Wed., Apr. 11 notes

Wednesday Apr. 11, 2007

Today was the 1st of the 1S1P Assignment #3 due dates (you could turn in one or two reports today). The 2nd due date is next Wed., Apr. 18. You can only turn in one report on the 2nd due date. You can't do more than two reports as part of Assignment #3.

One of the reasons for studying the Coriolis force was the question about water draining from sinks in the southern hemisphere. The suggestion is that because the winds that spin around centers of high and low pressure change direction in the northern and southern hemisphere, the direction of the spinning motion of water draining from a sink or toilet bowl will also change direction. This is not the case. Draining water can spin in either direction in either hemisphere and at the equator where the Coriolis force is zero. This is discussed on p. 130 in the photocopied Class Notes.

Spinning winds do change directions around large high and low pressure centers. The PGF starts the air moving (in toward low, out and away from high pressure) then the Coriolis force bends the wind to the right (N. hemisphere) or to the left (S. hemisphere).

There are situations where the PGF is much stronger than the CF and the CF can be ignored.

In this case spinning motion is only possible around low pressure.

Water can spin in either direction and drain from a bucket in the N. hemisphere, at the Equator itself, and in the S. hemisphere (despite what the short demonstration on the video shown in class seemed to show).

What provides the necessary inwardly directed force needed to keep water spinning and draining from a bucket or sink (here we go "beating a topic to death" again)? The answer is shown in the figure below.

If you look closely at water spinning in a bucket or sink you will notice that the surface of the water has a bowl shape. The water piles up and is deeper along the sides of the bucket than it is in the center. An inwardly directed pressure gradient force is created. The deeper water near the sides of the bucket produces a little higher pressure inside the water than the shallower water near the center of the bucket. This radial difference in pressure is what keeps the box of water spinning in a circular path.

A quick reminder that only the PGF and the CF are important when considering upper level winds. Upper levels winds blow parallel to the contours.

In the figure above, we drew in the pressure gradient force arrow (perpendicular to the contour lines and pointing toward low pressure). Then we can drew in an equal and oppositely directed CF so that the net force would be zero. We then saw that the CF was to the right of the wind and we could then say this was a N. hemisphere chart.

When air blows across the surface of the earth you must include the frictional force.

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 (there is less friction when winds blow over the ocean than when blowing over land).

This figure summarizes most 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.

Note the locations and directions of motion of the southern hemisphere warm and cold fronts. The winds spin clockwise around low pressure in the southern hemisphere. The coldest air is found in the south in the southern hemisphere.

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. You'll find the following discussion 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.

The surface winds blow from high to low. The surface winds and upper level winds are blowing in opposite directions.

You can complete the circulation loop by adding rising air above the surface low pressure at left and sinking air above the surface high at right. The surface winds which blow from the ocean onto land are called a sea breeze (the name tells you where the winds come from). Since this air is likely to be moist, cloud formation is likely when the air rises over the warm ground.

At night the ground cools more quickly than the ocean and becomes colder than the water. The thermal circulation pattern reverses direction. Surface winds blow from the land out over the ocean. This is referred to as a land breeze.

You can use the thermal circulation concept to understand these circulation patterns. The following examples weren't shown in class on Wednesday.

Cities are often warmer than the surrounding countryside, especially at night. This is referred to as the urban heat island effect. This difference in temperature can create a "country breeze."

In the summer India and SE Asia become warmer than the oceans nearby. Surface low pressure forms over the land, moist winds blow from the ocean onshore, and very large amounts of rain can follow. This Asian monsoon (monsoon refers to a seasonal change in the direction of the prevailing winds) is a large scale circulation pattern and is more than a thermal circulation. However you can use the thermal circulation concept to get a basic understanding of what is going on.

The winds change directions in the winter when the land becomes colder than the ocean.