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For teachers · Worksheet 3 of 5

Reading a pressure map

The lines around the letters H and L on the weather map tell you where the wind comes from, how strongly it blows and whether clouds or clear skies are on the way.

Aim

To recognise isobars, highs or anticyclones (H) and lows or depressions (L), link the spacing between isobars to the strength of the wind and know which way the air turns in each case.

A low on the left, marked L, with tightly packed isobars and the air turning anticlockwise, and a high on the right, marked H, with widely spaced isobars and the air turning clockwise. LH 996 hPa1028 hPa

Activity

  1. 10 min
    Find the letters. In the Enthusiast view, “Isobars · MSLP” (one line every 4 hPa) and “Highs and lows” are already switched on. Find the high and the low nearest to the Iberian Peninsula and read their pressure.
    Open the pressure map
  2. 10 min
    Isobars and wind. In “Colour field”, choose wind. Compare an area where the isobars are tightly packed with another where they are far apart. Where is the wind stronger?
  3. 10 min
    The map moves. Press ▶ on the time bar and follow a pressure centre over the next few days. Which way does it move? Will it pass close to the school?
  4. 10 min
    From the map to the sky. Look at the pressure at the school's location (“MSL pressure”) and relate it to the weather you can see out of the window. In Bachillerato, add the “500 hPa height contours” and look for a trough or a cut-off low.

Questions

  1. What do all the points on the same isobar have in common?
  2. What is the pressure of the high you found? And of the low? Are they above or below the average pressure?
  3. Where is the wind stronger: where the isobars are close together or where they are far apart? Why?
  4. In the northern hemisphere, which way does the air turn around a low? And around a high?
  5. What weather does a high usually bring? And a low?

Answers for teachers
  1. The same atmospheric pressure, reduced to sea level (MSLP) so that places at different altitudes can be compared. On the dashboard there is one isobar every 4 hPa.
  2. It depends on the day. The average sea-level pressure is about 1013 hPa; highs usually exceed 1020 hPa and deep lows drop below 1000 hPa.
  3. Where they are close together: the pressure changes a lot over a short distance (a steeper gradient) and the air speeds up more. It is like a steeper slope.
  4. Because of the Earth's rotation (the Coriolis force), in the northern hemisphere the air turns anticlockwise around a low and clockwise around a high. Near the ground, friction deflects it slightly towards the low pressure.
  5. In a high the air sinks and warms up, and no clouds form: settled weather (in winter, sometimes fog). In a low the air rises, cools and forms clouds and precipitation.