Pressure Belts and Pressure Cells
Geography › Pressure, Winds and Atmospheric Moisture · Topic 7.2
The Earth has seven pressure belts: the equatorial low, the two subtropical highs near 30 degrees, the two subpolar lows near 60 degrees and the two polar highs. The equatorial low and polar highs are thermal in origin, while the subtropical highs and subpolar lows are dynamic. The belts shift north and south with the Sun, and break up into pressure cells over land and sea.
5 min read · UPSC Prelims · Geography · Pressure, Winds and Atmospheric Moisture
At a Glance
- Equatorial low 0 to 5 degrees; doldrums; thermal
- Subtropical highs About 30 degrees; horse latitudes; dynamic
- Subpolar lows About 60 degrees; dynamic
- Polar highs Poles; thermal
- Shift Follows the apparent movement of the Sun
- Exam link UPSC Prelims, Geography: Pressure, Winds and Atmospheric Moisture
Where Does It Fit?
This is the second topic of the category on pressure, winds and atmospheric moisture. The previous topic described pressure and isobars, and this one maps the global pattern of pressure. The next topic covers the forces that shape wind movement. Previous topic: Atmospheric Pressure, Isobars and Vertical Variation. Next topic: Forces Affecting Wind Movement.
What Are the Pressure Belts of the World?
In short: if the Earth were uniform, the pressure would form seven alternating belts of low and high pressure between the equator and the poles.
| Belt | Latitude | Origin | Features |
|---|---|---|---|
| Equatorial low pressure belt | About 5 degrees north to 5 degrees south | Thermal: intense heating makes air rise | Calms and light variable winds, called the doldrums; heavy convectional rain; the Intertropical Convergence Zone lies here |
| Subtropical high pressure belts | About 30 to 35 degrees north and south | Dynamic: air that rises at the equator cools and sinks here | Calm, dry, clear weather; the “horse latitudes”; the great deserts lie beneath |
| Subpolar low pressure belts | About 60 to 65 degrees north and south | Dynamic: air is drawn away by rotation and circulation | Stormy weather; meeting of warm and cold air; temperate cyclones |
| Polar high pressure belts | About 90 degrees north and south | Thermal: very cold dense air sinks | Cold, dry, outblowing winds |
The name doldrums comes from sailors who found sailing ships becalmed in the weak winds there. The horse latitudes are said to be named after the horses that were thrown overboard from sailing ships stranded in the calm to save water. The vocabulary is part of the standard Prelims questions on the world pressure system.
Why Are Some Belts Thermal and Others Dynamic?
The belts have two types of origin. The thermal belts are caused by temperature: the equatorial low arises because the intense solar heating makes air expand and rise, and the polar highs arise because the intense cold makes the air dense and sinking. The dynamic belts are not caused directly by temperature: air rising at the equator moves poleward at high levels, cools and sinks near 30 degrees to create the subtropical highs. The Coriolis force turns this high-level air so that it piles up near 30 degrees, adding to the sinking. At about 60 degrees, the westerly winds from the subtropical highs and the cold polar easterlies meet and the air is forced to rise, creating the subpolar lows. The forces are discussed in Forces Affecting Wind Movement, and the full circulation in General Circulation of the Atmosphere.
How Do the Belts Shift with the Seasons?
The pressure belts follow the apparent movement of the Sun, as the zone of maximum heating moves between the Tropics, as in Solstices, Equinoxes and Variation in Day Length. In the northern summer (July) the belts shift north, and in the northern winter (January) they shift south, by about 5 to 10 degrees of latitude. The shift is bigger over land than over the sea. This movement brings the Mediterranean region under the subtropical high in summer, giving dry summers, and under the westerlies in winter, giving winter rain. It also pulls the equatorial low, and the rain it brings, across the tropics: the movement of the Intertropical Convergence Zone far north over South Asia in summer is the foundation of the Indian monsoon, as described in Mechanism of the Indian Monsoon.
What Are Pressure Cells and Semi-Permanent Pressure Centres?
