Reading the upper air
The surface map shows what’s happening now. The maps several kilometres up show where it’s going. Forecasters always look up before they look ahead.
In this lesson
- Why upper-air maps use pressure levels instead of heights
- The 500 hPa map: ridges, troughs and the steering flow
- The 850 hPa map, and why it’s tricky at Lethbridge’s elevation
- Thickness, and two rules of thumb for rain or snow
- Weather balloons: where Alberta’s come from
Maps at pressure levels
Upper-air maps are drawn on a pressure surface: the height where the pressure equals, say, 500 hPa. The lines on the map show how high that surface is, in decametres (tens of metres). “552” means the 500 hPa level is 5,520 metres up.
Warm air is less dense, so the pressure surfaces sit higher over warm air and lower over cold air. High heights mean warmth; low heights mean cold.
- 850 hPa: about 1.5 km above sea level. The low-level temperature and moisture.
- 700 hPa: about 3 km. Moisture and the mid-level wind.
- 500 hPa: about 5.5 km. The steering level, and the forecaster’s favourite map.
- 250 hPa: about 10 km. The jet stream.
The 500 hPa map: ridges and troughs
At 500 hPa, the height lines make big waves around the hemisphere.
- Ridges (the lines bulge north): warm air, sinking motion, usually settled weather.
- Troughs (the lines dip south): cold air, rising motion ahead of them, cloud and storms.
- The wind at this level blows along the lines and steers surface lows and highs. Watch which way the waves are moving, and you can see what’s coming in the next two or three days.
- Look for smaller ripples riding through the big waves (“shortwaves”). They often trigger snow or thunderstorms.
A ridge over British Columbia, with strong west to southwest winds aloft across the Rockies, is a classic chinook setup. A deep trough dropping down from the north is how the Arctic air arrives.
The 850 hPa map, and our elevation
On most of the continent, 850 hPa is a kilometre or more above the ground. Lethbridge is already about 900 metres above sea level, so here the 850 hPa level is only a few hundred metres above our heads, and in deep Arctic air it can sit very close to the ground.
That makes 850 hPa temperatures a good clue to what’s happening right above us, but use them with care. Forecasters on the Prairies often look at 700 hPa as well.
Rain or snow? Two rules of thumb
- 850 hPa temperature: snow is much more likely when the 850 hPa temperature is below about 0°C (32°F). Well below, and it’s snow; near zero, it could be rain, snow or both.
- Thickness: the distance between the 1000 hPa and 500 hPa levels shows how warm the whole lower atmosphere is. A thickness of 540 decametres is the traditional rain-or-snow line: below it, snow is more likely.
Weather balloons
Upper-air maps start with real measurements from weather balloons, launched twice a day at 00 and 12 UTC (6 pm and 6 am in Alberta) from sites around the world. Each one carries a small instrument, a radiosonde, that radios back temperature, humidity and wind as it climbs. The plot of what it measures is called a sounding.
- Alberta’s balloon site is at Stony Plain, west of Edmonton (station 71119, “WSE”).
- The closest to us is Great Falls, Montana (“TFX”).
- You can view both on the University of Wyoming’s free sounding page.
Learn to read a sounding in Weather School: Stability and Soundings.
Try it yourself
- Open Environment Canada’s model forecast charts and find a 500 hPa map.
- Find the nearest ridge and trough to Alberta. Which way is the flow over the Rockies?
- Step through the forecast hours. Does a trough move toward us? When?
- Write one sentence: “In the next 48 hours, the upper pattern suggests…”
🏫 For teachers and parents
- Time: 45 to 60 minutes.
- Demonstration: warm air expands. Put a balloon over the mouth of a bottle and stand it in warm water, then in cold water. That’s why pressure surfaces sit higher over warm air.
- Activity: students trace the 500 hPa height lines on a printed chart and colour ridges red and troughs blue.
- Quick check: “Where would you expect cloud and storms: ahead of a trough or under a ridge?”
Key terms
- Pressure surface
- The height at which the pressure equals a set value, such as 500 hPa.
- Ridge
- A northward bulge in the upper-air pattern, warm and settled.
- Trough
- A southward dip in the pattern, cold and unsettled.
- Thickness
- The depth of the layer between two pressure levels. Warmer air, thicker layer.
- Sounding
- The plot of temperature, moisture and wind measured by a weather balloon.
Check your understanding
1. Why is the 500 hPa map called the “steering level”?
The flow at about 5.5 km guides where surface weather systems go.
2. Where would you expect cloud and precipitation?
Air rises ahead of a trough, which builds cloud and precipitation.
3. A thickness well below 540 decametres suggests…
Low thickness means a cold lower atmosphere, which favours snow.
4. Why is the 850 hPa level tricky to use at Lethbridge?
At our elevation, the 850 hPa level is only a few hundred metres overhead, and sometimes near the surface.
Sources: Environment and Climate Change Canada model forecast products; University of Wyoming upper-air soundings; University of Alberta EAS 270 weather-map guide. Rain and snow rules are standard forecasting rules of thumb.
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).