Stability and Soundings Made Simple
Understand why some days build towering thunderstorms and others stay flat and hazy, and learn to read a weather balloon chart without fear.
In this lesson
- What “stable” and “unstable” air really mean, using a simple rising bubble of air
- How lapse rates and inversions decide whether air rises or stays put
- How weather balloons measure the atmosphere twice a day, all over the world
- How to read the basics of a Skew-T chart, including CAPE, the “fuel” for thunderstorms
Stable or unstable: a ball in a bowl
Put a marble in a bowl and give it a nudge. It rolls up the side, then settles back to the bottom. That is stable. Now balance the marble on top of an upside-down bowl and nudge it. It rolls off and keeps going. That is unstable.
Air behaves the same way. Meteorologists imagine a “parcel” of air, a bubble about the size of a house, with an invisible skin around it. Something gives it a nudge upward: the sun heating a field, a front, or wind pushing it up a hill. The question is simple: once it starts rising, does it keep going, or sink back down?
The answer depends on one comparison. A parcel that is warmer than the air around it is lighter, so it keeps rising, like a hot-air balloon. A parcel that is colder than its surroundings is heavier, so it sinks back.
Rising air cools at a predictable rate
As a parcel rises, the pressure around it falls and it expands. Expanding takes energy, so the parcel cools, even though no heat leaves it. For dry (unsaturated) air this happens at about 10°C for every kilometre of rise, or roughly 1°C per 100 metres. Once the parcel cools enough to form cloud, condensing water vapour releases heat, so it cools more slowly from then on, often roughly half as fast in warm weather.
Lapse rates: comparing the bubble with its surroundings
The real atmosphere around the parcel also gets colder with height, on average about 6.5°C per kilometre, but this changes a lot from day to day. How quickly the surrounding air cools with height is called the lapse rate.
- If the surrounding air cools slowly with height, a rising parcel soon finds itself colder than its neighbours and sinks back. The air is stable: think flat layers of cloud, haze and smoke that hangs low.
- If the surrounding air cools quickly with height, a rising parcel stays warmer than its neighbours and keeps accelerating upward. The air is unstable: think puffy cumulus building into thunderstorms.
Inversions: the lid on the pot
Normally the air gets colder as you go up. An inversion is a layer where it does the opposite: it gets warmer with height. Warm air sitting on top of cold air is the most stable arrangement there is. It acts like a lid on a pot, trapping whatever is underneath: cold air, fog, smoke and pollution.
Inversions form in a few common ways. On clear, calm nights the ground loses heat to space and chills the air right next to it. Dense cold air can also slide in underneath milder air, or sinking air under a big high pressure system can warm and form a lid higher up. A warm layer aloft over cold surface air is also the recipe for freezing rain (Lesson 7).
Inversions are everyday events here. On calm, clear nights, cold air drains into the Oldman River valley and the coulees, so the valley bottom can be noticeably colder than the west-side or north-side benches above it. In winter, shallow Arctic air can get trapped near the ground while milder Pacific or chinook air flows over the top. Then you can see the chinook arch overhead and feel the cold still parked at ground level. When the chinook wind finally mixes down and scours out the cold air, the temperature can jump sharply.
Weather balloons: measuring the sky twice a day
How do forecasters know the temperature five kilometres up? They send up a weather balloon. A balloon filled with hydrogen or helium carries a small instrument package called a radiosonde, which radios back temperature, humidity and pressure as it climbs. Tracking its position also gives the wind speed and direction at every level.
The balloon expands as it rises into thinner air, until it eventually bursts, often well above 30 kilometres. The radiosonde then drifts down on a small parachute. The full record from one flight is called a sounding.
Hundreds of stations around the world launch balloons at the same moments: 00 UTC and 12 UTC, every day. In Alberta that is 6 p.m. and 6 a.m. Alberta Time, all year now that our clocks no longer change. Launches happen a little before those times so the balloon is aloft at the official hour. The results feed straight into the computer models that make our forecasts.