In the real world the belts are not continuous. Land and sea heat unequally, so the belts are broken into cells of high and low pressure. These semi-permanent centres of action control the climate of large regions.
| Centre | Type | Season and location |
|---|---|---|
| Siberian (Asiatic) High | High | Winter; over cold Siberia; the source of cold dry winter winds towards India and China |
| Aleutian Low and Icelandic Low | Low | Winter; over the North Pacific and North Atlantic; subpolar lows |
| Azores (Bermuda) High and Hawaiian High | High | Year-round, stronger in summer; over the oceans at about 30 degrees north |
| Thermal lows over Asia and southwestern USA | Low | Summer; over hot land, including the Tibetan Plateau and northwest India, which draw in the monsoon |
| Mascarene High and Australian Low | High and low | Southern hemisphere: the Mascarene High over the southern Indian Ocean in the southern winter, which feeds the southwest monsoon |
As an analytical point, the seven-belt model is an average, and the actual pressure on a day may differ greatly; the transitions of the pressure cells between seasons are what drive the seasonal reversal of winds, the monsoon, discussed in Planetary, Seasonal and Local Winds.
Why Is This Topic Important?
- Foundation Underlies global winds
- India Monsoon and Siberian High
- Concepts Thermal versus dynamic
What Came Next?
The next topic describes the forces that affect wind movement. Next topic: Forces Affecting Wind Movement.
Key People and Terms
- Doldrums Calm equatorial belt
- Horse latitudes Calm subtropical high belt
- ITCZ Intertropical Convergence Zone
- Thermal origin Caused by heating or cooling
- Dynamic origin Caused by air movement
- Pressure cell Closed high or low centre
Exam Corner
Points to Remember
- Seven belts: 1 equatorial low, 2 subtropical highs, 2 subpolar lows, 2 polar highs.
- Equatorial low and polar highs are thermal.
- Subtropical highs and subpolar lows are dynamic.
- Belts shift north in July.
- Siberian High forms in winter.
Do Not Confuse With
- Doldrums and horse latitudes Equatorial calm versus subtropical calm
- Thermal and dynamic belts Heat-caused versus motion-caused
- Belt and cell Continuous zone versus closed centre
Memory Hook
Low at the equator, high at thirty, low at sixty, high at the poles.
Mains Angle
Questions ask for the pressure system and its seasonal shift. These points add depth.
- Origin Thermal and dynamic
- Shift Follows the Sun
- India Monsoon link
A Question You May Face
An original practice question, not a past paper question.
“Describe the pressure belts of the world and explain their origin and seasonal shift.”
How to Answer
- Belts Seven belts.
- Origin Thermal and dynamic.
- Shift North in July.
- Cells Land-sea effects.
GS Relevance
Frequently Asked Questions
What are the pressure belts of the world?
The Earth has seven pressure belts: the equatorial low near the equator, the two subtropical highs near 30 degrees, the two subpolar lows near 60 degrees and the two polar highs at the poles. They control the direction of the global winds.
What are the doldrums?
The doldrums are the equatorial low pressure belt, within about 5 degrees of the equator, where air rises strongly and winds are light and variable. Sailing ships were often becalmed there, and the belt has heavy convectional rainfall and thunderstorms.
What are the horse latitudes?
The horse latitudes are the subtropical high pressure belts near 30 to 35 degrees north and south, where air sinks, giving calm, dry and clear weather. The world's great hot deserts lie in these belts, and the name is said to come from horses thrown overboard from becalmed ships.
Why are subtropical highs called dynamic?
The subtropical high pressure belts are dynamic because they are not caused by temperature but by the movement of air. Air that rises at the equator moves poleward aloft, is turned by the Coriolis force, and sinks near 30 degrees, creating high pressure at the surface.
How do pressure belts shift with the seasons?
The pressure belts follow the apparent movement of the Sun and shift northward by about 5 to 10 degrees in the northern summer and southward in the northern winter. This shift moves rainfall and wind systems, and it underlies the Indian monsoon.
PYQ Practice — Statement Analysis
1 The equatorial low pressure belt is thermal in origin.
Intense heating makes air rise.
2 The subtropical high pressure belts are caused by intense cold.
They are dynamic; sinking air causes them.
3 The doldrums lie close to the equator.
They are calm and rainy.
4 The pressure belts remain fixed throughout the year.
They shift with the Sun.
Related Topics
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