In this article we will discuss about the horizontal and regional distribution of temperature.
Temperature decreases with increasing height in the troposphere but the rate of decrease varies according to seasons, duration of sunshine and location. On an average, the rate of decrease of temperature with increasing altitudes in a stationary column of air with absence of any vertical motion is 6.5°C per 1000 metres.
This decrease of temperature is called vertical temperature gradient or normal lapse rate which is 1000 times greater than the horizontal lapse rate (decrease of temperature with increasing latitudes). The decrease of temperature upward in the atmosphere proves the fact that the atmosphere gets heat from the earth’s surface through the processes of conduction, radiation and convection.
It is, thus, obvious that as the distance from the earth’s surface (the source of direct heat energy to the atmosphere) increases (i.e., as the altitude increases), the air temperature decreases.
The following are the reasons for decrease of temperature with increasing altitudes in the troposphere:
(i) Heat is transferred to the atmosphere from the earth’s surface through the processes of conduction, radiation and convection. Thus, as the altitude increases the amount of heat transported upward decreases. Consequently, every air layer receives less heat than the air layer lying below.
(ii) The air pressure is higher in the lower portion of the atmosphere near the earth’s surface because of weight of all the air layers lying above and thus the air density is maximum in the lower atmosphere but it decreases rapidly upward and the air becomes thin.
(iii) The quantity of water vapour, dust particles, water droplets, carbon dioxide etc., which absorb outgoing long-wave terrestrial radiation, is more concentrated in the lower portion of the atmosphere and decreases rapidly with increasing altitude.
Thus, the temperature of lower atmosphere becomes more than the air layers lying above because of more and more absorption of terrestrial radiation in the lower air layers. In other words, the temperature decreases upward because of decrease of absorption of terrestrial radiation with increasing height in the troposphere.
It may be pointed out that the decrease of temperature with increasing height is confined to the troposphere only. The height of troposphere is 16km and 6 km over the equator and the poles respectively but this height also varies in different seasons i.e., it becomes higher in summer than in winter. The upper limit of troposphere is called tropopause.
It is interesting to note that the temperature at tropopause increases from over the equator towards the poles because the height of tropopause decreases from over the equator towards the poles. The height of tropopause during July and January over the equator is -17km while temperature at the top of tropopause is – 70°C.
The height of tropopause decreases to 15km in July and 12.5km in January over 45°N latitude but the temperature at the top of tropopause increases to- 60°C in July and – 58°C in January. The height of tropopause further decreases to 10km in July and 9 km in January over the poles but the temperature increases to- 45°C in July and -58°C in January.
Upward from tropopause the temperature is reported to increase with increasing height in the stratosphere wherein it becomes 0°C or 32°F at the height of 50km from sea level. This is the upper limit of the stratosphere and is called stratopause. Temperature again decreases with increasing height in the mesosphere (50km- 80km).
The temperature becomes -80°C at mesopause, the upper limit of the mesosphere. Beyond mesopause temperature again increases with increasing height in the thermosphere. It is estimated that the temperature at its upper limit (height undecided) becomes 1700°C. It may be pointed out that this temperature cannot be measured by ordinary thermometer because the gases become very light due to very low air density.
Sometimes, temperature increases with increasing height in the troposphere. In other words, sometime warm air lies over cold air. This plenomenon is called inversion of temperature.
Horizontal Distribution of Temperature:
On an average, temperature decreases from the equator towards the poles and thus low latitudes are characterized by highest temperature whereas high latitudes record lowest temperature. It may be noted that highest temperature is never recorded at the equator instead it is recorded near the tropics of Capricorn and Cancer because a sizeable portion of incoming solar radiation is reflected by clouds and a large part of heat is spent in the process of evaporation near the equator.
On the other hand, most of the incoming solar radiation reaches the ground surface due to clear sky near the tropics of Cancer and Capricorn. Normally, temperature decreases from equator towards the poles. This change of temperature rather decrease of temperature pole-ward is called temperature gradient.
The rate of decrease of temperature between the tropics of Cancer and Capricorn is rather low and therefore temperature gradient is insignificant but temperature decreases more rapidly from the tropics pole-ward and therefore temperature gradient is also very high. The horizontal distribution of temperature is represented and studied with the help of isotherms.
