Atmospheric pressure is the force exerted onto a surface by the weight of the air above it, measuring how much the atmosphere is pushing down at a given point. Pressure varies across the globe because the atmosphere is constantly in motion. An area of higher pressure indicates a greater mass of air concentrated overhead compared to surrounding regions. These pressure differences drive all weather systems.
The Direct Answer: High Pressure Sinking
High pressure systems are characterized by a large-scale downward movement of air toward the Earth’s surface. Meteorologists refer to this sinking motion as subsidence, which defines the system. This means air in the upper atmosphere accumulates and pushes down on the air beneath it.
The accumulation of air aloft increases the weight of the atmospheric column, registering as a higher pressure reading at the surface. As the air descends, it spreads out near the ground, a process known as divergence. This outward flow accommodates the continuous downward push of air from above.
Why Does Air Sink in a High Pressure System
The main mechanism behind the sinking motion is a thermodynamic process called adiabatic warming. As air descends from higher altitudes, it encounters increasing pressure. This pressure compresses the air parcel without exchanging heat with the surrounding environment.
Compression causes air molecules to collide more frequently, converting the work of compression directly into heat energy. This warms the air parcel at a specific rate, known as the dry adiabatic lapse rate (approximately 10 degrees Celsius for every 1,000 meters of descent).
The decrease in relative humidity prevents cloud formation and precipitation. For clouds to form, air must cool to its dew point, allowing water vapor to condense. Since the descending air is constantly warming, it moves further away from its dew point, suppressing the vertical lift of moisture and stabilizing the atmosphere. This stability reinforces the downward motion and the high-pressure system.
How Sinking Air Affects Local Weather
The stability and dryness caused by subsiding air have predictable effects on ground-level weather. The most recognizable sign of a high-pressure system is fair weather and clear skies. Because cloud formation is suppressed, sunlight reaches the surface unimpeded, often leading to warmer daytime temperatures.
However, the lack of cloud cover allows heat to escape efficiently into space at night, causing temperatures to drop sharply after sunset. The outward-spreading air near the surface generally results in light, calm winds, contributing to stable conditions.
The stability of the atmosphere can also lead to air quality concerns when sinking air traps pollutants near the ground. This trapping occurs because the warming, sinking air creates a temperature inversion, where a layer of warm air sits above cooler air near the surface. This inversion acts like a lid, preventing the mixing of air and concentrating vehicle exhaust and industrial emissions. This results in poor visibility and the accumulation of haze or smog in urban areas.