7.2: Single-Cell Model

Hadley Cells

The first model we’ll examine is the single-cell model. With this model, we make the following assumptions.

1. The Earth is entirely covered with water. This is to remove any land-sea interactions.
2. There are no seasons, and the sun is always shining directly over the equator. This removes seasonal wind shifts.
3. There is no Coriolis force. While the Earth rotates to spread heat along latitudinal lines, this allows us to be concerned only with the pressure gradient force.

With these assumptions in place, Earth’s global circulation would be as illustrated in Figure \(\PageIndex{1}\), with one giant vertically overturning cell in each hemisphere. The excess heating at the equator is transported poleward by rising warm air, which is replaced by cold sinking polar air moving equatorward. This circulation is known as the Hadley cell. The Hadley cell is known as a thermally direct circulation because warm air rises and cold air sinks.

Figure \(\PageIndex{1}\): The single-cell model of Hadley cells on a planet. Warm air on the equator rises, generating low surface pressure, while cold air on the poles sinks, generating high surface pressure. At the surface, winds blow from the high pressure at the pole to the low pressure at the equator, while aloft, winds blow from the rising air at the equator to the sinking air at the poles. (CC-BY-SA 4.0; Suhari At Tamfury, via Wikimedia Commons).

The circulation can be thought of in two ways. In the first, hot air at the equator rises because it is warm and buoyant. It reaches the tropopause, spreading laterally north and south at high elevations. To compensate for the rising air, surface air flows toward the equator, resulting in convergence and further uplift. Continuity of this circulation results in a global circulation with rising air at the equator and sinking air at the poles.

A second way to view global circulation is that excess heating of air at the equator creates a large area of low pressure at the planet's surface, while excess cooling at the poles creates a high-pressure area at the surface. This global horizontal pressure gradient causes air to flow from high to low at the surface (pole to equator), where it subsequently rises at the equator, flows back to the poles, and sinks.

Both reasons are plausible; it's a matter of whether you focus on temperature or pressure. The temperature and pressure differences are intertwined, and both are important for the general circulation. While this single-cell model can explain some phenomena and works in some ways (and on some planetary bodies), it is not the reality on Earth. Because Earth is rotating, we need to consider the Coriolis force in addition to the pressure gradient force. In the single-cell model, as upper-level air flows from the equator toward the poles, it would be deflected by the Coriolis force. In the Northern Hemisphere, for example, this deflection would be to the right, resulting in a wind from west to east at upper levels. In this way, the air moving from the equator to the poles would never make it there because of the rotation of the Earth. A different model is needed.