Geography · Ch 14 — Solar Radiation, Heat Balance and Temperature
Distribution of Temperature
Distribution of Temperature
The map shows the world in January, with surface air temperatures drawn as isotherms — lines joining places that share the same temperature. These lines are labelled at five-degree intervals: −25, −12.5, 0, 10, 20 and 30 degrees Celsius. Across most of the map the isotherms run roughly parallel to the lines of latitude, which tells you that in January, temperature is largely controlled by the sun’s position — the southern hemisphere is tilted toward the Sun, so it is summer there, and the warmest air sits in the tropics and subtropics of the southern continents and oceans.
The most striking feature is a dashed line labelled the Thermal Equator. This is not the geographic Equator; it is the line joining the places with the highest average temperature at the surface. In January it weaves north and south of the geographic Equator, dipping into the southern hemisphere over the landmasses of South America, Africa and Australia, where the summer Sun heats the ground intensely. Over the oceans it shifts northward, because water warms and cools more slowly than land, so the warmest belt stays closer to the geographic Equator there.
The isotherms themselves bend in ways that reveal how land and ocean currents redistribute heat. Over the North Atlantic Ocean, the lines bend sharply northward — toward the pole. This is the signature of the Gulf Stream and the North Atlantic Drift, warm ocean currents that carry tropical heat into high latitudes, keeping the sea surface and the air above it far warmer than the latitude alone would suggest. In contrast, over Europe and especially over the Siberian interior of Asia, the isotherms bend southward, away from the pole. Here the continental landmass cools enormously in winter, and January temperatures plunge to between −18 and −48 degrees Celsius. The cold is so intense that the isotherms are pushed far south, wrapping around the heart of Siberia. …
The July map shows the world’s surface air temperature as a set of smooth, roughly east–west isotherms, each labelled in degrees Celsius. Four main lines appear: 0°C, 10°C, 20°C, and 30°C. Because it is July, the Northern Hemisphere is in summer, so the warmest air has shifted well north of the Equator. The isotherms are not perfectly straight — they bend over continents and oceans — but their general run is parallel to latitude, which tells you that solar heating, which varies with latitude, is the dominant control on temperature.
The most striking feature is the dashed Thermal Equator, the line joining the hottest places on Earth at each longitude. In July it lies north of the geographic Equator, looping over the heated landmasses of the Northern Hemisphere. Over the subtropical continental interior of Asia, the 30°C isotherm wraps around the region along roughly 30°N latitude — meaning that vast stretch of land, far from any ocean, is the hottest zone on the planet at this time. This is the heart of the Asian landmass in peak summer, where intense insolation and a dry continental climate push surface air temperatures above 30°C.
Two other isotherms anchor the map’s symmetry. Along 40°N, the 10°C isotherm runs across the map, and along 40°S the 10°C isotherm also appears. Notice what this implies: at the same latitude in the two hemispheres, the temperature is identical (10°C), yet the seasons are opposite. In July, 40°N is in summer and 40°S is in winter — so the fact that both show 10°C is not a sign of equal heating, but of the moderating effect of the oceans in the Southern Hemisphere, where water warms and cools slowly, keeping winter temperatures milder than they would be over land. …
The way heat is spread across the Earth’s surface is not uniform — it changes with latitude, season, and the nature of the underlying surface. The clearest way to see this pattern is to compare the temperature distribution in the two extreme months of the year: January and July. January represents the winter hemisphere, while July represents the summer hemisphere, so together they show the full annual swing of temperature.
The temperature distribution is generally studied through maps called isotherms — lines joining places of equal temperature. By looking at how these lines run, bend, and space themselves out in January and July, you can understand which parts of the Earth are warm, which are cold, and why.
January — the winter pattern
In January, the Sun’s vertical rays fall in the Southern Hemisphere, so the southern continents are warmer and the northern landmasses are cold. The isotherms in the Northern Hemisphere bend sharply southward over the continents, because land cools rapidly in winter. Over the oceans, the isotherms bend northward, since water retains heat longer than land.
The most striking feature of January is the intense cold over the interiors of Asia and North America. A strong thermal high-pressure cell develops over Siberia, and the isotherms crowd closely together there, indicating a very steep temperature gradient. The coldest regions on Earth in January are the interiors of these northern continents, not the poles themselves.
In the Southern Hemisphere, January is summer, so the isotherms are more widely spaced and run more nearly parallel to the latitudes. The temperature gradient is gentler because the Southern Hemisphere has more ocean surface, which heats and cools slowly.
July — the summer pattern
In July, the Sun’s vertical rays shift to the Northern Hemisphere, reversing the pattern. The northern landmasses now heat up rapidly, and the isotherms bend northward over the continents. The interiors of Asia and North America become the warmest regions, with very high temperatures.
Over the oceans, the isotherms bend southward, because water warms slowly. The temperature gradient in the Northern Hemisphere is gentler in July than in January, because the land has had time to warm and the contrast between land and sea is less extreme.
In the Southern Hemisphere, July is winter. The isotherms crowd together over the southern continents, especially over the landmasses, where temperatures drop sharply. The temperature gradient becomes steeper than in January, and the isotherms are more closely spaced.
Key contrasts between the two months
- In January, the isotherms are more closely spaced in the Northern Hemisphere; in July, they are more closely spaced in the Southern Hemisphere.
- The temperature gradient is steeper over continents than over oceans in both months. …
The global distribution of temperature can well be understood by studying the temperature distribution in January and July. The temperature distribution is generally shown on the map with the help of isotherms -- lines joining places having equal temperature. In general, the effect of latitude on temperature is well pronounced on the map, as the isotherms are generally parallel to the latitude. The deviation from this general trend is more pronounced in January than in July, especially in the northern hemisphere, because the northern hemisphere's land surface area is much larger than the southern hemisphere's -- so the effects of land mass and ocean currents are well pronounced.
In January the isotherms deviate to the north over the ocean and to the south over the continent -- seen on the North Atlantic Ocean, where the warm ocean currents Gulf Stream and North Atlantic Drift make the Northern Atlantic Ocean warmer, bending the isotherms towards the north. Over the land the temperature decreases sharply and the isotherms bend towards the south in Europe; this is much pronounced in the Siberian plain, where the mean January temperature along 60E longitude is minus 20 degree C both at 80N and 50N latitudes. The mean monthly temperature for January is over 27 degree C in equatorial oceans, over 24 degree C in the tropics, 2 degree C to 0 degree C in the middle latitudes, and -18 degree C to -48 degree C in the Eurasian continental interior. Over land, more than 30 degree C is noticed in the subtropical continental region of Asia along 30N latitude; along 40N runs the isotherm of 10 degree C and along 40S the temperature is 10 degree C. …