Atmospheric pressure is the force exerted by the air around us. For weather, the most useful information is not a fixed number on its own, but how that number compares with nearby places and how it changes over time.
What atmospheric pressure measures
Gravity holds the atmosphere around Earth. The air above any point has mass, and the weight of that air column creates pressure at the surface and at every level above it. Pressure acts in all directions rather than only downwards. The fluids and tissues in the body exert pressure too, which is why normal surface pressure does not crush us.
Pressure always decreases with height because less air remains above the measuring point. The decrease is not linear. Air is most compressed near the surface, so pressure falls more quickly through the lowest part of the atmosphere than it does higher up. Temperature also affects the spacing between pressure levels: the same pressure surface generally lies higher in a warm air column than in a cold one.
Units, standard pressure and barometers
Weather services usually report pressure in hectopascals (hPa). One millibar (mbar) has exactly the same numerical value as one hPa. Inches of mercury (inHg) remain common in US weather reports and aviation.
Standard sea-level pressure is 1013.25 hPa, equal to 29.92 inHg. It is a reference for calculations, instrument calibration and aviation. It is not a universal dividing line between high and low pressure in real weather. A centre at 1018 hPa could be a high in one pressure pattern and part of a low in another.
A barometer measures atmospheric pressure. An aneroid barometer contains a sealed metal capsule that expands or contracts as pressure changes. Electronic sensors now perform the same job in automatic weather stations, phones and outdoor instruments. A poorly calibrated sensor, an incorrect elevation setting or a move between floors can all shift a home reading.
Station pressure and sea-level pressure
Station pressure is the pressure at the instrument’s actual elevation. A mountain station therefore records a much lower value than a coastal station under the same broad weather pattern. That difference mostly reflects the amount of atmosphere above each instrument, not a stronger weather system at the coast.
For comparison, meteorologists calculate mean sea-level pressure, or MSLP. They reduce the station reading to an estimate of the pressure that would exist at sea level beneath that location. Surface charts and many public forecasts use MSLP so that highs, lows and horizontal pressure differences can be compared across uneven terrain.
The reduction is an estimate and depends partly on the temperature of the air column. It becomes less straightforward at high-elevation stations, where the calculated layer between the station and sea level does not physically exist. Before comparing a home station with an app or map, check whether it shows station pressure, sea-level pressure or an aviation altimeter setting. These values serve different purposes and need not match.
Reading highs, lows and isobars
On a surface weather chart, a high is a local pressure maximum and a low is a local minimum. The labels describe the shape of the surrounding pressure field. Isobars join points with equal sea-level pressure and make that shape visible.
Closely spaced isobars show a stronger horizontal pressure gradient, which often supports stronger wind. Widely spaced lines usually indicate a weaker gradient. The relationship is not a direct conversion from spacing to the wind at one street or runway. Friction, terrain, elevation and the structure of the atmosphere can all change local speed and direction. Wind speed therefore needs to be read alongside the pressure map.
Air accelerates towards lower pressure, but Earth’s rotation deflects the flow and friction modifies it near the ground. Surface winds generally circulate clockwise around highs and anticlockwise around lows in the Northern Hemisphere; the directions reverse in the Southern Hemisphere. Near the surface, wind usually crosses isobars at an angle rather than running exactly along them or directly from the centre of a high to a low.
What pressure patterns can bring
High-pressure systems often contain broad areas of descending air. As the air sinks, it is compressed and warms, which can lower its relative humidity and limit the growth of deep cloud. This often produces settled conditions, particularly near the centre of a high.
High pressure does not guarantee sunshine or dry weather. Moist air can remain trapped beneath an inversion, leading to fog, low cloud or persistent overcast. Weak vertical mixing can also allow pollutants to accumulate near the ground. Season, terrain, nearby water and the source of the air all affect the result.
Low-pressure systems are often associated with rising air, cloud and unsettled weather. Mid-latitude lows may carry warm, cold and occluded fronts with different patterns of cloud, precipitation, wind and temperature. Central pressure alone does not reveal how severe a system will be. A modest low with tightly packed isobars can produce stronger local winds than a deeper low with a broad pressure gradient. Tropical cyclones are classified by their structure and sustained winds, not by one pressure threshold.
Pressure tendency and a home barometer
Pressure tendency describes the amount and character of pressure change during a stated period, commonly three hours in surface observations. Falling pressure can accompany an approaching low or front. Rising pressure can follow a departing system or the arrival of a ridge. The rate of change and the regional pattern usually say more than a solitary reading.
These clues are not universal rules. Atmospheric tides, local heating, thunderstorms and terrain-driven circulations can also alter pressure. There is no single fall-rate threshold that works everywhere as a storm warning. Compare like with like, record readings at regular intervals, and combine the trend with wind, cloud, radar and official warnings.
Pressure above the surface
Meteorologists also analyse constant-pressure levels such as 850, 700 and 500 hPa. These are not fixed altitudes. The height of each surface changes as the atmosphere warms, cools and redistributes mass. The 500 hPa level is often near 5 to 6 kilometres above sea level, but its height varies from place to place and from day to day.
Upper-air charts show troughs, ridges and winds that help steer weather systems. Forecast models calculate this three-dimensional structure rather than extending the surface pattern straight upwards. How weather forecasting works explains how observations and model calculations become a forecast.
Pressure altimeters and forecast limits
An aircraft pressure altimeter is a sensitive barometer calibrated to display altitude in a standard atmosphere. Pilots set it using an official local altimeter setting and update that setting as conditions change. Non-standard temperature still introduces error, while an out-of-date pressure setting can make the indicated altitude differ from the aircraft’s actual height. Consumer forecast pressure is not a substitute for official aviation observations, procedures or instruments.
Pressure is equally limited as a stand-alone weather forecast. It helps identify the broad pattern and its movement, but it cannot specify when rain will begin, where fog will clear or how strong a gust will be at a particular location. On Airpult, read pressure together with temperature, humidity, wind, cloud and precipitation, then follow the relevant official warning service when hazardous weather is possible.
