Atmospheric pressure is one of the key variables behind changing weather. High- and low-pressure systems influence vertical air motion, cloud formation, precipitation, wind direction and wind speed.

But the familiar rule that “high pressure means good weather and low pressure means rain” is only a starting point. Actual weather depends on how pressure is distributed, temperature, humidity, air masses, fronts, wind and conditions higher in the atmosphere.

For the underlying quantity itself, see what atmospheric pressure is and how it is measured.

How does high pressure affect the weather?

A region where atmospheric pressure is higher than in the surrounding area is described as a high-pressure system.

Air within a high-pressure system often tends to sink from higher levels of the atmosphere. As descending air moves into higher pressure, it is compressed and warms. Its relative humidity can then fall, making cloud formation less likely.

High pressure is therefore often associated with:

• Clearer or less cloudy skies
• More stable weather
• Lower probability of widespread precipitation
• Relatively light surface winds in some situations

However, high pressure does not always mean warm, sunny weather. During winter, persistent high pressure can also trap cold air near the surface and favour fog, low cloud or temperature inversions.

How does low pressure affect the weather?

A region where pressure is lower than its surroundings is called a low-pressure system.

Near the surface, air tends to converge toward lower pressure. This can support rising motion. As air rises into lower surrounding pressure, it expands and cools.

If enough water vapour is present, that cooling can increase relative humidity and lead to condensation. The result may be:

• Cloud formation
• Rain or snow
• More changeable weather
• Stronger winds or storms in some systems

Low pressure therefore does not “pull” moisture out of the atmosphere or guarantee rain by itself. Rather, low-pressure systems are commonly associated with rising air and atmospheric structures that favour cloud and precipitation when sufficient moisture is available.

Why do pressure differences create wind?

When atmospheric pressure is not the same from one location to another, a horizontal force acts on the air. This is known as the pressure gradient force.

It accelerates air from higher pressure toward lower pressure. In the real atmosphere, however, wind does not normally travel in a straight line from a high-pressure centre to a low-pressure centre because the Coriolis effect and surface friction alter its direction.

The important quantity is not simply the pressure at two locations, but how quickly pressure changes with distance.

For that relationship in detail, see how pressure affects wind.

What do isobars tell us about the weather?

On a weather map, lines joining locations with the same atmospheric pressure are called isobars.

When isobars are packed closely together, pressure is changing rapidly over a short distance. This usually indicates a stronger pressure gradient and potentially stronger winds.

Widely spaced isobars usually indicate a weaker pressure gradient.

That is why meteorologists do not look only at numbers such as 1010 hPa or 1025 hPa. The shape and spacing of the isobars are equally important.

Does falling barometric pressure mean rain is coming?

Not necessarily. Falling pressure by itself does not guarantee rain.

A steady pressure fall can indicate that a low-pressure system or weather front is approaching. If that system also contains sufficient moisture and rising air, the probability of cloud, stronger winds and precipitation may increase.

Similarly, rising pressure can be associated with more stable conditions, but a single barometer reading should never be treated as a complete forecast.

For more on pressure trends, see what barometric pressure can indicate.

Why does atmospheric pressure change?

Atmospheric pressure at the surface is related to the mass and structure of the air column above a location.

As air masses move, temperatures change and the atmosphere expands, contracts, rises or sinks, surface pressure can change as well.

Temperature is important because it affects air density and vertical motion, but the statement “warm air always creates low pressure and cold air always creates high pressure” is too simplistic for real weather.

The atmosphere is three-dimensional and constantly moving. Surface pressure depends on the behaviour of the whole air column and the surrounding circulation.

For the controlled gas-physics relationship, see the relationship between pressure and temperature.

Is every cyclone a hurricane?

No. In meteorology, a cyclone is a broad term for a large-scale circulation around a region of relatively low pressure.

Many low-pressure systems in the mid-latitudes are cyclones, but they are not hurricanes.

A tropical cyclone is a particular type of storm system that develops over warm tropical or subtropical waters. Depending on the region and intensity, strong tropical cyclones may be called hurricanes or typhoons.

They can contain very low central pressure, intense winds and heavy rainfall, but low pressure and hurricane are not interchangeable terms.

