Atmospheric pressure is the weight of the air column above a location. High pressure areas, or anticyclones, occur where the air mass is heavier than the surrounding air. This increased weight is associated with the downward movement, or subsidence, of air from the upper atmosphere toward the surface. Sinking air compresses and warms, suppressing cloud formation and leading to stable, clear weather. Global patterns of air movement, driven by Earth’s rotation and uneven solar heating, create alternating bands of high and low pressure that encircle the planet.
The Relationship Between Vertical Air Movement and Surface Pressure
The vertical motion of the air column directly overhead governs surface pressure. When air rises (convection), it reduces the mass pressing down, creating low pressure often associated with cloud formation. Conversely, high pressure results when air sinks (subsidence) from the upper atmosphere, increasing the total air mass. Sinking air is compressed and warms adiabatically—warming due to pressure increase—which causes moisture to evaporate. Global high pressure belts form through two distinct mechanisms: dynamic forcing (large-scale circulation patterns) and thermal forcing (driven by extreme cold).
The Dynamic Formation of Subtropical High Pressure Belts
The most extensive and prominent high pressure areas are the Subtropical High Pressure Belts, located near 30 degrees North and South latitude. These belts are dynamically induced by the massive global circulation system known as the Hadley Cell, which transports heat from the equator toward the poles.
The process begins near the equator, where intense solar heating causes air to warm and rise forcefully in the Intertropical Convergence Zone (ITCZ). This air reaches the top of the troposphere and moves poleward at an altitude of approximately 12 to 15 kilometers.
As this air travels poleward, it cools and begins to pile up, increasing the mass in the upper atmosphere. Simultaneously, the Earth’s rotation introduces the Coriolis effect, which deflects the moving air. Due to the conservation of angular momentum, the air accelerates eastward, forming strong upper-level westerly winds.
The combination of upper-level air convergence and the deflection caused by the Coriolis effect prevents the air from continuing its journey. Around the 30-degree mark, this accumulated air mass is forced to descend toward the surface, creating the persistent subtropical high pressure belts. This strong subsidence warms and dries the landscape, which is why the world’s major deserts are found near these latitudes. The resulting high pressure at the surface then drives air back toward the equator as the trade winds, completing the Hadley Cell loop.
The Thermal Formation of Polar High Pressure Belts
In contrast to the dynamically forced subtropical highs, the high pressure areas found at the Earth’s poles, centered near 90 degrees North and South, are primarily thermally induced. These Polar High Pressure Belts are a direct consequence of the extremely low temperatures experienced in the polar regions throughout the year.
The lack of direct solar radiation leads to intense and persistent cooling of the air near the surface. As air cools, it becomes significantly denser and heavier than the air at lower latitudes. This cold, dense air naturally sinks directly to the surface due to its weight, creating a continuous zone of high pressure.
The sinking air in the polar regions is responsible for the clear skies and minimal precipitation, resulting in the characteristic polar deserts. Air flowing outward from these polar high pressure zones toward the sub-polar low pressure areas forms the surface winds known as the polar easterlies.