Wed., Feb. 13 notes

Wednesday Feb. 13, 2008

A busy start to class today: students were turning in 1S1P reports and checking out the materials for Experiment #2.

Quiz #1 is one week from today. Quiz #1 will cover material on both the Practice Quiz Study Guide and the new Quiz #1 Study Guide.

What looks to be a fairly strong, cold, and fast moving storm is approaching. You may get a chance to experience passage of a cold front. If so here is some of what you can look for.

The front is approaching from the west in the picture above. At this time we are located in the warm air mass. This air may also be moist, though that is often not the case in Arizona. Winds generally blow from the southwest.

If the warm air is moist, then arrival of the front will cause the warm air to rise. Rising air expands and cools, so a band of clouds will often be seen along a cold front. Winds can get gusty as the front passes through.

The low pressure center has moved toward the NE. The cold front has rotated around the low pressure center. The front has passed through and we are now in the cold air behind the cold front. The winds are now blowing from the NW. The air behind the front is often much drier than the air ahead of the front. The coldest air may still be quite a ways behind the front and might not arrive for a day or two. Skies often clear quickly following passage of a cold front and it can get much colder at night in the clear dry air behind the front.

A short time lapse video of a cold front passing through Tucson on Easter Sunday, 1999, was shown in class.

We spent the rest of the period learning about upper level charts. These maps show conditions at various altitudes above the ground. Conditions up there are important because they can strongly influence conditions at the ground.

We started with three basic things to know about upper level charts. First the overall appearance is somewhat different from a surface weather map. On a surface map you generally find circular (more or less) centers of high and low pressure. You can also find closed high and low pressure centers at upper levels, but more generally you find a wavy pattern like sketched below (the figures below are from a previous class and were used because they are a little clearer than the figures shown in class).

The u-shaped portion of the pattern is called a trough. The n-shaped portion is called a ridge.

Troughs are produced by large volumes of cool or cold air (the cold air is found between the ground and the upper level that the map depicts). The western half of the country in the map above would probably be experiencing colder than average temperatures. Large volumes of warm or hot air produce ridges.

The winds on upper level charts blow parallel to the contour lines. On a surface map the winds cross the isobars slightly, spiralling into centers of low pressure and outward away from centers of high pressure. The winds generally blow from west to east.

Next we will look at some of the interactions between features on surface and upper level charts

On the surface map you see centers of HIGH and LOW pressure. The low pressure center, together with the cold and warm fronts, is a middle latitude storm.

Note how the counterclockwise winds spinning around the LOW move warm air northward (behind the warm front on the eastern side of the LOW) and cold air southward (behind the cold front on the western side of the LOW). Clockwise winds spinning around the HIGH also move warm and cold air. The winds are shown with thin brown arrows on the surface map.

Note the ridge and trough features on the upper level chart. We learned that warm air is found below an upper level ridge. Now you can begin to see where this warm air comes from. Warm air is found west of the HIGH and to the east of the LOW. This is where the two ridges on the upper level chart are also found. You expect to find cold air below an upper level trough. This cold air is being moved into the middle of the US by the northerly winds that are found between the HIGH and the LOW.

Note the yellow X marked on the upper level chart directly above the surface LOW. This is a good location for a surface LOW to develop and strengthen (to strengthen the surface low pressure in the center of the storm must get even lower than it is now) We will find that this is frequently a location where there is upper level divergence. Similary the pink X is where you often find upper level convergence. This could cause surface high pressure to get even higher.

Now we need to look in a little more detail at how upper level winds can affect the development or intensification of a surface storm.

The figure shows a cylinder of air (see p. 42 in the photocopied Classnotes). The pressure at the in the center at the bottom of the cylinder is determined by the weight of the air overhead. The surface winds are spinning counterclockwise and spiralling in toward the center of the surface low. This adds air to the cylinder of air. Adding air to the cylinder means the cylinder will weigh more and you would expect the surface pressure at the bottom of the cylinder to increase.

We'll just make up some numbers, this might make this clearer.

You'll find this figure on p. 42a in the Class Notes. At the top we will assume the surface low has 960 mb pressure. Imagine that each of the surface wind arrows winds brings in enough air to increase the pressure at the center of the LOW by 10 mb. You would expect the pressure at the center of the LOW to increase from 960 mb to 1000 mb.

This is just like a bank account. You have $960 in the bank and you make four $10 dollar deposits. You would expect your bank account balance to increase from $960 to $1000.

But what if the surface pressure decreased from 960 mb to 950 mb as shown in the following figure?

The next figure shows us what could be happening (back to p. 42 in the Class Notes).

There may be some upper level divergence (more arrows leaving the cylinder than going in ). Upper level divergence removes air from the cylinder and would decrease the weight of the cylinder.

We need to determine which of the two (converging winds at the surface or divergence at upper levels) is dominant. That will determine what happens to the surface pressure.

Again some actual numbers might help (see p. 42b in the Class Notes)

The 40 millibars worth of surface convergence is shown at Point 1. Up at Point 2 there are 50 mb of air entering the cylinder but 100 mb leaving. That is a net loss of 50 mb. At Point 3 we see the overall result, a net loss of 10 mb. The surface pressure should decrease from 960 mb to 950 mb. That change is reflected in the next picture (found at the bottom of p. 42b in the Class Notes).

The surface pressure is 950 mb. This means there is more of a pressure difference between the low pressure in the center of the storm and the pressure surrounding the storm. The surface storm has intensified and the surface winds will blow faster and carry more air into the cylinder (the surface wind arrows each now carry 12.5 mb of air instead of 10 mb). The converging surface winds add 50 mb of air to the cylinder (Point 1), the upper level divergence removes 50 mb of air from the cylinder (Point 2). Convergence and divergence are in balance (Point 3). The storm won't intensify any further.

Click here for some additional examples. By working through some additional examples you might increase your understanding of this material and build up your confidence (of course there's always a chance that more examples will just make this topic more confusing - the choice is yours)

We're almost done, one last figure (it's the figure on p. 41 in the photocopied Class Notes again with some new information added (redrawn here for improved clarity). This figure wasn't shown in class.

Now that you have some idea of what upper level divergence looks like (more air leaving than is going in) you are in a position to understand another one of the relationships between the surface and upper level winds.

One of the things we have learned about surface LOW pressure is that the converging surface winds create rising air motions. The figure above gives you an idea of what can happen to this rising air (it has to go somewhere). Note the upper level divergence in the figure: two arrows of air coming into the point "DIV" and three arrows of air leaving (more air going out than coming in is what makes this divergence). The rising air can, in effect, supply the extra arrow's worth of air.

Three arrows of air come into the point marked "CONV" on the upper level chart and two leave (more air coming in than going out). What happens to the extra arrow? It sinks, it is the source of the sinking air found above surface high pressure.