PPL Tester

Highs, lows & isobars

On a surface chart, isobars join places of equal pressure. Two centres matter (Northern Hemisphere):

Low (cyclone) High (anticyclone)
Air motion converges and rises subsides and diverges
Surface flow counterclockwise, inward clockwise, outward
Weather cloud, precipitation, poorer fair, settled

Shapes on the chart:

Isobar spacing sets wind strength: close together = strong wind, far apart = light wind. The wind blows nearly along the isobars, not across them.

North American surface analysis with isobars, high and low centres and station plots
Surface analysis: closely-packed isobars mean strong wind, widely spaced mean light; H and L mark the pressure centres. NOAA/WPC, public domain; WMO-standard, identical in Canada.

Buys Ballot's law: stand with your back to the wind and low pressure is on your left.

Prognostic (forecast) charts carry the same fronts and isobars projected forward in time:

12-hour surface prognostic chart
+12 h surface prognostic chart
24-hour surface prognostic chart
+24 h surface prognostic chart
36-hour surface prognostic chart
+36 h surface prognostic chart
48-hour surface prognostic chart
+48 h surface prognostic chart

NOAA/WPC surface prognostic charts, public domain. The frontal and isobar reading skill transfers to Canadian charts; the Canadian GFA is a separate, differently formatted product (see its lesson).

Trap: the two rotations. Northern-Hemisphere lows spin counterclockwise, highs clockwise. Both reverse in the Southern Hemisphere.

Source note: the weather theory here is grounded in the FAA Aviation Weather Handbook (FAA-H-8083-28), a free public-domain text; the physics is universal and matches the Canadian material (equivalent to TP 9352). Canada-specific products/rules stay cited to the TC AIM.

References

Practice