The consistent sinking of air near 30 degrees North and 30 degrees South latitude is a fundamental feature of Earth’s atmospheric circulation, creating a band of persistent high pressure. Historically known as the Horse Latitudes, this pattern dictates climate across vast regions. This atmospheric behavior is part of a larger, continuous system that moves heat and moisture away from the tropics. Understanding why air sinks precisely at these subtropical boundaries requires examining the forces that govern the movement of air high above the surface.
Initiating the Global Air Pump Equatorial Ascent
The circulation responsible for the sinking air begins near the equator, where solar energy is most intense throughout the year. This concentrated heating warms the Earth’s surface and the air directly above it, causing the air to become highly buoyant and less dense. The warm air rises rapidly, carrying a massive amount of moisture upward into the atmosphere. This vigorous upward motion occurs in a narrow, low-pressure belt known as the Intertropical Convergence Zone (ITCZ), characterized by frequent thunderstorms. The air continues its ascent until it hits the tropopause, the boundary with the stratosphere, acting as a lid roughly 12 to 15 kilometers high. Once the air reaches this ceiling, it is forced to diverge horizontally toward the poles in both the Northern and Southern Hemispheres.
The Poleward Journey and Cooling
After the air masses diverge at the top of the troposphere, they begin their slow, high-altitude journey away from the equator toward higher latitudes. As this air moves poleward, it travels away from the primary tropical heat source that initially powered its ascent. During this extended horizontal transit, the air continually radiates heat out into space, a process known as radiational cooling. This gradual cooling causes the air mass to lose buoyancy and increase in density, making it progressively heavier than the surrounding upper-atmosphere air. The poleward flow is a transitional phase where the air prepares for its eventual descent, transporting energy away from the tropics.
Why the Air Stops and Sinks at 30 Degrees
The air mass does not simply sink wherever it gets cold; its descent is sharply defined around 30 degrees latitude by two powerful physical forces.
Angular Momentum and Convergence
The first is the conservation of angular momentum, which governs the air’s eastward speed relative to the Earth’s rotation. Air starting at the equator has a high rotational speed, and as it moves poleward, it gets closer to the Earth’s axis of rotation. To conserve its angular momentum, the air must accelerate eastward, creating a powerful high-altitude wind known as the subtropical jet stream. This intense eastward flow acts as a dynamic barrier, preventing the air from traveling much farther poleward and forcing it to converge and pile up around the 30-degree mark.
Adiabatic Warming
The second factor is the convergence and compression of this accumulated air. As the air is forced downward, the pressure from the massive column of air above it causes compression. This compression leads to a process called adiabatic warming, where the air heats up as it is squeezed without exchanging heat with the surrounding atmosphere. This warming effect drastically lowers the air’s relative humidity, making it extremely dry and stable, which suppresses all cloud formation and precipitation. The continuous downward pressure of this warm, dry air creates the persistent high-pressure zones observed at the surface in the subtropics.
The Subtropical Highs Climate and Weather Impacts
The persistent sinking and warming of air at these latitudes create the semi-permanent high-pressure systems known as the subtropical highs. The atmosphere within these systems is exceptionally stable, which effectively inhibits the rising motion necessary for cloud formation and rain. This atmospheric stability results in clear skies, abundant sunshine, and very low annual precipitation over the affected regions. Consequently, the world’s major hot deserts, such as the Sahara in North Africa, the Arabian Desert, and the Australian Outback, are predominantly located along these subtropical high-pressure belts. The air flowing out from the base of these high-pressure zones creates the prevailing surface winds, including the reliable trade winds that flow back toward the equator.