What is an anticyclone?

An anticyclone is a large-scale circulation associated with relatively high atmospheric pressure.

Anticyclones can develop over both land and ocean; they are not restricted to continental areas.

Because they are commonly associated with descending air, they often favour stable conditions. Depending on season, moisture and surface conditions, however, an anticyclone may also bring fog, low cloud or persistent periods of unusually hot or cold weather.

How are weather fronts related to pressure?

A weather front is the boundary between air masses with different temperature and moisture characteristics.

Mid-latitude low-pressure systems are often linked with warm fronts, cold fronts or occluded fronts.

As a front passes, conditions can change rapidly:

• Atmospheric pressure may rise or fall
• Wind direction may shift
• Temperature may change quickly
• Clouds and precipitation may develop

Meteorologists therefore analyse a low-pressure centre together with its surrounding fronts rather than treating the central pressure value as the whole weather system.

Why is pressure reduced to sea level on weather maps?

Atmospheric pressure naturally decreases with altitude. Directly comparing station pressure from a mountain location with pressure measured at sea level would therefore be misleading.

A high-altitude station can show a much lower measured pressure even when both stations are under the same broad weather pattern.

For synoptic weather maps, pressure is therefore commonly adjusted to an equivalent mean sea-level pressure. This allows pressure patterns from stations at different elevations to be compared more meaningfully.

The physical reason pressure decreases with height is explained in why atmospheric pressure falls with altitude.

Which unit is used for atmospheric pressure?

Meteorology commonly expresses atmospheric pressure in hectopascals (hPa).

1 hPa = 100 Pa

A hectopascal is numerically equal to a millibar:

1 hPa = 1 mbar

This is why older meteorological references using millibars can often be read directly alongside modern hPa values.

Standard atmospheric pressure is:

1013.25 hPa = 1013.25 mbar = 101,325 Pa

For other conversions, see pressure units and conversions.

How do meteorologists measure atmospheric pressure?

Atmospheric pressure has traditionally been measured with a barometer.

A mercury barometer balances atmospheric pressure against the height of a mercury column. An aneroid barometer uses the deformation of a sealed flexible metal capsule.

Modern automatic weather stations commonly use electronic pressure sensors instead.

For a broader look at measuring instruments, see how pressure is measured.

Can pressure changes be used to predict the weather?

Yes, but pressure alone is not enough for a reliable forecast.

A steady fall over several hours can be a useful indication that the pressure pattern is changing and that a low-pressure system or front may be approaching. A rapid fall can sometimes accompany increasingly unsettled conditions.

Rising pressure can indicate the arrival or strengthening of a more stable air mass.

Modern forecasting, however, combines pressure information with:

• Pressure maps and isobars
• Temperature and humidity observations
• Wind speed and direction
• Weather radar
• Satellite data
• Upper-air observations
• Numerical weather-prediction models

So a rule such as “the barometer is falling, therefore it will definitely rain” is not a reliable forecasting method.

Atmospheric pressure and weather in simple terms

Atmospheric pressure is closely linked to both vertical and horizontal air motion. High-pressure systems often involve descending air that supports more stable conditions, while low-pressure systems commonly involve rising air that can favour cloud and precipitation.

Pressure differences also help drive wind. But pressure is only one part of the weather system: temperature, humidity, fronts, Earth's rotation, upper-level circulation and local geography all contribute to the conditions we experience.

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Related pressure topics

Atmosphere and weather:
Atmospheric pressure and measurement | Barometric pressure | Pressure and wind | Pressure at altitude

Pressure fundamentals:
What is pressure? | How is pressure measured? | Pressure units | Calculating pressure | Pressure and temperature | Pressure and density | Pressure in gases

Water and depth:
Hydrostatic pressure | Divers and underwater pressure | Deep-sea pressure

Pressure measurement and control:
Pressure measurement topics | Pressure indicators and controllers | Pressure FAQ | Pressure sensors and control applications

Further reading:
Reading pressure graphs | Pressure equations | Other meanings of pressure | Pressure in film and media

The atmosphere is a dynamic system influenced by many interacting variables. This page explains the basic relationship between atmospheric pressure and weather and should not be treated as a local weather forecast or warning service.

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