Atmospheric pressure is the weight of the air above us, spread over an area. At sea level a column of air weighs more than ten tonnes per square metre, yet we do not feel it because the pressure also acts inside our bodies. What matters to us is how it varies: by a few hectopascals from one day to the next, and those changes decide the weather.
Units and the standard value
In meteorology, pressure is measured in hectopascals (hPa): one hectopascal is 100 pascals and equals one millibar, the older unit you still meet. The reference value is that of the standard atmosphere, 1,013.25 hPa at sea level. It is called "standard" because it serves as a benchmark: above it, pressure is judged high; below it, low.
On maps and in forecasts, the figure given is pressure reduced to sea level, meaning what you would measure if the station were at zero height. Without that correction, mountain stations would always look like depressions and no map would be readable. The real reading at the station, lower at altitude, is the station pressure.
Why pressure falls with height
The higher you go, the less air remains above you, and the air itself, compressed under its own weight, is denser at the bottom than at the top. Pressure therefore does not fall evenly: near sea level it drops by about 1 hPa every 8 metres, whereas higher up you must climb much further to lose the same amount. It is halved at about 5,500 m, and a little over 200 hPa is left at 11,000 m, the cruising height of airliners.
At around 3,000 m the pressure is about 700 hPa in the standard atmosphere. That is why walkers feel altitude: with each breath they take in less oxygen, since the proportion of oxygen stays the same but the air is less dense.
Highs and lows
On a weather map, lines called isobars join points of equal pressure. Areas of high pressure are anticyclones: the air sinks, warms and dries, which dissolves clouds. You generally find fair weather, light winds and, in winter, mist or fog under a layer of low cloud. Areas of low pressure are depressions: the air rises and cools, the vapour condenses, and clouds and precipitation form.
Wind blows from high towards low pressure, but the Earth's rotation deflects it. In the northern hemisphere it circulates clockwise around a high and anticlockwise around a low; in the southern hemisphere it is the other way round. The closer together the isobars, the stronger the wind.
Reading the trend
It is less the value of the pressure that counts than its trend, meaning how it has changed over the last few hours. Pressure rising steadily generally signals improvement and drier weather. Pressure falling signals deterioration: the faster the fall, the more active the disturbance, and a drop of several hectopascals in three hours often comes with a strong wind.
A steady pressure, even a high one, says only that the weather is not changing. Compare it with the reading from three hours ago and twelve hours ago: that is what sailors did with their recording barometer.
A worked example: Toronto
For Toronto, using the forecast of 2026-11-02 08:13: the day's mean pressure, reduced to sea level, is 1024.2 hPa, which is 11.0 hPa above the standard value of 1013.25 hPa, so more of an anticyclonic situation. The city lies at 161 m above sea level: a barometer at the station would read about 1004.8 hPa.
Frequently asked questions
What is normal atmospheric pressure?
The reference value is 1,013.25 hPa at sea level, that of the standard atmosphere. Actual pressure varies by a few tens of hectopascals around it, depending on highs and lows.
Does falling pressure always mean rain?
Not always, but it is a good signal. A steady fall points to an approaching depression or front, so more cloud and a risk of precipitation; the faster the fall, the sharper the change.
Why does the forecast pressure not depend on altitude?
Because it is reduced to sea level so that places can be compared. Station pressure itself drops about 1 hPa every 8 metres near sea level.
Sources
- ISO 2533:1975, Standard Atmosphere: the values of 1,013.25 hPa, a standard temperature of 15 °C at sea level and a lapse rate of 6.5 K per kilometre, which give the chart and the pressures quoted.
- World Meteorological Organization (WMO), Guide to Instruments and Methods of Observation, WMO-No. 8: pressure measurement and reduction to sea level.
- Today's example: mean daily pressure from the Open-Meteo forecast, converted to station pressure with the standard atmosphere.