Thu., Nov. 9 notes

Thursday Nov. 9, 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 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 cold air moves northward, the cold air is found in the south, 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.

Here are some thermal circulation like examples

Cities will sometimes become warmer than the surrounding countryside, especially at night. 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.

In the winter, high pressure forms over the land, and dry winds blow from land out over the ocean.

This is an example of a monsoon wind system, a situation where the prevailing winds change directions with the seasons.

Now we'll apply the thermal circulation concept to the earth as a whole and learn about the "one-cell model" of the earth's global pressure and wind circulation pattern. You'll learn what the "one-cell" refers to shortly. A model is just a simplified depiction or representation of the earth's global scale circulation.

The incoming sunlight shines on the earth most directly at the equator. The equator will become hotter than the poles. By allowing the earth to rotate slowly we spread this warmth out along the entire length of the equator rather than concentrating it in a spot on the side of the earth facing the sun.

You can see the wind circulation pattern that would develop (really just the same situation as the second sample problem studied earlier). The term one cell just means there is one complete loop in the northern hemisphere and another in the southern hemisphere.

Next we will remove the assumption concerning the rotation of the earth. We won't be able to ignore the Coriolis force now.

Here's what a computer would predict you would now see on the earth. Things are pretty much the same at the equator in the three cell and one cell models: low pressure and rising air. At upper levels the winds begin to blow from the equator toward the poles. Once headed toward the poles the upper level winds are deflected by the Coriolis force. There end up being three closed loops in the northern and in the southern hemispheres. There are belts of low pressure at the equator (equatorial low) and at 60 degrees latitude (subpolar low). There are belts of high pressure (subtropical high) at 30 latitude and high pressure centers at the two poles (polar highs).

We will look at the surface features in a little more detail because some of what is predicted, even with the unrealistic assumptions, is actually found on the earth.

We'll first look at surface pressures and winds on the earth from 30 S to 30 N. Then we'll look at the region from 30 N to 60 N, where most of the US is located.

This is the first map. Let's start at 30 S. Winds will begin to blow from High pressure at 30 S toward Low pressure at the equator. Once the winds start to blow they will turn to the left because of the Coriolis force. Winds blow from 30 N toward the equator and turn to the right in the northern hemisphere (you need to turn the page upside down and look in the direction the winds are blowing). These are the Trade Winds. They converge at the equator and the air there rises (refer back to the crossectional view of the 3-cell model). This is the cause of the band of clouds that you can often see at or near the equator on a satellite photograph.

The Intertropical Convergence Zone or ITCZ is another name for the equatorial low pressure belt. This region is also referred to as the doldrums because it is a region where surface winds are often weak. Sailing ships would sometimes get stranded there hundreds of miles from land. Fortunately it is a cloudy and rainy region so the sailors wouldn't run out of drinking water.

Hurricanes form over warm ocean water in the subtropics between the equator and 30 latitude. Winds at these latitudes have a strong easterly component and hurricanes, at least early in their development, move from east to west. Middle latitude storms found between 30 and 60 latitude, where the prevailing westerly wind belt is found, move from west to east.

You find sinking air, clear skies, and weak surface winds associated with the subtropical high pressure belt. This is also known as the horse latitudes. Sailing ships could become stranded there also. Horses were apparently either thrown overboard (to conserve drinking water) or eaten if food supplies were running low. Note that sinking air is associated with the subtropical high pressure belt so this is a region on the earth where skies are clear (Tucson is located at 32 N latitude, so we are affected by the subtropical high pressure belt).

Here's the other map, it's a little simpler. Winds blowing north from H pressure at 30 N toward Low pressure at 60 N turn to the right and blow from the SW. These are called the "prevailing westerlies." In the southern hemisphere the prevailing westerlies blow from the northwest. The 30 S to 60 S latitude belt in the southern hemisphere is mostly ocean. The prevailing westerlies there can get strong, especially in the winter. They are sometimes referred to as the "roaring 40s" or the "ferocious 50s."

The subpolar low pressure belt is found at 60 latitude. Note this is also a convergence zone where the cold polar easterly winds and the warmer prevailing westerly winds meet. The boundary between these two different kinds of air is called the polar front and is often drawn as a stationary front on weather maps. A strong current of winds called the polar jet stream is found overhead. Middle latitude storms will often form along the polar front.

Despite the simplifying assumptions in the 3-cell model, some of the features that it predicts (particularly at the surface) are found in the real world. This is illustrated in the next figure.

The 3-cell model predicts subtropical belts of high pressure near 30 latitude. What we really find are large circular centers of high pressure. In the northern hemisphere the Bermuda high is found off the east coast of the US (feature 3 in the figure), the Pacific high (feature 4) is positioned off the west coast. Circular low pressure centers, the Icelandic (feature 2) and Aleutian low (feature 1), are found near 60 N. In the southern hemisphere you mostly just find ocean near 60 S latitude. In this part of the globe the assumption of the earth being of uniform composition is satisfied and a true subpolar low pressure belt as predicted by the 3-cell model is found near 60 S latitude.

The equatorial low or IRCZ is shown in green. Notice how it moves north and south of the equator at different times of the year.

The winds that blow around these large scale high and low pressure centers create the major ocean currents of the world. If you remember that high pressure is positioned off the east and west coast of the US, and that winds blow clockwise around high in the northern hemisphere, you can determine the directions of the ocean currents flowing off the east and west coasts of the US. The Gulf Stream is a warm current that flows from south to north along the east coast, the California current flows from north to south along the west coast and is a cold current. A cold current is also found along the west coast of South America (a disruption of this current often signals the beginning of an El Nino event); winds blow counterclockwise around high in the southern hemisphere. These currents are shown in the enlargement below.