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Reading the weather · 5 min · 4 steps

Highs, lows and the spin

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Goal. Read isobars, find highs and lows, and link them to wind direction, wind speed and weather.

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Steps

  1. Predict

    In the northern hemisphere, which way does the wind blow around a low-pressure area?

    • Clockwise
    • Anticlockwise
    • Straight into the centre
    Show the answer

    Anticlockwise. Anticlockwise, and slightly inwards near the ground.

    The grey lines are isobars: they join places with the same air pressure. L marks a low, H a high. Look for an L, or if there is none in view, an H, and follow the wind streaks around it.

  2. Air is pushed from high towards low pressure, but the Earth's spin turns it, so it circles the centres: anticlockwise round lows and clockwise round highs in the north. Near the ground friction lets it spiral slightly into the low.

  3. Where the isobars crowd together, the pressure changes quickly over a short distance and the wind is strongest. Compare the streak colours where the lines are close and where they are far apart.

  4. South of the equator everything turns the other way: clockwise round lows. Everywhere, lows bring rising air, cloud and rain, while highs bring sinking air and usually settled weather.

The common mistake

Myth

High pressure means strong wind.

Fact. Wind depends on how quickly the pressure changes across the map, shown by how close the isobars are, not on how high the pressure is. The centre of a high is often calm.

Check yourself

Question 1

What do isobars join?

  • Places with the same temperature
  • Places with the same air pressure
  • Places with the same rainfall
Show the answer

Places with the same air pressure. Isobars join places where the sea-level pressure is the same, drawn here every 4 hPa.

Question 2

Where is the wind strongest?

  • Where the isobars are close together
  • Where the isobars are far apart
  • At the centre of a high
Show the answer

Where the isobars are close together. Closely spaced isobars mean a steep pressure difference, which drives strong wind.

Question 3

What weather does a low usually bring?

  • Cloud, rain and wind
  • Clear, calm skies
  • Snow every time
Show the answer

Cloud, rain and wind. Air rises in a low, cools and forms cloud and rain.

Words in this lesson

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Curriculum

Taiwan 12-year curriculum (108 課綱), natural science
Ib-Ⅳ-2 · Ib-Ⅳ-3 · EIb-Vc-5 · EIb-Vc-6
US Next Generation Science Standards
MS-ESS2-5
England national curriculum
KS3 Geography: weather and climate