Mon., Apr. 28 notes

Monday Apr. 28, 2008

The 1S1P Topic #7 (Why the Wind Blows) reports have been graded. So have the In class Optional Assignments from last Friday.
Answers will appear online sometime Tuesday.

Quiz reviews this afternoon and tomorrow afternoon. See the Quiz #4 Study Guide for time and locations. There has been a small change to the Study Guide. A very small amount of material was removed.

I had an interesting time at the Stevie Eller Dance Theater last Saturday night. Click here for more information.

The figure above shows that if you just remember that the PGF points toward low pressure and that the CF is the right and left of the wind in the northern and southern hemispheres, you could easily figure out which hemisphere each of the examples above belongs in.

This question on the optional assignment caused a lot of people some problems. The net force is inward in all three cases.

Today 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 (note the middle latitude storm in the southern hemisphere is misplaced. It should be south of 30 S)

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.