16-1    Factors affecting wind:

1. Air pressure: 

   • Pressure = force/area. force = mass times g (gravitational acceleration). The pressure at sea level = 1kg/cm² = 1 atmospheric pressure = 1013 minibar = 760 mm Hg height.

   • As the elevation increases, pressure decreases.

   • Air pressure is measured by barometers.

   • Air flow from high P to low P.

   • Wind is horizontal air flow, caused by air pressure difference.

   • contour maps used to describe the pressure distribution quantitatively. The lines are called isobars.

   • The difference between lines indicates pressure change. The line spacing indicates the gradient.

   • Closer the space, higher the gradient ==> high wind. 

   • On a map, the flag indicates wind direction.

2. Coriolis effect:

   • Due to the rotation of the Earth.

   • The air, like water, also show the westward movement near the equator.

3. Friction with Earth's surface.

   • Most important in several km near the surface. Frictions lower the wind speed, reduces the Coroilis effect.

   • Results: the upper jet stream move parallel to isobar, while the surface wind cross the isobar with an angle. Fig. 16.5, 6.

16-2    Cyclone and anticyclone:

1. Near the surface:

2. Cyclone: low pressure center, wind blow inward, counterclockwise on Northern hemisphere, called convergence. Fig. 16.8.

3. Anticyclone: opposite to cyclone, wind blow outward, divergence.

4. Aloft: 

   • Above L is divergence and above H is convergence, resulting in a net pattern in Fig. 16.9.

5. At low pressure center: air uplifting, cloudy.

6. At high pressure center: air subsidence, clear sky.

16-3    Circulation of atmosphere:

1. Uneven heat on Earth's surface: near the equator more heat received, less gone; near pole, less received, more gone. Thus, warm air flow pole ward to compensate the heat loss. Cool air to flow equator ward to cool down.  

2. Net circulation without rotation:

   

    

3. With rotation (idealized) Fig. 6.12:

   

4. Influence of continents: In continents like Asia, during winter: high pressure, in summer: low pressure. this seasonal change in wind direction is called monsoons, resulting hot summer and cold winter.

16-4    Circulation in mid-latitude:

Jet stream in south in winter and north in summer, resulting cold winter and hot summer.

16-5    Local winds:

Due to local T and P changes..

1. Sea and land breeze.

   

2. Valley and mountain breeze. Fig. 16.15.

   • Day: warm air uplift ==> valley breeze.

   • Night: cool air subside ==> mountain breeze.

16-6    Measurement of wind:

• Use wind vane to measure the direction and cup anemometer to measure the speed. Speed high, spin fast.

Homework:

• Read chapter summary on p.459.

• Use your own word to explain the key terms on page 460.

• Answer the review questions on page 460.