Pression Atmosphérique

What Is Air Pressure?

Air pressure — also called atmospheric pressure or barometric pressure — is the weight of the atmosphere pressing down on Earth's surface. While air feels weightless in everyday life, the column of air stretching from your head to the edge of space has significant mass. At sea level, the atmosphere exerts a pressure of approximately 1,013 millibars (or 101.3 kPa), equivalent to the weight of roughly 10 tonnes pressing on every square metre of surface.

The key insight is that air pressure is not constant. It varies with altitude, temperature, and the movement of large air masses. These variations drive virtually every weather phenomenon you experience — from gentle breezes to violent storms.

Pressure and Altitude

Air pressure decreases with altitude because there is less atmosphere above you. At the summit of Mount Everest (8,849m), pressure is less than one-third of sea level pressure — which is why climbers require supplemental oxygen. This relationship is why aircraft cabins are pressurised, and why your ears pop in ascending lifts or aeroplanes.

This also explains why weather forecasters adjust pressure readings to sea level before comparing stations. A weather station at 2,000 metres would naturally show lower pressure than one at the coast — adjusting to sea level removes this altitude effect and allows meteorologists to identify genuine pressure systems.

High Pressure Systems

A high pressure system (anticyclone) forms when air descends and spreads outward at the surface. Descending air compresses and warms, which suppresses cloud formation and precipitation. High pressure systems are therefore typically associated with fair, settled weather — clear skies, light winds, and warm or cold temperatures depending on season and location.

In the Northern Hemisphere, air circulates clockwise around high pressure centres; in the Southern Hemisphere, anticlockwise (due to the Coriolis effect). High pressure systems can be stationary or slow-moving, explaining why fair spells can last for days or weeks.

Low Pressure Systems

A low pressure system (cyclone or depression) forms when air converges at the surface and rises. Rising air cools, leading to cloud formation, precipitation, and unsettled weather. Low pressure systems bring wind, rain, and dramatic weather changes — from mid-latitude depressions to tropical cyclones and hurricanes.

In the Northern Hemisphere, air spirals anticlockwise into low pressure centres. The gradient between high and low pressure determines wind speed — the steeper the gradient (closer together the isobars on a weather map), the stronger the wind.

How Pressure Changes Predict Weather

A barometer measures air pressure. Falling pressure generally signals deteriorating weather — approaching fronts, rain, wind, or storms. Rising pressure signals improving conditions. Rapid pressure falls (more than 1 millibar per hour) indicate rapidly developing storms.

Traditional barometers use mercury columns; modern digital barometers and weather stations measure electronically. Many smartphones contain barometric sensors — some weather apps use your phone's sensor to track local pressure trends in real time.

Pressure on the SunorSnow Dashboard

SunorSnow's multi-city weather dashboard displays current atmospheric pressure alongside temperature, humidity, wind and UV index for each of your tracked cities. Watching pressure trends across multiple cities simultaneously gives you a uniquely comprehensive view of approaching weather systems — particularly useful if you are planning travel or outdoor activities across different locations.