Draw your own weather map
Forecasters have analyzed maps by hand for more than a century, and many still do, because drawing the lines yourself makes you notice things a computer map hides. Grab a pencil.
In this lesson
- Drawing isobars: the rules
- Finding highs and lows, and what the spacing tells you about wind
- Finding cold, warm and Arctic fronts from the station plots
- The prairie special: the lee trough and the “Alberta clipper”
Step 1: Draw the isobars
Isobars are lines joining places with the same sea-level pressure, like contour lines on a hiking map.
- Draw them every 4 hPa: 1000, 1004, 1008, 1012, 1016, 1020 and so on.
- Use a pencil and draw lightly. You will erase.
- Pass between stations: a 1012 line runs between a station reading 1010.6 and one reading 1013.8, closer to the 1013.8.
- Isobars never cross and never end in the middle of the map. They close into loops or run off the edge.
- Keep them smooth, and label each line at the edge.
Good starting trick: find the highest and lowest pressures on the map first, then work outward.
Step 2: Mark the highs and lows
- Put a blue H in the centre of the highest closed loop and a red L in the lowest.
- Lows bring rising air, cloud and precipitation. Highs bring sinking air, and usually clearer, quieter weather.
- In the Northern Hemisphere, wind blows counterclockwise around lows and clockwise around highs, nearly along the isobars but angled a little in toward the low near the ground.
- Close isobars mean strong wind. Where the lines bunch up, the pressure changes quickly over a short distance, and the wind blows hard.
Check yourself: do the wind barbs on the map roughly follow your isobars? If not, take another look at your lines.
Step 3: Find the fronts
A front is the boundary between two air masses. Look for places where several things change at once across a short distance:
- Cold front (blue line with triangles pointing the way it moves): a sharp temperature and dew point drop behind it, a wind shift (often from south or southwest to west or northwest), pressure falling ahead and rising behind, and the isobars bending into a “V” along it.
- Warm front (red line with half-circles): warmer, moister air behind it, with a wide band of cloud and steady precipitation ahead of it.
- Arctic front: on the Prairies, the most dramatic of all. Bitterly cold, dense air from the north slides south. Across it, temperatures can differ by 15 or 20 degrees, and the wind swings to the north or northeast.
In winter, an Arctic front often stalls right across southern Alberta, with frigid air on the plains and mild air near the mountains. On those days, Lethbridge can be the dividing line, and towns an hour apart can be 20 degrees different. When the mild west wind wins, it’s a chinook.
Prairie specials
- Lee trough: when strong west winds cross the Rockies, a trough of low pressure forms just east of the mountains, right over southern Alberta. It’s often drawn as a dashed line. It brings the gusty, mild west winds of a chinook.
- Alberta low: lows often form, or strengthen, east of the Rockies, then race east across the Prairies. The fast-moving winter ones are called “Alberta clippers”, famous for wind and blowing snow on their way to the rest of the continent.
- Upslope snow: when a high sits to the north or east, it pushes moist air from the east or northeast up the rising land toward the foothills. Rising air cools, and the result can be steady, heavy snow even without a big storm overhead.
Try it yourself
- Print the free Western Canada station map from Weather Graphics.
- Decode the pressures and draw isobars every 4 hPa.
- Mark the highs and lows.
- Look for fronts: a sharp temperature change, a wind shift and a pressure trough together.
- Now compare with the professional analysis: Environment Canada’s surface analysis charts, or Weather Graphics’ version with the isobars already drawn (Western Canada with terrain and isobars). How close did you get?
🏫 For teachers and parents
- Time: 60 minutes, or two classes.
- Materials: printed station maps (one per pair), pencils, erasers, blue and red pencils.
- Warm-up: practise with contour lines on a hill first: same idea, different numbers.
- Assessment: pairs swap maps and check each other’s isobars against the three rules (every 4 hPa, never cross, between the stations).
- Extension: analyze the same map three days in a row and track how the low moved.
Key terms
- Isobar
- A line joining places with the same sea-level pressure.
- Pressure gradient
- How quickly pressure changes over distance. Steep gradient, strong wind.
- Front
- The boundary between two air masses.
- Lee trough
- A trough of low pressure just downwind of a mountain range.
- Upslope flow
- Air pushed uphill by the wind, cooling as it rises.
Check your understanding
1. What do closely packed isobars tell you?
Bunched isobars mean the pressure changes quickly over a short distance, which drives strong wind.
2. In the Northern Hemisphere, how does wind blow around a low?
Wind circles a low counterclockwise, with friction near the ground angling it slightly in toward the centre.
3. Which change is NOT a sign of a cold front passing?
Behind a cold front the air is usually colder and drier, so the dew point drops.
4. Why can easterly winds bring heavy snow to southern Alberta?
That’s upslope flow: rising air cools, and its moisture becomes cloud and snow.
Sources: Environment and Climate Change Canada, surface analyses; University of Alberta, What you should learn to deduce from weather maps (EAS 270).
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).