GEOG101: Physical Geography

Lecture 08: Wednesday, February 18, 2009

Topics
1. Wind Essentials
2. Driving Forces Within the Atmosphere
3. Atmospheric Patterns of Motion

Wind Essentials
• Winds spread the natural and anthropological pollutions worldwide
• Winds form because of 1) the gravitational force and 2) the pressure gradient
• Winds Change directions and strengths because of 1) the Coriolis force and 2) friction forces
• Winds as a driving mechanism for ocean currents

• Air Pressure and Its Measurement
      Mercury barometer
      Aneroid barometer
      Units: Millibar (mb) and Kilopascal (kPa): 1 kPa = 10 mb
      Units: mm, inches, referring to height of Hg.
      At sea level, the pressure is: 1013.2 mb (101.325 kPa; 760 mm; 29.92 in.) or ~1.0 kg/square cm (14.7 lb/square inch)

• Wind measurement: Speed (Anemometer) and direction (Wind Vane)
      Wind names: Winds are named for the direction from which they originate. Westerly winds means from west to east

Driving Forces Within the Atmosphere
• Motion: energy transport from equator to poles
• Pressure gradient: The earth surface is unequally heated. Cold, dense air at poles --> higher pressure; Warm, less dense air at equator --> lower pressure;
• Isobars: isolines connecting points of equal pressure
      Closer isobars denote higher pressure gradient
      Wider isobars indicate more gradual pressure gradient

• If winds flow simply because of the pressure gradient, they would travel along straight lines
• The Coriolis force causes apparent deflection:
      In northern hemisphere, apparent deflection to right
      In southern hemisphere, apparent deflection to left

• Geostrophic wind: wind blows in rigth angles to pressure gradient (disregarding friction)
      high pressure to the right, low pressure to the left looking downwind in northern hemisphere: pressure gradient force is balanced by Coriolis force
      Noethern hemisphere: anticyclone (clockwise) and Cyclone (anticlockwise)
      In southern hemisphere, the direction is in reverse. Why?

• Surface Winds - Friction
      Surface winds experience friction force
      Friction force depends on terrain
      Inflow (convergence) around a Low
      Outflow (divergence) around a High

Atmospheric Patterns of Motion
• Equatorial low-pressure trough (ITCZ-intertropical convergence zone)
• Polar high-pressure cells (around 90 degree N and 90 degree S, not shown)
• Subtropical high-pressure cells (20-35 degree N and 20-35 degree S)
• Subpolar low-pressure cells (around 60 degree N and 60 degree S)

• Jet stream
      Jet streams at collison zone between cold and warm air masses; the best known one is the polar jet stream (usually just called the 'jet stream') between polar air and tropical air masses typically found in our latitudinal region, further to the north in the summer, to the south in the winter; Other jet streams incldues the subtropic jet streams
• Rossby waves: Meanders in jet streams and similar winds are called Rossby waves, generated by cyclonic an anticyclonic wind cells 

• Hadley Cells
      Rising air in the heart of the tropics and sinking air in the subtropics
      Rising branch of the Hadley Cell: thick deep convective clouds similar to our summer thunderstorms
      Sinking branch of the Hadley Cells: deserts over land, and over ocean by sheets of low level cloud near the surface with clear air above

• Read Fig. 6.12, p. 155 (it's also in the slides) to understand the general atmospheric circulation pattern