Q.Draw a simplified diagram to show the general circulation of the atmosphere over the globe. What are the possible reasons for the formation of subtropical high pressure over 30 degree N and S latitudes?

The general circulation of the atmosphere is best understood as a set of three connected cells in each hemisphere. Near the equator, intense heating causes air to rise, forming the equatorial low-pressure belt (the Intertropical Convergence Zone). This rising air moves poleward aloft, cools, and sinks around 30° latitude, creating the subtropical high-pressure belts. From there, surface air flows back toward the equator (the trade winds) and also toward the poles (the westerlies). At around 60° latitude, the warmer subtropical air meets colder polar air, forcing the polar air to rise and creating the subpolar low-pressure belt. Finally, the cold, dense air over the poles sinks, forming the polar high-pressure cells. …

Picture the Earth as a spinning ball wrapped in a moving ocean of air. If the planet didn’t rotate, the simplest circulation would be a single giant loop: hot air rising at the equator, flowing poleward aloft, sinking at the poles, and returning along the surface. But the Earth does rotate, and that rotation — the Coriolis effect — bends every moving air current. The result is not one loop but three distinct cells in each hemisphere, stacked like gears between the equator and the poles.

At the equator, intense solar heating makes the air expand, become light, and rise almost continuously. This rising air creates the equatorial low-pressure belt, a zone of heavy rainfall and towering clouds. As this air climbs to the upper troposphere, it cannot keep flowing straight toward the poles because the Coriolis force deflects it eastward. By about 30° latitude, the air has cooled, become denser, and begins to sink. This sinking motion is the heart of the Hadley cell — a closed loop of rising at the equator, poleward flow aloft, sinking at 30°, and equatorward flow at the surface.

The sinking air at 30° N and S is the direct reason for the subtropical high-pressure belts. As air descends, it is compressed by the weight of the air above it, and compression warms it. Warmer air can hold more moisture, so any clouds that existed evaporate. The sky clears, rain stops, and the surface pressure rises because the column of air above is heavier. This is why the world’s great deserts — the Sahara, the Arabian, the Australian outback — sit roughly along these latitudes. The sinking air also diverges at the surface: part flows back toward the equator as the trade winds, and part flows poleward as the westerlies.

But there is a second, subtler reason tied to the planet’s rotation itself. The Coriolis effect does more than bend winds; it also influences where air can pile up. At the equator, the Earth’s surface moves fastest eastward. Air moving poleward from the equator carries that high eastward speed with it, but the ground beneath it moves slower at higher latitudes. This mismatch creates a net deflection that literally pushes air toward 30° latitude, piling it up. Think of a spinning merry-go-round: if you try to walk straight outward while it spins, you get shoved sideways. The atmosphere experiences the same shove, and the result is a belt of accumulated, sinking air at 30° N and S.

The diagram below shows the general circulation in a simplified cross-section. The equator is in the middle, the poles at the edges, and the three cells are drawn as loops of rising and sinking air.

        POLAR HIGH (sinking air)
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        Polar cell:  rising at 60°, sinking at pole
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        SUBPOLAR LOW (rising air at 60°)
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        Ferrel cell:  sinking at 30°, rising at 60°
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        SUBTROPICAL HIGH (sinking air at 30°)
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        Hadley cell:  rising at equator, sinking at 30°
              |
        EQUATORIAL LOW (rising air)
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