prevent cold winds of Central Asia from entering India, during winter. Because of this Kolkata is not as cold as Guangzhou (Canton) in winter though both are situated almost on the same latitude.
As we move upwards from sea level, we experience gradual decrease in temperature. Temperature decreases at an average rate of 60C per 1000 m. altitude. It is known as normal lapse rate. The air at lower elevations is warmer than that of higher elevations because it is closest to the heated surface of the earth. As a result mountains are cooler than the plains even during summers. It is worth remembering that the rate of decrease of temperature with altitude varies with time of day, season and location.
Quito and Guayaquil are two cities of Ecuador (South America) situated near the equator and relatively close to each other. Quito is at 2800 metres. high from mean sea level while Guayaquil is just at 12 metres altitude. However because of difference in altitude. Quito experiences annual mean temperature of 13.3°C while in Guayaquil it is 25.5°C.
(iv) Ocean Currents
Ocean currents are of two types – warm and cold.
Warm currents make the coasts along which they flow warmer, while cold currents reduce the tempeeture of the coasts along which they flow.
The North-Western European Coasts do not freeze in winter due to the effect of North Atlantic Drift (a warm current), while the Quebec on the coast of Canada is frozen due to the Cold Labrador Current flowing along it, though the Quebec is situated in lower latitudes than the North-West European Coast.
Winds also affect temperature because they transport heat from one region to the other. The temperature of a place will rise if it lies on the path of winds coming from warmer regions. The temperature will fall if the place lies on the path of the winds blowing from cold regions. This process of horizontal transport of heat by winds is known as advection.
(vi) Vegetation Cover
Soil devoid of vegetation cover receives heat more rapidly than the soil under vegetation cover. Because vegetation cover absorbs much of sun’s heat and then prevents quick radiation from the earth whereas the former radiates it more rapidly. Hence the temperature variations in dense forested areas are lower than those in desert areas.
For example annual range of temperature in equatorial regions is about 5°C while in hot deserts, it is as high as 38°C.
(vii) Nature of the Soil
Colour, texture and structure of soils modify temperature to a great degree. Black, yellow and clayey soils absorb more heat than sandy soils. Likewise heat radiates more rapidly from sandy soils than from black, yellow and clayey soils. Hence temperature contrasts are relatively less in black soil areas than those of sandy soils.
(viii) Slope and Aspect
Angle of the slope and its direction control the receipt of insolation. The angle of incidence of sun’s rays is greater along a gentler slope and smaller along a steeper slope. The ray in both the cases carry an equal amount of solar energy.
Greater concentration of solar energy per unit area along gentler slope raises the temperature while its lesser concentration along steeper slopes lowers the temperature. For such reasons, the southern slopes of the Himalaya are warmer than the northern ones.
At the same time the slopes, in terms of aspect, exposed to the sun receive more insolation and are warmer than those which are away from the direct rays of the sun. The northern slopes of the Himalaya for example, not facing the sun are exposed to cold northernly winds are obviously colder. On the other hand the southern slopes of the Himalaya are sun-facing and are also shelter from the northernly cold winds are warmer. Hence we observe settlements and cultivation largely on the southern slopes of the Himalaya while the northern slopes are more under forest area.
|Latitude, land and sea contrast, relief and altitude, oceans currents, winds, vegetation cover, nature of soil, slope and aspect control the distribution of temperature in the world.|
The horizontal distribution of temperature over the globe can be studied easily from the maps of January and July months, since the seasonal extremes of high and low temperature are most obvious in both northern and southern hemispheres during these months.
Horizontal Distribution of Temperature in January
In January, the sun shines vertically overhead near the Tropic of Capricorn. Hence it is summer in southern hemisphere and winter in northern hemisphere.
High temperature is found over the landmasses mainly in three regions of the southern hemisphere. These regions are North-west Argentina, East, Central Africa, and, Central Australia. Isotherm of 30°C closes them. In northern hemisphere landmass arc cooler than oceans. During this time Northeast Asia experiences lowest temperatures.
As the air is warmer over oceans than over landmasses in the northern hemisphere, the Isotherms bend towards poles when they cross the oceans. In southern hemisphere, the position of the isotherms is just reverse. They bend towards poles when they cross the landmasses and towards equator when they cross oceans.
