Maarg Manthan · Topic 6.5

Horizontal Distribution of Temperature and Latitudinal Heat Balance

Geography › Atmosphere and Temperature · Topic 6.5

The horizontal distribution of temperature is shown on maps by isotherms, which generally run east-west and show a fall from the equator towards the poles, but bend where land meets sea. The thermal equator lies north of the geographic equator. The tropics have a heat surplus and the poles a deficit, and winds and ocean currents carry heat poleward, which holds the latitudinal heat balance.

6 min read · UPSC Prelims · Geography · Atmosphere and Temperature

At a Glance

Isotherms Lines of equal temperature
Pattern Fall from equator to poles; bend over land and sea
Thermal equator Belt of highest temperature, shifting with the Sun
Greatest annual range Siberian interior
Heat balance Surplus in tropics, deficit at poles
Exam link UPSC Prelims, Geography: Atmosphere and Temperature

Where Does It Fit?

This is the fifth topic of the category on the atmosphere and temperature. The previous topic listed the factors that control temperature, and this one shows the result on the map of the world. The next topic covers the vertical distribution of temperature and inversion. Previous topic: Factors Affecting Temperature Distribution. Next topic: Vertical Distribution of Temperature and Temperature Inversion.

How Is Temperature Shown on Maps?

In short: isotherms are lines that join places of equal temperature, reduced to sea level so that altitude does not mislead.

The horizontal distribution of temperature is shown by isotherms, lines joining places having the same temperature at a given time, corrected to sea level so that the effect of altitude is removed and the effect of other factors is seen. Where isotherms are close together, the temperature changes quickly, and where they are far apart, it changes slowly. If the Earth were uniform, isotherms would run parallel to the latitudes. In reality they bend because of differences in the heating of land and water, and the effect of ocean currents, as in Factors Affecting Temperature Distribution. Isotherms are drawn for the whole year or for the extreme months of January and July.

What Are the January and July Patterns?

The January isotherms and the July isotherms show the seasonal shift of heat between the hemispheres.

Feature	January	July
Season	Winter in the north, summer in the south	Summer in the north, winter in the south
Thermal equator	Shifts south of the geographic equator	Shifts north, to about 10 to 20 degrees north
Isotherms over the northern continents	Bend equatorward over land, which is very cold; bend poleward over oceans, which are warmer	Bend poleward over land, which is hot; bend equatorward over oceans, which are cooler
Southern hemisphere	Isotherms are straighter, since there is mostly ocean	Straight and widely spaced
Hot spots	Australia, southern Africa, Argentina	Sahara, Arabia, northern India, southwestern USA
Cold spots	Siberia, northern Canada	Antarctica

The North Atlantic Drift makes the isotherms bend sharply poleward over the northeast Atlantic in winter, so that northern Europe is far warmer than other places at the same latitude, such as Labrador. The southern hemisphere has more water and so has smaller seasonal swings.

What Is the Thermal Equator and the Annual Range of Temperature?

The thermal or heat equator is the line joining the places of highest mean temperature in each meridian. It does not coincide with the geographic equator, since land heats more than sea, and the northern hemisphere has more land. It lies at about 5 degrees north on average, and shifts to about 10 to 20 degrees north over the continents in July.

The annual range of temperature, the difference between the warmest and coolest month, is small near the equator, often only a few degrees, and increases poleward, but it is largest in the interiors of the large landmasses in the middle and high latitudes. Verkhoyansk in Siberia has an annual range of over 60 degrees. The range is much smaller over the oceans and on the western margins of continents.

How Is Heat Balanced Between Latitudes?

The Earth as a whole is in balance, but the tropics, roughly between 40 degrees north and 40 degrees south, receive more heat than they lose, and the regions poleward of this lose more heat than they receive. If there were no transfer, the tropics would get hotter and the poles colder. The surplus is moved poleward by the atmosphere and the oceans. The atmosphere carries heat by the general circulation, by warm air masses, storms and cyclones, as in General Circulation of the Atmosphere and Temperate Cyclones. The ocean carries heat by currents, such as the Gulf Stream, as in Ocean Currents – Causes and Types. Together they transport heat at a very high rate, with the atmosphere responsible for a larger share in the middle latitudes. It is this transfer that makes the latitudinal heat balance and keeps the climates of the middle latitudes liveable.