The lines drawn on maps joining the places of equal temperature reduced to sea level are called isotherms. It is necessary to reduce the actual temperatures of all places at sea level before drawing isotherms. It is, thus, obvious that isotherms do not represent the real temperature of the places through which they pass rather they show temperature of the places at sea level.
This is why the isotherm maps are not useful for farmers because they need real temperature of a particular place for growing crops. Normally, isotherms run east-west and are generally parallel to latitudes. This trend shows strong control of latitudes on the horizontal distribution of temperature.
Generally, isotherms are straight but they bend at the junction of continents and oceans due to differential heating and cooling of land and water. Isothermal lines are more irregular in the northern hemisphere because of large extent of continents but they are more regular in the southern hemisphere due to over-dominance of oceans. Isotherms are generally closely spaced in the northern hemisphere but they are widely spaced in the southern hemisphere.
The closely spaced isotherms denote rapid rate of change of temperature and steep temperature gradient. On the other hand, widely spaced isotherms indicate slow rate of temperature change and low temperature gradient. On an average, isotherms trending from land towards the ocean bend equator-ward during summer and pole-ward during winter.
On the other hand, isotherms trending from the oceans to the continents bend pole-ward during summer and equator-ward during winter. The isotherms during the months of January and July are taken as representatives for the study of horizontal distribution of temperature during winter and summer seasons respectively because they represent seasonal extremes.
July and January:
If we look at the world map representing horizontal distribution of temperature in January and July (figs. 34.3 and 34.4) the following facts come to light:
(1) The months of July and January are warmest and coldest in the northern hemisphere whereas the warmest and coldest months in the southern hemisphere are January and July respectively.
(2) Both the maps (fig. 34.3 and 34.4) show latitudinal shifts of isotherms in accordance with seasonal shifting of overhead sun but this shifting of isotherms is more pronounced on the continents.
(3) The maximum temperatures in January and July are always recorded on the continents. Minimum temperature in January is observed in Asia and North America.
(4) January isotherms suddenly bend poleward while passing through warm portions of the oceans and bend equator-ward while passing through the cold portions of the oceans in January in the northern hemisphere while the trend is opposite in July, On the other hand, the isotherms are more or less regular and straight in the southern hemisphere because of over-dominance of oceans.
(5) Temperature gradient is more pronounced during winter than summer.
Regional Distribution of Temperature:
According to ancient Greek thinkers the globe is divided into three temperature zones on the basis of latitudes (fig. 34.5) e.g.:
(1) Tropical zone,
(2) Temperate zone, and
(3) Frigid zone.
(1) Tropical zone extends between the tropics of Cancer (23.5°N) and Capricorn (23.5°S). The Sun’s rays are more or less vertical on the equator throughout the year. The remaining areas are also characterized by vertical sun’s rays at least once every year.
There is no winter around the equator because of high temperature prevailing throughout the year but as one approaches the tropics of Cancer and Capricorn summer and winter are clearly observed and differentiated.
(2) Temperate zone extends between 23.5° and 66.5° latitudes in both the hemispheres. Though the duration of day and night is longer in this zone but it is never more than 24 hours. There are marked seasonal contrasts with the northward and southward (summer and winter solstices) migration of the overhead sun and thus the range of temperature between summers and winters becomes exceptionally very high.
(3) Frigid zone extending between 66.5° latitude and the poles in both the hemispheres is characterized by more oblique sun’s rays throughout the year resulting into exceptionally very low temperature. The length of day and night is more than 24 hours. Days and nights are of 6 months duration at the poles. Sun is never vertical and the ground is covered with snow as temperature more or less remains below freezing point.
It may be pointed out that the Greeks gave undue importance to latitudes in determining different temperature zones and overlooked the controls of contrasting nature of continents and oceans in terms of their heating and cooling, prevailing winds, ocean currents, nature of ground surface etc.
Taking all these factors in consideration Soupan divided the globe into temperature zones on the basis of isotherms. According to him the outer limit of tropical zone should be determined on the basis of annual isotherm of 68°F (20°C). The boundary between temperate and frigid zones in the northern hemisphere should be demarcated by 50°F (10°C) isotherm of July while January isotherm of 50°F (10°C) should separate temperate zone from the frigid zone in the southern hemisphere.