Large expanse of water exists in southern hemisphere. Hence, isotherms are regular and widely spaced in the southern hemisphere. While they are irregular and closely spaced in northern hemisphere due to large expanse of landmasses.
For these reasons no extreme seasonal contrasts between land and water are found in middle and higher latitudes in the southern hemisphere as they exist north of equator.
Horizontal Distribution of Temperature in July
During this period the sun shines vertically overhead near the Tropic of Cancer. Hence, high temperatures are found in the entire northern hemisphere. Isotherm of 30°C passes between 10° N and 40° N latitudes.
The regions having this temperature include South Western USA, the Sahara, the Arabia, Iraq, Iran, Afghanistan, desert region of India and China.
However, lowest temperature of 0°C is also noticed in the Northern Hemisphere during summer in the central part of Greenland.
During summer in the northern hemisphere, isotherms bend equatorward while crossing oceans and polewards while crossing landmasses. In southern Hemisphere the position of isotherms is just opposite. lsotherms are wide spaced over oceans while they are closely spaced over landmasses.
A comparison between the January and July isotherm maps reveals the following important characteristics –
- The latitudinal shifting of highest temperature as a result of migration of the vertical rays of the sun.
- The occurance of highest values in the low latitudes and the lowest value in the high latitudes is due to the decreasing insolation from equator to the poles.
- In northern hemisphere the isotherms on leaving the land usually bend rather sharply towards poles in winter and towards the equator in the summer. This behaviour of the isotherms is due to the differential heating and cooling of landmasses. The continents are hotter in the summer and colder in the winter than the oceans.
- Difference between the average temperatures of warmest and the coldest months is known as annual range of temperature. Annual range of temperature is larger in the interior parts of the continents in middle and high latitudes of the northern hemisphere. Verkhoyansk in Siberia records 66°C the highest annual range of temperature in the world. Its lowest average winter temperature is-50°C. Hence it is aptly called ‘cold pole’ of the earth.
|The difference between average temperature of the warmest and the coolest months is known as annual range of temperature.|
Vertical Distribution of Temperature
The permanent snow on high mountains, even in the tropics, indicate the decrease of temperature with altitute. Observations reveals that there is a fairly regular decrease in temperature with an increase in altitude. The average rate of temperature decrease upward in the troposphere is about 60C per km, extending to the tropopause. This vertical gradient of temperature is commonly referred to as the standard atmosphere or normal lapse rate, but is varies with height, season, latitude and other factors. Indeed the actual lapse rate of temperature does not always show a decrease with altitude.
Inversion of Temperature
Long winter night, clear sky, dry air and absence of winds leads to quick radiation of heat from the earth’s surface, as well as from the lower layers of the atmosphere. This results in the cooling of the air near the earth’s surface.
The upper layers which lose their heat not so quickly are comparatively warm. Hence the normal condition in which temperature decreases with increasing eight, is reversed. The cooler air is nearer the earth and the warmer air is aloft.
In other words, temperature increases with increasing height temporarily or locally. This phenomena is termed as inversion of temperature.
Sometimes the cold and dense air remains near the surface for number of days. So the phenomenon of inversion of temperature is also seen for days together. The phenomenon of inversion of temperature is especially observed in intermontane valleys. During winters the mountain slopes cool very rapidly due to the quick radiation of heat. The air resting above them also becomes cold and its density increases. Hence, it moves down the slopes and settles down in the valleys. This air pushes the comparatively warmer air of valleys upwards and leads to the phenomenon of inversion of temperature. Sometimes the temperature falls below freezing point in the valleys leading even to the occurance of frost.
In contrast, the higher slopes remain comparatively warmer. That is why mulberry planters of the Suwa Basin of Japan and the apple growers of Himachal Pardesh avoid the lower slopes of the mountains to escape winters frost. If you have been to any hill station you would have seen that most of the holiday resorts and the houses of affluent persons are built on the upper slopes.
|Temperature usually decreases with increasing altitude.
The normal lapse rate is 60C per 1000m metres ascend.
The phenomenon in which temperature increases with increasing altitude temporarily and locally under certain conditions is known as inversion of temperature.
Bibliography : NIOS Geography Book