What Are the Temperature Extremes of the World and India?

The highest air temperature accepted by the World Meteorological Organization is 56.7 degrees Celsius at Furnace Creek (Death Valley), California, on 10 July 1913. The lowest reliable temperature is minus 89.2 degrees at the Vostok station in Antarctica on 21 July 1983. In India, the highest official temperature is 51 degrees at Phalodi in Rajasthan on 19 May 2016, and the Himalayan places such as Dras in Ladakh record winter minima of about minus 40 degrees and below. The year 2024 was the warmest year on record globally, about 1.55 degrees above the 1850 to 1900 average. As an analytical point, global averages hide large regional contrasts, and the poles have warmed much faster than the tropics, a pattern called polar amplification, which makes the study of heat distribution important for climate change.

Why Is This Topic Important?

Maps Isotherm questions
Concepts Thermal equator and range
Climate change Polar amplification

What Came Next?

The next topic describes the vertical distribution of temperature and temperature inversion. Next topic: Vertical Distribution of Temperature and Temperature Inversion.

Key People and Terms

Isotherm Line of equal temperature
Thermal equator Belt of highest temperature
Annual range Warmest minus coolest month
Heat surplus Gain more than loss
Heat deficit Loss more than gain
Polar amplification Faster warming near poles

Exam Corner

Points to Remember

Isotherms are reduced to sea level.
Thermal equator is about 5 degrees north.
Siberia has the largest annual range.
Heat surplus is between about 40 north and 40 south.
Highest in India: Phalodi, 51 degrees.

Do Not Confuse With

Thermal equator and geographic equator Hottest belt versus 0 degrees latitude
Isotherm and isobar Temperature versus pressure
Heat surplus and deficit Tropics versus poles

Memory Hook

Isotherms bend at coasts; hot belt wanders north; winds and currents carry the surplus to the poles.

Mains Angle

Questions ask for patterns and the reasons for them. These points add depth.

Pattern Fall poleward, bends at coasts
Seasons July and January contrast
Balance Poleward heat transport

A Question You May Face

An original practice question, not a past paper question.

“Describe the horizontal distribution of temperature in January and July and explain how the latitudinal heat balance is maintained.”

How to Answer

Isotherms Definition and bends.
Seasons January and July.
Balance Winds and currents.
Extremes World and India records.

GS Relevance

Prelims: isotherms, thermal equator, temperature records, heat transport. GS Paper 1: climatology.

Frequently Asked Questions

What are isotherms?

Isotherms are lines on a map joining places that have the same temperature at a given time or on average. They are drawn after reducing the readings to sea level to remove the effect of altitude, and they show the horizontal distribution of temperature.

What is the thermal equator?

The thermal equator is the line joining the places with the highest mean temperature on each meridian. It does not coincide with the geographic equator, but lies at about 5 degrees north on average, and shifts to 10 to 20 degrees north over land in July.

Why do isotherms bend over continents and oceans?

Isotherms bend because land and sea heat and cool at different rates. In winter, the isotherms bend toward the equator over cold continents and toward the poles over warmer oceans, and in summer the reverse. Warm ocean currents add to this bending.

Which region has the highest annual range of temperature?

The interiors of the middle- and high-latitude continents have the highest annual range, especially northeastern Siberia, where Verkhoyansk has a range of over 60 degrees Celsius. Maritime regions and the equator have a much smaller range of only a few degrees.

How does heat flow from the equator to the poles?

The tropics have a surplus of heat and the poles a deficit. The imbalance is corrected by the atmosphere, through winds, air masses and storms, and by the oceans, through currents like the Gulf Stream, which carry heat poleward and moderate temperatures.

PYQ Practice — Statement Analysis

1 Isotherms are lines joining places of equal temperature.

True

They are reduced to sea level.

2 The thermal equator always coincides with the geographic equator.

False

It lies about 5 degrees north on average.

3 The annual range is largest in continental interiors of high latitudes.