Module 13 - Meteorology
Pressure Systems
High pressure (anticyclone) brings generally settled, fair weather with light winds. Wind circulates clockwise around a high in the northern hemisphere. Low pressure (depression or cyclone) brings unsettled, wet, windy weather. Wind circulates anticlockwise around a low in the northern hemisphere.
Isobars are lines of equal pressure on a synoptic chart. Closely spaced isobars mean strong winds; widely spaced isobars mean light winds. The closer the isobars, the steeper the pressure gradient, and the stronger the wind.
Surface wind does not flow exactly along isobars — friction with the sea surface causes it to cross isobars slightly toward low pressure. Over the sea the wind typically crosses isobars at about 10–15°; over land the angle is greater (up to 30°).
Key points
- High pressure = fair weather, light winds, clockwise circulation (N hemisphere)
- Low pressure = unsettled, windy, anticlockwise circulation (N hemisphere)
- Close isobars = strong winds
- Wide isobars = light winds
- Pressure measured in millibars (mb) or hectopascals (hPa) — same unit
- Surface wind crosses isobars slightly toward low pressure (~10–15° over sea)
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