The principal pressure areas
Global atmospheric circulation showing the principal high and low-pressure belts.
Types of Anticyclones (High Pressure)
There are five main types: warm, cold, temporary cold, ridges, and blocking.
Cold Anticyclones
Form over continents in winter and polar areas.
- Characteristics: Strong temperature inversions, clear air, sharp frosts, and very cold surface.
- Variation with Height: They weaken with increasing altitude.
- At surface: Isobars curve upwards (high pressure).
- Aloft: Due to low temperature, pressure decreases rapidly, so at high levels they may become low-pressure areas (isobars curved downwards).
Warm Anticyclones
Associated with warm air and subsidence aloft (e.g., Azores High).
- Characteristics: Stable weather, often warm. Usually quasi-stationary or permanent.
- Variation with Height: They intensify with altitude.
- Isobaric surfaces curve upwards at all levels (both at surface and aloft).
Pressure Systems in the Northern Hemisphere
| System | Seasonality | Location / Effect |
|---|---|---|
| Azores High | Year-round | Subtropical High. 30ºN (Winter) - 35ºN (Summer). Affects Europe and N. Africa. |
| Icelandic Low | Year-round | Deep low pressure over Iceland/Greenland. Stronger in winter. |
| Siberian High | Winter Only | Very cold high pressure over Asia and Eastern Europe. |
| Siberian Low | Summer Only | Weak thermal low pressure over Asia. |
| Mediterranean | Variable | Frontal depressions in Winter. Anticyclonic and hot in Summer. |
Weather Map Terminology
- Ridge: An elongated extension of a high-pressure system (inverted U-shape). Good weather.
- Trough: An elongated extension of a low-pressure system (V-shape). Bad weather, convergence.
- Col: A neutral zone between two highs and two lows. Calm and variable winds.
Identification on Maps (Southern Hemisphere)
In the Southern Hemisphere:
- Low Pressure (Cyclone): Winds rotate clockwise.
- High Pressure (Anticyclone): Winds rotate counter-clockwise.
- Fronts: Cold and warm fronts are associated with depressions.
General Rules of Pressure and Height
- High Pressure: Isobaric surfaces bulge upwards.
- Low Pressure: Isobaric surfaces bulge downwards.
- Warm Anticyclone: High pressure at surface and aloft.
- Cold Depression: Low pressure at surface and aloft.