Weather reports usually mention low and high pressure systems, fronts and troughs because these terms relate to atmospheric air pressure which is a major driver of wind and weather.

Air pressure at any point is the total weight of the air above that point. This is determined by the number of molecules present. Dense humid air contains more molecules than less dense, dry air and therefore exerts more pressure.

One cubic metre of air weighs about one kg. At the surface of the earth, this is about 10,000 kg (10 tonnes) per square metre, or 14 lb per square inch. Pressure can be expressed in millibars, the height in millimetres, of a column of mercury needed to balance the weight of the air in a mercury barometer. Or as hectopascals which is the international unit for measuring atmospheric pressure (1 hPa = 100 Pascals = 1 mb). At sea level normal air pressure is 1,013.25 mb.

Due to unequal heating, the atmospheric air pressure also becomes unequal, and the pressure gradient created by this inequality creates air movement, wind and weather.

Parcels of air that are homogenous areas of pressure, temperature and humidity are subject to processes which can transform them into areas of low or high pressure.

Low pressure systems are formed when a parcel of air is lifted. As the air rises, the pressure falls causing expansion, which pushes back against the surrounding air which expends energy causing the air parcel to lose heat. The rising air spirals inward in a clockwise direction in the Southern Hemisphere (anticlockwise in Northern Hemisphere). As the air cools, water vapour condenses, causing clouds, precipitation, storms and high winds. The clouds act as a blanket so low pressure systems do not have extremes of temperatures.

High pressure systems are formed when a parcel of air sinks down. As the air subsides, it is subject to compressional heating by the surrounding air which evaporates water vapour and dries the air parcel. The sinking air spirals outwards in an anti clockwise direction in the Southern Hemisphere (clockwise in the Northern Hemisphere). High pressure systems are associated with clear skies and dry weather, because there are no clouds to blanket the atmosphere high pressure systems have more extremes of temperature.

Across the globe there are several consistently low and high pressure regions: Equatorial low – 0-10 degrees, Subtropical high – 20-35 degrees, Sub polar low – 60 degrees, Polar high – 90 degrees. Air pressure is also an important factor in the creation of regular cyclical weather patterns such as the monsoons.

High and low pressure systems are mobile and move over the Earth's surface, the leading edges between the moving air masses are delineated as cold or warm fronts.

Weather maps indicate pressure as isobars drawn at 4 hPa intervals, black arrows indicate wind direction, barbed lines show cold fronts with the barbs indicating the direction of movement.