Atmospheric Circulation and Pressure Scale of Winds Scale

Atmospheric Circulation and Pressure

Scale of Winds Scale Time Distance Planetary weeks global -years Example Westerlies Synoptic days 100’s -weeks -1000’s km Cyclones Mesocale minutes -days sea breeze thunderstorm 1 -100 km Microscale seconds <1 km -minutes wind gust

• General circulation - term that refers to planetary scale winds • Westerly winds - winds blowing west to east. • Easterly winds - winds blowing east to west.

Single Cell Model George Hadley (1738)

Single Cell Model • George Hadley (1685 -1768) • 1738 - proposed a simple model to explain global circulation – Wanted to explain why sailors often easterly winds in the lower latitudes • Single Cell Model – Air is heated at the equator – expands upward and diverges toward the poles – Sinks back down at the poles – Returns toward the equator

Three Cell Model William Ferrel (1865) Polar Cell Ferrel Cell Hadley Cell

Image after Lutgens & Tarbuck 10 th Ed. Prentice Hall

Three Cell Model • William Ferrel (1817 -1891) • 1865 - developed the three cell model 1) Hadley Cell - circulates air between the tropics and subtropics (0 o-30 o) 2) Ferrel Cell - circulates air between the subtropics and subpolar regions (mid-latitudes 30 o to 60 o) 3) Polar Cell - circulates air between the subpolar and polar regions. (60 o-90 o)

Hadley Cell • Equatorial low - zone of low pressure along the equator – – – • Subtropical high - zone of high pressure at ~30 o latitude. – • Also called intertropical convergence zone (ITCZ) Rainiest zone in the world. Also called doldrums Horse latitudes Northeast tradewinds - develop from the PGF between the equatorial low and the subtropical high.

Ferrel Cell - circulates air between the subtropics and subpolar regions (mid-latitudes 30 o to 60 o) • Subpolar low - zone of low pressure at ~60 o latitude. • Westerlies - develop from the PGF between the subtropical high and the subpolar low and the effects of the coriolis force. – mid-latitudes

Polar Cell - circulates air between the subpolar and polar regions. (60 o-90 o) • Polar high - zone of high pressure at the poles. • Polar easterlies - result from the PGF that develops between the subpolar low and polar high. • Polar front - transition zone between the cold polar air and the warmer mid-latitudes. • Jet stream - streams of air within the westerlies that travel at speeds of 200 km/hr. in the winter and 100 km/hr. in the summer.

Polar Cell • Polar Jet stream - Jet stream that develops above the polar front, resulting from the strong pressure gradient. Subtropical Jet Stream - a jet stream that develops just north of the equator.

Polar Cell Polar Front Theory for mid-latitude cyclones

Cyclones and Anticyclones • Sea level pressure across the globe are divided into high and low pressure centers.

Upper Troposphere Winds Jet Streams - streams of fast moving air in the upper tropospere. Ridges and Troughs Ridges - upward bulges in the pressure surface. Troughs - depressions in the pressure surface. Wave-like flow Speed divergence Speed convergence

Ocean Currents - horizontal movement of the surface waters found along the rims of the major ocean basins - winds are the primary driving force of the currents - Coriolis force also affects the currents Equatorial Current - “backbone” of the ocean current system - derive energy from the tradewinds.