Wed., Nov. 8 notes

Wednesday Nov. 8, 2006

Now we'll look briefly at surface winds. We must now include the frictional force.

In the figure at top left, we start at Point 1 by drawing in the pressure gradient force (perpendicular to the contour lines and pointing toward low pressure). Then we can draw in an equal and oppositely directed CF so that the net force will be zero. Since the CF is the right of the wind we can say this is a NH chart.

The frictional force will always point in a direction opposite the wind. Friction always try to slow moving objects (it doesn't cause you to speed up on your bicycle or to veer suddenly to the right or left). The strength of the frictional force depends on wind speed (stronger when the winds are fast and zero when the wind isn't blowing at all). Friction also depends on the type of surface the wind is blowing over.

The friction will slow the wind. That in turn weakens the CF (remember the strength of the CF depends on wind speed. The CF no longer balances the PGF, and the wind turns slightly and blows across the contours toward low pressure. On a chart like this with straight contours you end up with a new balance among the forces. Together the CF + F = PGF. The wind will blow in a straight line at constant speed across the contours toward low pressure.

This figure summarizes everything we have done. The upper level winds are shown in the figure at left. The upper level winds blow parallel to the contour lines.

At right surface winds around centers of high and low pressure are shown. You should remember from early in the semester that winds blow counterclockwise and inward around low pressure in the NH. They blow clockwise and outward around high pressure.

In the southern hemisphere the directions of spin change (clockwise around low and counterclockwise around high). Winds still blow converge into low pressure and diverge from centers of high pressure. This means that rising air (which expands and cools) and clouds will be found with centers of low pressure in both the northern and southern hemispheres.

This figure compares middle latitude storms in the NH and SH. In the NH cold air moves southward from higher latitudes on the west side of the low, warm air moves northward on the east side. The fronts spin counterclockwise around the low pressure center.

In the SH the cold air is found in the south and moves northward, again on the west side of the low. Warm air moves southward on the east side of the low. The fronts rotate clockwise around the low.

Differences in temperature such as might develop between a coast and the ocean or between a city and the surrounding country side can create horizontal pressure differences. The horizontal pressure gradient can then produce a wind flow pattern known as a thermal circulation. These are generally relatively small scale circulations and the pressure gradient is so much stronger than the Coriolis force that the Coriolis force can be ignored. We will learn how thermal circulations develop and then apply to concept to the earth as a whole in order to understand large global scale pressure and wind patterns. What follows is a slightly different version of what you will find on p. 131 in the photocopied class notes.

A beach will often become much warmer than the nearby ocean during the day (the sand gets hot enough that it is painful to walk across in barefeet). Pressure will decrease more slowly with increasing altitude in the warm low density air than in the cold higher density air above the ocean. Even when the sea level pressures are the same over the land and water (1000 mb above) an upper level pressure gradient can be created.

The upper level pressure gradient force will cause upper level winds to blow from H (910 mb) toward L (890 mb).

The movement of air above the ground can affect the surface pressures. As air above the ground begins to move from left to right, the surface pressure at left will decrease (from 1000 mb to 990 mb in the picture above). Adding air at right will increase the surface pressure there (from 1000 to 1010 mb). This creates a surface pressure gradient and surface winds begin to blow from right to left (the opposite of what is going on above the ground).

You can complete the picture by adding rising air above the surface low and sinking air above the surface high. Because the surface winds come from the ocean they are referred to as a sea breeze. These winds would probably be pretty moist so clouds would be likely over land above the surface low.

At some point during the night, the ocean often ends up warmer than the land. The thermal circulation reverses direction. The surface winds are then called a land breeze and clouds and rain form out over the ocean.