This figure is found at the bottom of p. 40 c in the photocopied ClassNotes.  You should be able to sketch in the direction of the wind at each of the three points and determine where the fastest and slowest winds would be found. (you'll find the answers at the end of today's notes).

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3.
The pressure pattern determines the wind direction and wind speed.  Once the winds start to blow they can affect and change the temperature pattern.  The figure below shows the temperature pattern you would expect to see if the wind wasn't blowing at all or if the wind was blowing straight from west to east.  The bands of different temperature are aligned parallel to the lines of latitude.  Temperature changes from south to north but not from west to east.  The figures below are more cleaner and neater versions of what was drawn in class (the figures in class may also have had 60 N latitude instead of the 45 shown here - don't worry about that).

This isn't a very interesting picture.   It gets a little more interesting if you put centers of high or low pressure in the middle.

The clockwise spinning winds move warm air to the north on the western side of the High.  Cold air moves toward the south on the eastern side of the High.  The diverging winds also move the warm and cold air away from the center of the High.

Counterclockwise winds move cold air toward the south on the west side of the Low.  Warm air advances toward the north on the eastern side of the low.

The converging winds in the case of low pressure will move the air masses of different temperature in toward the center of low pressure and cause them to collide with each other.  The boundaries between these colliding air masses are called fronts.  Fronts are a second way of causing rising air motions (rising air expands and cools, if the air is moist clouds can form)

Cold air is moving from north toward the south on the western side of the low.  The leading edge of the advancing cold air mass is a cold front.  Cold fronts are drawn in blue on weather maps.  The small triangular symbols on the side of the front identify it as a cold front and show what direction it is moving.  The fronts are like spokes on a wheel.  The "spokes" will spin counterclockwise around the low pressure center (the axle).

A warm front (drawn in red with half circle symbols) is shown on the right hand side of the map at the advancing edge of warm air.  It is also rotating counterclockwise around the Low.

This type of storm system is referred to as an extratropical cyclone (extra tropical means outside the tropics, cyclone means winds spinning around low pressure) or a middle latitude storm.   Large storms also form in the tropics, they're called tropical cyclones or more commonly hurricanes.

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Clouds can form along fronts (often in a fairly narrow band along a cold front and over a larger area ahead of a warm front).  We need to look at the crossectional structure of warm and cold fronts to understand better why this is the case.

The top picture below shows a crossectional view of a cold front

The Volkswagens aren't heavy enough to lift the Cadillac.  They run up and over the Cadillac. 

Fronts are a second way of causing air to rise.  Rising air cools and if the warm air is moist and cooled enough, clouds and precipitation can form.  That's why the clouds were drawn in along the fronts in the middle latitude storm picture above.

Here's the surface map that we were working with last week.  Cold and warm fronts have been added.  The cold front is almost certainly what is responsible for the rain shower along the Gulf Coast.  The warm front is probably the main cause of the rain and drizzle in the NE portion of the map. 

We will come back to the topic of fronts again.  We will, in particular, learn about some of the weather changes that take place as a front approaches and passes through.  We will also look at how fronts can be located on surface weather maps.

We had just enough time to learn three basic things about upper level charts.  This material won't be on 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 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.

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 didn't cover this next section in class (we didn't have enough time and I don't like covering potentially confusing material like this right before a quiz even if it won't be appear on the quiz).  This section examines in more detail 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, the 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.