General global circulation

050-02-03Pablo Asensio Martínez2026-04-012 min

Global atmospheric circulation is the large-scale movement of air that distributes heat across the Earth's surface.

Global Circulation

Three-Cell Model

To explain the general circulation, a three-cell model is used in each hemisphere:

1. Hadley Cell (Equator - Subtropics)

  • At the Equator: Warm air rises (Equatorial Low Pressure), creating storms and the Intertropical Convergence Zone (ITCZ).
  • Aloft: Air flows towards the poles.
  • In the Subtropics (~30° N/S): Air descends (subsidence), creating permanent Subtropical High Pressure zones (e.g., the Azores High). This descent generates dry and clear weather (desert zone).
  • At the Surface: Air returns to the Equator as the Trade Winds.

2. Ferrel Cell (Subtropics - Temperate Latitudes)

  • Located between 30° and 60° latitude.
  • It is a cell "induced" by the movement of the other two.
  • Surface Winds: Flow from the subtropics towards the poles, deflected by Coriolis to the east, forming the Westerlies.

3. Polar Cell (Temperate Latitudes - Poles)

  • At the Poles: Cold, dense air descends, creating Polar High Pressure.
  • At the Surface: Air flows towards the equator (Polar Easterlies).
  • At ~60° Latitude: Polar air collides with warmer air from the Ferrel cell, rising and creating Subpolar Low Pressure zones (Polar Front).

Pressure Systems and Isobaric Surfaces

Interpretation of Isobaric Surfaces

Constant pressure surfaces (e.g., 500 hPa) are not flat:

  • "Bulging Up": Indicate High Pressure (or warm air). The pressure surface is at a higher altitude.
  • "Bulging Down": Indicate Low Pressure (or cold air). The pressure surface is at a lower altitude.

Wind Speed Comparison (Gradient)

For the same isobar spacing (same pressure gradient):

  • Around a HIGH: The wind is faster (Super-geostrophic). Centrifugal force adds to the gradient force.
  • Around a LOW: The wind is slower (Sub-geostrophic). Centrifugal force opposes the gradient force.

Buys Ballot's Law (Flight Application)

Ley de Buys Ballot

  • Tailwind (Northern Hemisphere): If you fly with a tailwind, the Low Pressure is on your left.
  • Headwind (Northern Hemisphere): If you fly with a headwind, the Low Pressure is on your right.
  • Crossing a system:
    • If you pass from a Low to a High (in the NH), the wind will Veer (turn clockwise).