Tue., Apr. 29 notes

Tuesday Apr. 29, 2008

The 1S1P Topic #7 (Why the Wind Blows) reports have been graded.

Quiz reviews this afternoon and tomorrow afternoon. See the Quiz #4 Study Guide for time and locations.

I had an interesting time last Saturday evening. Click here for more details.

We're trying to understand why winds blow the way we do. It should be possible to do this in 10 fairly easy to understand steps. The first three steps were covered in class last Thursday. Here they are:

The winds above are what we will be trying to understand.

Upper level winds blow parallel to the contours. You should by the end of the class today be able to pick one of the 4 upper level winds and be able to show the directions of the PGF and the CF. Here's an example

Frictional force (which is absent at upper levels) causes surface winds to blow across the contours toward low pressure. Here's an example of what you should eventually be able to say about anyone of the 4 surface winds examples above.

The main point to take from Step #2 is that a net inward force is needed anytime an object is moving in a circular path.

The pressure gradient force always points toward low pressure. The PGF can cause stationary air to begin to move (move toward low pressure).

The Coriolis force points perpendicular to the wind. It can only change the wind's direction, it can't cause the wind to speed up or slow down. The direction of the CF depends on whether you're in the northern or southern hemisphere, there's no CF at the equator.

Now we start to put everything together. The PGF at Point 1 starts stationary air moving toward the center of low pressure. Once the air starts to move, the CF causes it to turn to the right (because this is a northern hemisphere chart). The wind eventually ends up blowing parallel to the contour lines and spin in a counterclockwise direction.

The situtation is similar in the southern hemisphere. The CF causes the wind to take a left turn, however. The wind ends up blowing in a clockwise direction around the low pressure center.

With high pressure the air starts moving outward. In this example the wind takes a right turn and ends up blowing in a clockwise direction around the high. Note there is a net inward force here just as there was with the two previous examples involving low pressure.

Winds spin counterclockwise around high pressure in the southern hemisphere.

Friction acts in a direction opposite the wind. Friction can only slow down the wind. Slowing the wind weakens the CF and it can no longer balance the PGF. The wind ends up blowing across the contours, always toward low pressure. Together the frictional force and the Coriolis force balance the PGF.

The winds are spiralling inward in the top and bottom examples. These must be surface centers of low pressure. The middle two examples are high pressure. Converging winds cause air to rise. Diverging winds created sinking wind motions.

Next we will cover the last section on the Quiz #4 Study Guide: 3-cell model features. The features referred to are large scale (global scale) pressure belts and surface winds found on the earth. Don't worry about what the term "3-cell" is referring to.

This figure tries to explain the origin of the surface high and low pressure belts that we will be looking at.

First incoming sunlight strikes the equator perpendicularly. It strikes the poles at a steep angle. The equator and the air above the equator becomes warmer than at higher latitudes.

1.   Pressure will decrease with increasing altitude above the equator. The rate of pressure decrease will be relatively slow in the warm low density air. As a result you end up with high pressure at upper levels (not higher than at the surface but higher than the pressure you would find at the same altitude to the north or south).

2.   Upper level winds will be begin to blow away from the equator and toward he north and south.

3.   As soon as air begins to blow north and south from Point 1(air is being removed from the atmosphere above the equator), the surface air pressure at Point 3 will decrease.

Note this is exactly the same thing that happens with huricanes. High pressure and diverging winds at the top center of the hurricane lower the surface pressure at the bottom center of the hurricane. Surface winds converge and the storm intensifies.

4. Air moving north and south from Point 1 won't travel all the way to the poles. As soon as the air starts to head north or south and away from the equator it will be bent by the Coriolis force. By the time it gets to 30 degrees latitude, the wind is blowing parallel to the lines of latitude. As upper level winds add air to the atmosphere above 30 degrees latitude the surface pressure will start to increase.

5. Suface winds begin to blow from high pressure near 30 degrees toward lower pressure at the equator.

Here are the surface features.

You should be able to start with a blank sheet of paper and reproduce this figure. Really.

Start by drawing in the low pressure belt at the equator. This is called the Equatorial low.

Then draw in belts of high pressure at 30 N and 30 S. These are the subtropical highs.

Draw in surface winds blowing from high pressure toward low pressure. Once the winds start to blow, the Coriolis force will cause them to bend (to the right in the northern hemisphere, to the left in the southern hemisphere, be sure to look in the direction the wind is blowing).

You will find easterly winds in the subtropics. These are the Trade winds. They converge at the Equator producing the intertropical convergence zone (ITCZ). This is a cloudy region on the earth (the converging winds cause air to rise). Sailing ships would sometimes lose their wind in the doldrums and be stuck out in the open ocean without enough food (they could collect rain water to drink).

Westerly winds, the prevailing westerlies are found north of 30 N and south of 30 S.

The "horse latitudes" centered at 30 N is another region of calm surface winds. Sailing ships would lose their winds and become stranded. This time there wasn't a ready supply of fresh water. The term horse latitudes refers to a situation where the horses were either eaten or thrown overboard rather than being allowed to drink fresh water.

Hurricanes form in the subtropics and move from east to west. Middle latitude storms form at higher latitude and move from east to west .

We spent the last few minutes of the class discussing a common misconception - that water drains from sinks and toilets spinning in one direction in the northern hemisphere and in the other direction in the southern hemisphere.