Upper Air: The Jet Stream, 500 mb and 850 mb
Discover the fast river of air high overhead that steers our storms, warm spells and cold snaps, and the two upper-air maps forecasters check first.
In this lesson
- Why forecasters look kilometres above the ground to predict the weather at ground level
- What the jet stream is, and how its ridges and troughs bring warm spells and cold snaps
- Why the 500 mb map is called the “steering wheel” and the 850 mb map the “thermal truth”
- What the polar vortex really is, and what it is not
The weather has an upstairs
Most of us experience weather at ground level: the wind on our face, the rain on the windshield. But the systems that bring that weather are organised and steered by what happens several kilometres overhead. A forecaster who only looked at surface maps would be like a sailor who watched the waves but ignored the wind.
Instead of heights in metres, meteorologists usually describe the upper air by pressure levels. Remember from Lesson 3 that air pressure falls as you go up. So “the 500 millibar level” simply means “the height where the pressure has dropped to 500 millibars.” (A millibar, or mb, is exactly the same as a hectopascal, hPa. Upper-air charts still often use the older name.)
Think of the atmosphere as a building with floors. Each floor has a job, and forecasters walk up and down the stairs every day to check on them.
The jet stream: a river in the sky
The jet stream is a narrow, fast-moving band of wind high in the atmosphere, usually about 9 to 12 kilometres up, near the cruising altitude of airliners. Its winds commonly blow at well over 100 km/h and can be much faster in winter. It generally flows from west to east around the globe.
Why does it exist? The jet stream forms along the boundary between cold polar air and warmer air to the south. The bigger the temperature contrast, the stronger the jet. That is why it is strongest in winter, when the Arctic is coldest compared with the south, and why it shifts north and weakens in summer.
The jet rarely flows in a straight line. It meanders like a river crossing a flat plain, with big northward bulges and southward dips. Those bends are the key to our weather.
- A ridge is a northward bulge. Warm air from the south pushes up underneath it. Ridges bring sinking air, settled skies and warmer-than-normal temperatures.
- A trough is a southward dip. Cold air from the north spills down into it. Troughs bring rising air, clouds, precipitation and cooler temperatures.
Ridge overhead: think warm and quiet. Trough approaching from the west: think change, cloud and a chance of precipitation. Trough overhead or just passed: think cooler.
500 mb: the steering wheel
The 500 mb level sits around 5.5 kilometres up, roughly halfway through the atmosphere by weight. Half of all the air is above it and half below. That makes it a good “average” of the whole atmosphere, and it is the single map many forecasters check first.
Surface lows and highs tend to be carried along by the flow at this level, much like leaves floating on a stream. If the 500 mb winds blow from the northwest over Alberta, systems tend to arrive from the northwest. That is why it is nicknamed the steering wheel.
The 500 mb map also shows the ridges and troughs clearly. Forecasters watch for small ripples, called shortwaves, moving through the bigger pattern. Each shortwave can trigger a round of cloud, showers or snow as it passes.
Sometimes a very strong ridge stalls and the jet stream detours around it for days. This is called a blocking pattern. A stubborn summer ridge can become a “heat dome,” with sinking air baking the region underneath day after day.
850 mb: the thermal truth
Surface temperatures can be misleading. A sunny afternoon heats the ground, a clear night chills it, and cold air drains into valleys. The 850 mb level, about 1.5 kilometres above sea level, sits above most of that daily noise. Its temperature reflects the true character of the air mass passing over, which is why forecasters call it the thermal truth.
On 850 mb maps, forecasters look for warm air advection (winds carrying warmer air in) and cold air advection (winds carrying colder air in). They also use 850 mb temperatures as one of several clues for deciding between rain and snow.
Because Lethbridge sits at about 910 metres, the 850 mb level is only several hundred metres above our heads, so it is closer to “ground truth” here than it is near sea level. Forecasters on the high prairies also lean on the 700 mb level, about 3 kilometres up. In winter, a ridge over the West often brings mild air aloft while shallow cold air stays trapped near the ground. That is why the Oldman River valley can sit in fog and frost while the air a few hundred metres up is far milder. When a trough digs south through western Canada, the door opens for Arctic air to pour down the east side of the Rockies (Lesson 8). Strong westerly winds aloft, often ahead of an approaching trough, help drive our chinooks.
The polar vortex, minus the hype
The polar vortex is a large area of low pressure and very cold air that spins around the North Pole. It is always there, stronger in winter and weaker in summer. It is not a storm, and it does not “arrive” like one.
When the vortex is strong and tight, the cold tends to stay bottled up in the Arctic. When it weakens or gets stretched out of shape, lobes of very cold air can sag far to the south, sometimes over the Prairies. News stories then say “the polar vortex is coming,” which really means a piece of that Arctic cold has slipped south along a deep trough.
Whatever it is called, a deep Arctic outbreak can bring dangerous wind chills. Dress in layers, cover exposed skin, keep a winter kit in the vehicle and watch for Environment Canada extreme cold warnings.
Try it yourself
Look at today’s sky and the forecast together. Long streaks of high cirrus moving quickly from the west or southwest often mean the jet stream is close and a trough may be approaching. A week of clear, calm, unusually warm days usually means a ridge is parked overhead. Check the barometer trend on the live station and see whether it matches what the upper pattern suggests.
West Lethbridge now (3:14 am): 0°C, wind S 2 km/h, humidity 95%. Live station ›
Key terms
- Millibar (mb)
- A unit of air pressure, identical to the hectopascal (hPa); used to name upper-air levels.
- Jet stream
- A narrow band of very fast wind about 9 to 12 kilometres up, flowing generally west to east.
- Ridge
- A northward bulge in the upper-level flow, usually bringing warm, dry, settled weather.
- Trough
- A southward dip in the upper-level flow, usually bringing cooler, unsettled weather.
- 500 mb level
- About 5.5 kilometres up; its winds steer weather systems, so it is called the steering wheel.
- 850 mb level
- About 1.5 kilometres above sea level; its temperature shows the true character of the air mass.
- Shortwave
- A small ripple moving through the larger upper-air pattern that can trigger cloud and precipitation.
- Polar vortex
- A large, persistent area of cold low pressure circling the North Pole; pieces can break off and push south.
Check your understanding
1. What makes the jet stream stronger in winter?
The jet forms along the boundary between cold and warm air, so the larger the contrast, the faster it blows.
2. A large ridge in the jet stream settles over southern Alberta for several days. What weather is most likely?
Under a ridge, warm air bulges north and sinking air suppresses clouds, so the weather is usually warm and quiet.
3. Why is the 500 mb map called the “steering wheel”?
The 500 mb level is roughly halfway through the atmosphere by weight, and lows and highs tend to follow its flow.
4. Why do forecasters call the 850 mb temperature the “thermal truth”?
At about 1.5 kilometres above sea level, it reflects the real character of the air mass rather than local surface effects.
5. Which statement about the polar vortex is accurate?
The polar vortex is a permanent feature, strongest in winter; when it weakens or stretches, lobes of Arctic air can push south.
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).