Lecture 5: General Circulation of the Atmosphere EarthsClimate_Web_Chapter.pdf, p. 15-22 Questions? 1. Why do winds blow around a high or a low pressure center? Why does a low area mean more precipitation? Why does a high mean a dry area?

The Global Energy Budget: Driver of Atmospheric Motion A balance exists between the incoming solar and outgoing longwave energy averaged over the globe and the year However, the tilt of the Earth means this balance is not maintained for each latitude SURPLUS DEFICIT

To compensate for this energy imbalance, winds in the atmosphere and currents in the oceans transport cold air and water toward the equator About 1/3 of this transport occurs from the evaporation of tropical waters and subsequent transport into high latitudes, where it condenses and releases latent heat About 1/3 occurs from the poleward transport of warm waters by ocean currents The remaining 1/3 occurs from middle latitude cyclones and anticyclones

Global Wind Patterns Westerlies Polar easterlies Hadley Cell

Atmospheric Pressure The pressure at the base of this column of air results from the weight of the gasses above. The earth's atmosphere has a greater density of gases at its base due to gravity.

Forces & Motion Air responds similarly as water to this force, moving from higher pressure to lower pressure. Pressure forces are only one influence on the movement of atmospheric air.

PGF vs. Cyclonic Winds Pressure gradient force (PGF) winds acting alone would head directly into low pressure. Surface observations of winds, such as the cyclonic flow around this low, reveal that PGF winds are deflected by other forces.

Playing catch on a merry-go-round Clockwise Blue, Yellow, Red, Green Straight-line, watch from above Curved to the left, watch by the green on board

Coriolis Force Counterclockwise Earth's rotation transforms straight line motion for an outside viewer in space into curved motion for an viewer on Earth. The Coriolis force explains this apparent curvature of winds to the right due to rotation. Its magnitude increases with wind velocity and earth's latitude.

Cyclones and Anticyclones Three forces: PGF Coriolis Frictional

Vertical Air Motion Winds converging into a low pressure center generate upward winds that remove the accumulating air molecules. These updrafts may cause cloud formation. Likewise, diverging air molecules from a high pressure area are replenished by downward winds.

Northern Hemisphere Flow Winds blow counterclockwise around low pressure systems in the Northern Hemisphere, but clockwise around lows in the Southern Hemisphere. Figure 9.27A

Coriolis Force and Nature’s Greatest Storms

Hurricane Movement Formation Low-pressure areas (easterly waves) Warm, moisture, unstable area Ocean surf. temp. > 26–27°C Low wind shear Sufficient Coriolis force (5-10 degrees away from the equator) Strong upper-level anticyclone. Their movement is guided by winds around subtropical high pressure systems.

R. Anthes http://www. ucar

R. Anthes http://www. ucar

R. Anthes http://www. ucar

R. Anthes http://www. ucar

R. Anthes http://www. ucar