Factors Controlling Temperature Distribution

The temperature of air at any place is not a matter of chance. It is controlled by a small set of physical factors, and the most important of them are the latitude of the place, its altitude, and its distance from the sea. Of these, latitude works as the master control because it decides how much solar energy a place receives. But the actual temperature pattern on a map is never perfectly neat — it is disturbed by the presence of land and water, by ocean currents, and by the shape of the continents. To read this pattern, geographers use isotherms, which are lines joining places having equal temperature. Once you can read isotherms, the whole story of temperature distribution opens up.

The effect of latitude is well pronounced on a world map of surface air temperature. In general, the isotherms run parallel to the latitudes, which shows that temperature falls steadily as you move away from the equator. But this general trend is not followed everywhere. The deviation is much more pronounced in January than in July, and it is especially strong in the northern hemisphere. The reason is simple: the northern hemisphere has a far larger land surface area than the southern hemisphere. Land heats and cools quickly, while the ocean heats and cools slowly. So wherever there is a big landmass or a strong ocean current, the isotherms get bent out of their neat east–west alignment.

In January, the isotherms deviate to the north over the ocean and to the south over the continent. The North Atlantic Ocean is the classic example. The warm ocean currents — the Gulf Stream and the North Atlantic Drift — make the northern Atlantic warmer than the latitude would suggest, so the isotherms bend towards the north over the water. Over the land, the temperature falls sharply, and the isotherms bend towards the south in Europe. This bending is most pronounced in the Siberian plain. A striking illustration of this is that along 60° E longitude, the mean January temperature is minus 20° C at both 80° N and 50° N latitudes — that is, a place 30 degrees of latitude apart records the same temperature because the interior of Eurasia is so brutally cold in winter.

The book gives a clear picture of January temperatures across the globe. The mean monthly temperature for January is over 27° C in the equatorial oceans, over 24° C in the tropics, 2° C to 0° C in the middle latitudes, and a bitter –18° C to –48° C in the Eurasian continental interior. Notice how the range widens as you move from ocean to continent — the ocean keeps things moderate, the continent lets the cold pile up.

The southern hemisphere tells a different story. Here the effect of the ocean is well pronounced, because the southern hemisphere is mostly water. The isotherms are more or less parallel to the latitudes, and the variation in temperature is more gradual than in the northern hemisphere. The book gives a neat set of parallels: the isotherm of 20° C runs parallel to 35° S, the isotherm of 10° C runs parallel to 45° S, and the isotherm of 0° C runs parallel to 60° S. The spacing is even and orderly — no Siberian plains to wreck the pattern.

In July, the picture changes. The isotherms generally run parallel to the latitude once again, and the equatorial oceans record warmer temperatures, 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 also 10° C. So in July, the northern landmass heats up fiercely, but the isotherms stay more regular than they were in January. …