Environment Canada runs a network of upper-air stations across the country. The nearest Canadian one to Lethbridge is near Edmonton, at Stony Plain. Just south of the border, the U.S. National Weather Service launches from Great Falls, Montana. Forecasters for southern Alberta look at both.
Reading a Skew-T without fear
Soundings are plotted on a chart called a Skew-T. It looks like a plate of spaghetti at first, but at heart it is just a graph of temperature against height, like the one above, with a twist: the temperature lines are tilted to the right. That tilt simply spreads the lines out so they are easier to read. You can ignore most of the background lines and focus on four things.
- Up the chart means up in the sky. The numbers on the side are pressure in hectopascals: 850 is low down, 300 is near jet stream height.
- The red line is the air temperature measured by the balloon. Follow it up to spot inversions, where it bends to the right (warmer) instead of to the left.
- The green line is the dew point. Where red and green nearly touch, the air is close to saturated, which usually means cloud. Where they are far apart, the air is dry.
- The dashed line is a rising parcel. It shows the path a bubble of surface air would follow if lifted. Wherever it sits to the right of (warmer than) the red line, the air is unstable at that level.
Most Skew-T charts also show wind barbs down the right-hand side, giving the wind direction and speed at each level.
CAPE in plain words
CAPE stands for Convective Available Potential Energy. In simple terms, it is the size of that shaded area: how much “push” a rising parcel would get from being warmer than its surroundings. More CAPE means stronger updrafts, taller storms and a better chance of hail. Low values support only modest showers, while large values on a summer day can fuel severe storms. CAPE is only the fuel, though. Storms also need a trigger to get the air rising, and a layer of warm air aloft (a “cap”) can hold everything down until something breaks it.
Subtract the dew point from the temperature and multiply by about 125. The answer is a rough height, in metres, where rising air will form cloud. A spread of 10 degrees suggests cloud bases near 1,250 metres above the ground.
On unstable summer days in southern Alberta, a sunny morning can turn into thunderstorms with hail by late afternoon. Watch the sky to the west, and heed Environment Canada watches and warnings.
Try it yourself
On a warm, sunny day, watch the cumulus clouds. If they stay small and flat-topped, the air above is fairly stable. If they bubble upward into tall towers by mid-afternoon, the air is unstable. Use the dew point tool and the current temperature to estimate the cloud base, then see if the flat bottoms of the clouds look about right.
West Lethbridge now (3:14 am): 0°C, wind S 2 km/h, humidity 95%. Live station ›
Key terms
- Air parcel
- An imaginary bubble of air that meteorologists follow as it rises or sinks.
- Stable air
- Air that resists vertical motion; a lifted parcel sinks back down.
- Unstable air
- Air that encourages vertical motion; a lifted parcel keeps rising.
- Lapse rate
- How quickly temperature falls with height.
- Inversion
- A layer where temperature rises with height, acting like a lid that traps air beneath it.
- Radiosonde
- The instrument package carried by a weather balloon that measures temperature, humidity and pressure.
- Skew-T
- A chart of a balloon sounding, with temperature lines tilted to make the profile easier to read.
- CAPE
- Convective Available Potential Energy: a measure of how much energy is available to fuel rising air and thunderstorms.
Check your understanding
1. A rising parcel of air finds itself warmer than the air around it. What happens next?
Warmer air is less dense, so like a hot-air balloon the parcel keeps rising. That is unstable air.
2. About how fast does dry, rising air cool?
Unsaturated air cools by expansion at roughly 10°C per kilometre, or about 1°C per 100 metres.
3. What is an inversion?
In an inversion, warmer air sits above cooler air, forming a stable lid that traps cold air, fog and smoke below.
4. When are weather balloons launched at stations around the world?
Stations worldwide launch at the same two times each day so the data line up for the computer models.
5. On a Skew-T, the temperature and dew point lines are nearly touching at one level. What does that suggest?
When temperature and dew point are close, the air is nearly saturated, which usually means cloud.
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).