1. Atmospheric wind systems

2. Hadley’s circulation model Heat supplied at low latitudes Temperatures steady at all latitudes → heat must be transported polewards at all latitudes Simplest wind model: Hadley (18th century): – equatorward winds at surface – poleward winds at altitude

3. Atmospheric cells Coriolis is important Three circulation cells are formed – Hadley cell: 0 o and 30 o lat – Ferrel cell: 30 o to 60 o degrees – Polar cell: N of 60 o degrees Trade winds (easterlies) – ~15 o N and 15 o S – Equatorward surface winds deflected to the west Westerlies – ~45 o N and 45 o S – Poleward surface winds deflected to the east

4. Subtropical high & polar front Subtropical high: – Intersection of the Hadley & Ferrel cells – ~30 o N and 30 o S – Sinking, dry air → great deserts, high ocean surface salinity – ‘Horse latitudes’ Polar front (Subpolar low) – Intersection of the Ferrel & Polar cells – ~60 o N and 60 o S

5. Intertropical convergence zone Near equator: Winds converge, air moves vertically between the S and N Hadley cells→ Intertropical convergence zone (ITCZ), doldrums Moist, rising air → heavy precipitation ITCZ varies seasonally Greatly affected by land/water T differences Distorted northwards over land in northern summer

6. Actual wind patterns Wind patterns complicated considerably by presence of continents and seasonal variations Seasonal variations much greater in NH due to greater land mass Monsoons – Seasonal prevailing wind that lasts several months – Variation in land/ocean heating – Africa and Asia monsoons associated with movement of ITCZ Storms – Disturbed state of the atmosphere – Wind speeds over 24.5 m/s – Thunderstorm, snowstorm, gale… – Mechanism for transporting large amounts of heat toward the poles

7. Flow around a low-pressure area in the NH. Blue: pressure-gradient force. Red: Coriolis acceleration (always perpendicular to the velocity). From wikipedia Cyclones Area of low atmospheric pressure → inward spiraling winds Spiral counterclockwise in the NH, clockwise in the SH 6 types – different characteristics, mechanisms & geographic locations – including tropical cyclones (tropical oceans, warm humid) and extratropical cyclones (mid-latitude, form at polar front) Polar low over the Barents Sea