Mon., Feb. 14 notes

Monday Feb. 14, 2011
click here to download today's notes in a more printer friendly format

What seemed like some nice Valentine's Day music from Pink Martini ( you heard "Let's Never Stop Falling in Love", and, once the projectors started working again, saw parts of "The Flying Squirrel". We didn't have nearly enough time for "Una Notte a Napoli", and "Amado Mio").

The first real take home Optional Assignment of the semester was collected today. I won't be able to get those graded and returned to you before the quiz on Wednesday, so here are some answers.

The first real quiz of the semester is this week. The quiz will cover material on both the Practice Quiz and Quiz 1 Study Guides. Reviews are scheduled for Monday (2-3 pm and 4-5 pm in SocSci 222) and Tuesday (4-5 pm in FCS 225) afternoons.

You'll find mention of an Optional Map Analysis Activity and an Optional Assignment on Upper Level Charts embedded in today's online notes.

We started class by listing some of the weather changes you might expect before and after passage of a warm front.

Here's a map view of a warm front

The front acts like a spoke on a wheel and spins counterclockwise around the low pressure center (axle). The arrows draw inside the warm and cold air masses show some of the possible wind directions.

Here's the crossectional view

Here ae the weather changes in advance of and following the frontal passage.

Weather Variable	Behind (after)	Passing	Ahead (before)
Temperature	warmer		cool
Dew point	may be moister		drier
Winds	from S or SW, maybe W		from S, SE, even the E
Clouds, Weather	clearing		wide variety of clouds that may precede arrival of the front by a day or two may be a wide variety of types of precipitation also (snow, sleet, freezing rain, and rain)
Pressure	rising	minimum	falling

And here is the surface map analysis:

We started by finding the lowest and highest pressure values actually plotted on the map (highlighted in yellow above). Then we choose from the list of allowed isobar values and which of them fall between the high and low pressure extremes on the map. In this case we needed to draw 992 and a 996 mb isobars. The isobars allow you to locate the center of low pressure. One end of the warm front will originate there.

Then we circled the warmest air on the map. Then on the basis of temperature change, shift in wind direction, big change in dew points and pressure change (pressure falling ahead and rising behind the front) we located the warm front at shown above. Note also the extensive cloud coverage ahead of the warm front.

There was also pretty clear evidence of a cold front on this map and one has been drawn in.

A new Optional Activity will give you a chance to draw in some isobars and locate a cold and warm front on your own. You'll be able to earn 1S1P points or extra credit points (click on one of the links to learn more). Your map analysis is due Wed., Feb. 23.

Before leaving this topic let's go back to the surface map example on page 39 in the ClassNotes. We were trying to figure out what was causing the clouds in the NE portion of the map and what was causing the rain shower along the Gulf Coast.

There is pretty clear indication of both a warm front and a cold front on this map and they have been drawn in above. The warm front is probably what is producing most of the widespread cloudiness and precipitation in the NE portion of the map (rising air motions caused by surface winds converging into the low pressure center are also contributing). The cold front is most likely producing the showers along the Gulf Coast.

We've been spending some time learning about surface weather maps. Maps showing conditions at various altitudes above the ground are also drawn. Upper level conditions can affect the development and movement of surface features (and vice versa)

We started with three basic things to know about upper level charts. This is the material you should concentrate on for this week's quiz.

First the overall appearance is somewhat different from a surface weather map. The pattern on a surface map can be complex and 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 mostly you find a relatively simple wavy pattern like sketched below. The busy looking figure drawn in class has been split into 3 figures for clarity.

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 upper level winds generally blow from west to east.

Next looked at some of the interactions between features on surface and upper level charts. This material is more difficult and confusing and there won't be questions about this material on the quiz. The figure below is found on p. 41 in the photocopied ClassNotes.

On the surface map you see centers of HIGH and LOW pressure. The surface 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 surface 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 form, develop, and strengthen (strengthening means the pressure in the surface low will get even lower than it is now. This is also called "deepening"). The reason for this is that the yellow X is a location where there is often upper level divergence. Similary the pink X is where you often find upper level convergence. This could cause the pressure in the center of the surface high pressure to get even higher.

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

This figure (see p. 42 in the photocopied Classnotes) shows a cylinder of air positioned above a surface low pressure center. The pressure at the bottom of the cylinder is determined by the weight of the air overhead. The surface winds are spinning counterclockwise and spiraling in toward the center of the surface low. These converging surface winds add air to the cylinder. 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 things clearer.

You'll find this figure on p. 42a in the Class Notes. We will assume the surface low has 960 mb pressure. Imagine that each of the surface wind arrows 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? Or in terms of the bank account, wouldn't you be surprised if, after making four $10 dollar deposits, your balance went from $960 to $950.

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 at some point above the ground than going in ). Upper level divergence removes air from the cylinder and would decrease the weight of the cylinder (and that would lower the surface pressure)

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.

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. The picture above wasn't shown in class today.

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.

A new Optional Assignment that covers some of this material on upper level charts is now available. You will need to read all three of the sections on Upper Level charts listed on the Lecture Notes Table of Contents but will be able to earn either extra credit points or a green card. The assignment is due Fri., Feb. 25.