Air Pressure: The Most Important Number
You will learn what a barometer is really measuring, what highs and lows mean for your day, and why the pressure reading at a Lethbridge station looks so different from the one in the forecast.
In this lesson
- What air pressure is, and the units you will see (hPa, mb and kPa)
- What high and low pressure systems are, and the weather each tends to bring
- Why a station at about 913 metres reads roughly 100 hPa lower than sea level, and how “sea-level pressure” fixes that
- How to read the barometer’s trend, which often says more than the number itself
The weight of the sky
In Lesson 1 we saw that air has weight. Air pressure is simply that weight, pressing down on every surface: the whole column of air above you, from your head to the edge of space.
Imagine lying on the floor under a pile of blankets. Add blankets and you feel more pressure; take some away and you feel less. The atmosphere is that pile. When more air piles up over a region, pressure rises. When air is removed, pressure falls. Pressure is the atmosphere’s way of keeping score of where the air is piling up and where it is draining away.
The units
- Hectopascals (hPa): the standard unit in meteorology. Average sea-level pressure is about 1013 hPa.
- Millibars (mb): exactly the same size as a hectopascal. 1013 mb equals 1013 hPa. Older charts and many weather enthusiasts still use millibars.
- Kilopascals (kPa): Environment Canada’s public forecasts use these. Divide hPa by 10, so 1013 hPa is 101.3 kPa.
- Inches of mercury (inHg): common on American equipment and in aviation, about 29.92 at average sea-level pressure.
Temperature tells you how it feels right now. Pressure, and especially how it is changing, tells you what the atmosphere is about to do. Differences in pressure are what make the wind blow, and the patterns of highs and lows are the skeleton of every weather map.
Highs and lows
On a weather map, a big blue H marks a centre of high pressure, and a red L marks a centre of low pressure. The curved lines around them are isobars, lines joining places with the same pressure, just like contour lines on a topographic map join places of equal height.
High pressure: the settled one
In a high, air is slowly sinking from above. Sinking air warms and dries out as it descends, which dissolves clouds. Highs usually bring clear or mostly clear skies and lighter winds near their centre. In winter, a strong high full of Arctic air can mean bitter cold, crisp blue skies and those very still, very cold nights we met in Lesson 2.
Low pressure: the stormy one
In a low, air near the ground is drawn inward from all sides. It has nowhere to go but up, and rising air cools. Cooling air eventually can’t hold all its water vapour, so clouds form, and with enough lift, rain or snow follows. Most of our significant storms, from soaking spring rains to big snowfalls, are tied to low pressure systems.
High pressure, fair weather. Low pressure, unsettled weather. It is not a guarantee, but it is right far more often than not.
Station pressure versus sea-level pressure
Here is something that puzzles almost every new weather station owner in Lethbridge. The forecast or the news talks about pressure near 1013 hPa, but the station’s raw reading says something closer to 900 hPa. Is the barometer broken?
Not at all. The West Lethbridge Weather station sits at about 913 metres above sea level. We already have that much air below us, so the column above us is shorter and lighter. Near the ground, pressure falls by roughly 1 hPa for every 8 to 10 metres you climb, so 913 metres works out to roughly 100 hPa less than at sea level.
That raw reading is called station pressure. It is real, but it is no good for comparing with other places. A station in the mountains would always look like a deep low, and a station on the coast like a strong high, just because of their elevation.
So weather services convert every reading to sea-level pressure: an estimate of what the pressure would be if the station sat at sea level, with an imaginary column of air filling in the space below. Once every station is on the same footing, the real highs and lows stand out on the map. That is the number you see in forecasts and on most station displays, including ours.
Converting to sea level involves guessing the temperature of that imaginary column of air beneath us. On the high prairie that guess gets tricky in extreme weather. In a deep Arctic outbreak the calculated sea-level pressure can look remarkably high, partly because the math assumes a very cold, dense column underneath. It is also common to see the barometer sag as a chinook develops, because the strong westerly flow over the Rockies tends to leave lower pressure on our side of the mountains. Watching pressure here is a great way to learn the personality of prairie weather.
The trend tells the story
A single pressure reading, on its own, says surprisingly little. Is 1008 hPa good or bad? It depends on what it was doing a few hours ago. The direction and speed of change, called the pressure tendency, is the real clue. Forecasters usually look at the change over the last three hours.
- Falling steadily: a low or a front is likely approaching. Clouds, wind or precipitation may be on the way.
- Falling fast: a strong system is coming, often with strong winds. Pay attention to forecasts and warnings.
- Rising after a storm: the system is moving away and a high is building in. Clearing usually follows, though in our part of the world a sharp rise can also arrive with a gusty north wind behind a cold front.
- Steady: no big change expected soon.
Pressure also wiggles a little on its own each day, with small, regular rises and falls of a hPa or so, caused by the sun heating the atmosphere. So do not read too much into a tiny change. Look for steady movement of a few hPa over several hours.
The speed of the change matters more than the number. A slow drift is routine. A fall of several hectopascals in just a few hours is the barometer saying “something is coming.”
Try it yourself
Check the station barometer now and note the reading and the trend arrow. Check again in three hours, then once more tomorrow morning. Did the pressure go up or down, and did the weather follow the rule of thumb? Over a couple of weeks you will start to see systems coming before the clouds do.
West Lethbridge now (3:14 am): 0°C, wind S 2 km/h, humidity 95%. Live station ›
Key terms
- Air pressure
- The weight of the column of air above a point, pressing on it from all directions.
- Hectopascal (hPa)
- The standard unit of air pressure in meteorology, equal to one millibar. Ten hectopascals make one kilopascal.
- Isobar
- A line on a map joining places with the same pressure.
- High pressure system
- An area where pressure is highest at the centre, with sinking air and usually settled weather.
- Low pressure system
- An area where pressure is lowest at the centre, with rising air and often cloud, wind and precipitation.
- Station pressure
- The actual pressure measured at the station’s own elevation.
- Sea-level pressure
- Station pressure converted to an estimate of what it would be at sea level, so places can be compared.
- Pressure tendency
- How the pressure has changed recently, usually over the last three hours.
Check your understanding
1. An Environment Canada forecast lists pressure as 101.6 kPa. What is that in hectopascals?
One kilopascal is ten hectopascals, so 101.6 kPa is 1016 hPa.
2. Why does a high pressure system usually bring clear skies?
Sinking air warms as it descends, which evaporates cloud droplets. Rising air, as in a low, does the opposite.
3. The raw barometer reading at a Lethbridge station says about 905 hPa. What is the most likely explanation?
At our elevation there is less air above us, so station pressure normally reads roughly 100 hPa below sea-level pressure.
4. Near the ground in the Northern Hemisphere, how does air move around a low?
Air spirals counterclockwise and inward toward a low, then rises. Clockwise and outward describes a high.
5. Which barometer change is the strongest sign that stormy weather is coming?
The speed of the change matters most. A fast fall usually means a strong low or front is approaching.
Weather School is for learning and enjoyment. It is not official guidance and may contain errors. For watches and warnings, always use Environment Canada (WeatherCAN app or weather.gc.ca).