Brief notes on the Mean Annual Temperature Distribution

While discussing the average annual temperature distribution over the globe, it should be kept in mind that the mean value of the annual temperature of a place is of little significance climatologically, because it is simply the mean of the seasonal extremes.

The equatorial regions, however, are an exception to this general rule, for there is little annual variation in temperature in these latitudes.

Salient features: The following are the salient features of the average annual distribution of temperature:

(1) The highest temperatures are found in the tropics and subtropics which receive the largest amounts of insolation all the year round. On the contrary, the lowest average temperatures are recorded in the Polar Regions, where the amount of solar energy received in the course of a year is very small.

(2) The isotherms within much of the tropics are widely spaced. In other words, the northern-south temperature gradients in the lower latitudes are rather insignificant.

(3) The isothermal lines generally extend east-west giving an evidence of the dominant control of latitude on the horizontal distribution of temperature.

However, the isotherms are relatively more irregular in the northern hemisphere than the southern hemisphere. The dominant control of latitude on temperature is more clearly seen in the southern hemisphere because of the preponderance of water over land.

(4) The isotherms, while passing from continents to oceans, get distorted at their margins. The magnitude of deviation in the east- west trend of the isotherms is larger in the northern hemisphere where there are a larger percentage of land surfaces.

(5) In the eastern part of the North Atlantic the isotherms show a larger amount odd devia­tion toward north due to the combined effect of the warm ocean currents of the Gulf Stream and the North Atlantic Drift, and the southerly winds.

Similar bending of isobars towards the pole is seen in the North Pacific, because of the presence of the Kuroshio warm ocean current and the North Pacific Drift as well as the prevailing waterlines.

(6) Wherever there are cold ocean currents, the isotherms bend towards the equator. The impact of the cold currents like the Labrador cold currents, Benguela cold currents, Californian cold current and Humboldt Current can be seen in the map in (Figure 24.2).

(7) The coldest temperatures in winter and the highest temperatures in summer are found over the continents. This is obviously the result of differential heating of land and water.

It may also be noted that since temperatures do not fluctuate as much over water as over land, the north- south migration of isotherms is greater over the continents than over the oceans, especially in the middle latitudes.

The western coastal regions of North America and North Europe are warmer than their central regions. On the contrary, the tropical western coastal regions of Africa, Australia and South America are colder than their middle parts.

(8) The belt of high mean annual temperature (26.7° C and above) varies in width in the tropical regions. It attains the maximum width over the continents. The northern hemisphere being the land hemisphere is warmer than the southern hemisphere. The thermal equator is found north of the geographical equator (0°).

(9) The temperature gradient is very steep in the higher latitudes as well as along the eastern margins of the continents.

(10) The mountain ranges seem to exert great influence on the horizontal distribution of temperature. The barrier effects of the Rockies in North America and the Andes in South America do not allow the oceanic influences to penetrate far inland. Since the Himalayas in Asia are oriented in east – west direction, there is a packing of isobars near them.