Skip to main content

Section 9.1 Global wind patterns

Subsection 9.1.1 Circulation scales

  • microscale (<1 km in size): PGF, CEN, friction (cumulus clouds, parcels)

  • mesoscale (1-1000 km): PGF, CEN, friction, Cor (thunderstorms, fronts)

  • synoptic scale (>1000 km): geostrophic balance (mid-latitude cyclones)

  • planetary scale (10,000 km): geostrophic balance (atmospheric circulation cells; prevail over longer timescales)

Subsection 9.1.2 Circulation models

  • model: simplified description of a complex system

  • used to explain and predict observations using cause-and-effect relationships

Any circulation model for the Earth should be able to explain:

  • steady and calm winds observed by mariners

  • Regions that lack winds

  • Global patterns of cloudiness

  • Midlatitude cloud patterns

  • Global patterns of precipitation

  • Jet streams

Global Circulation on Non-Rotating Earth

  • single cell circulation (Hadley model)

  • contrast in temperatures between the poles and the equator creates a large convection cell in both hemispheres.

Global Circulation on Rotating Earth

  • three-cell circulation: Hadley, Ferrel, and Polar cells

  • Coriolis effect deflects flow, poleward flows cool and sink

  • Not all cells are meridional (N-S motion), there are also zonal cells (E-W motion)

    • For example, Walker circulation is the zonal component of a Hadley cell

    • this behavior is observed in El Nino Southern Oscillation (ENSO)