In addition to surface weather maps, maps showing conditions at various altitudes above the ground are also routinely made.  We'll spend some time learning about these upper level charts.  Upper level conditions can affect the development and movement of surface features (and vice versa).

We'll start with some basic features then have a more careful and detailed look at upper level charts.   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.

Now on to a little more in depth look at upper-level charts. 

By the end of this section you should better understand what the title "850 mb Chart" on the upper level map above refers to. 

You should also understand what the numbers on the contour lines represent and what their units are.  On a surface map contours of pressure, isobars, are normally drawn.  That is usually not the case on upper level charts.  You'll also have a better idea of where the names trough and ridge come from and why they are associated with cold and warm air masses, respectively.

Note that the values on the contours decrease as you move from the equator toward higher latitude.   You should be able to explain why that happens. 

You really only need to remember two things from earlier in the course:  (1) pressure decreases with increasing altitude, and (2) pressure decreases more rapidly in cold high-density air than it does in warm low density air. 

Pressure drops from 1000 mb to 800 mb, a 200 mb change, when moving upward 1500 meters in the cold air in the picture above.  It decreases from 1000 mb to 900 mb, only 100 mb,  in the same distance in the warm low density air.

Isobars on constant altitude upper level charts
One way of depicting upper level conditions would be to measure pressure values at some fixed altitude above the ground.

his approach is shown above.  Pressures range from 800 mb to 900 mb at 1500 meters altitude. The pressure pattern could then be plotted on a constant altitude chart using isobars (figure below).  Note the lowest pressures are found in the cold air, higher pressures would be found in the warm air.

That would seem to be a logical way of mapping upper level atmospheric conditions.  Unfortunately that isn't how things are done.

Height contours on constant pressure (isobaric) upper level charts

Just to make life difficult meterologists do things differently.  Rather than plotting conditions at a constant altitude above the ground, meterologists measure and plot conditions at a particular reference pressure level above the ground. 

In the picture above you start at the ground (where the pressure is 1000 mb) and travel upward until you reach 850 mb pressure.  You make a note of the altitude at which that occurs.  In the cold dense air at the left pressure decreases rapidly so you wouldn't need to go very high, only 1200 meters.  In the warm air at right pressure decreases more slowly, you would have to go quite a bit higher, to 1800 m.

Every point on the sloping surface above has the same pressure, 850 mb.  The altitude above the ground is what is changing.  You could draw a topographic map of the sloping constant pressure surface by drawing contour lines of altitude or height.

The L and H on this map represent low and high altitude, respectively.

The two kinds of charts (constant altitude or constant pressure) are redrawn below.

The numbers on the contour lines have been left off in order to clearly see that both types of maps have the same overall pattern (they should because they're both depicting the same upper level atmospheric conditions).

In the example above temperature changed smoothly from cold to warm as you move from left to right (west to east).
See if you can figure out what temperature pattern is producing the wavy 850 mb constant pressure surface below.

This shouldn't be too hard if you remember that the 850 mb level will be found at relatively high altitude in the warm air where pressure decreases slowly with increasing altitude.  The 850 mb level will be found closer to the ground in cold air where pressure decreases rapidly with increasing altitude.  The temperature pattern is shown below.

Temperatures change from average, to warm, back to average, to cold, and then to average again at the eastern edge of the picture.

If you imagine hiking along the 850 mb surface you can begin to understand where the term ridge comes from.  In a ridge the reference pressure is found at higher than average altitude above the ground.  A trough is in effect a valley where the reference pressure is found at lower altitude, closer to the ground.

In the next figure we will add south to north temperature changes in addition to the west to east temperature gradient.

Here's what the temperature pattern will look like.

Temperature drops as you move from west to east (as it did in the previous pictures) and now it drops as you move from south to north.  What will the wavy 850 mb constant pressure surface look like now?

It's the wavy surface that we had in the previous example (where there was just a west to east temperature change) with the northern edge tilted downward because there is colder air in the north.   That's not much of a change.  But look at how the map has changed.  We now see an "n" shaped ridge and a "u" shaped trough.

The highest point on the 850 mb surface (1800 meters or so) is found above the hot air near the SW corner of the picture.  The lowest point (a little less than 1000 meters) is found in the coldest air near the NE corner of the picture.

Now let's go back to the figure that we started this section with.

1. The title tells you this is a map showing the altitude of the 850 mb constant pressure level in the atmosphere.

2.  Height contours are drawn on the chart.  They show the altitude, in meters, of the 850 mb pressure level at different points on the map.

3.  The numbers get smaller as you head north because the air up north is colder.  The 850 mb level is closer to the ground in the north where the air is colder, denser, and where pressure decreases more rapidly with increasing altitude.

Here's a figure with some questions to test your understanding of this material.

This is a 500 mb constant pressure chart not an 850 mb chart like in the previous examples.  The 500 mb pressure is found higher in the atmosphere than the 850 mb level.

Is the pressure at Point C greater than, less than, or equal to the pressure at Point D (you can assume that Points C and D are at the same latitude)?  How do the pressures at Points A and C compare?

Which of the four points (A, B, C, or D) is found at the lowest altitude above the ground, or are all four points found at the same altitude?

The coldest air would probably be found below which of the four points?  Where would the warmest air be found?

What direction would the winds be blowing at Point C?

You'll find the answers to these questions at the end of this lecture.

Here is a quick comparison of upper level charts in the northern and southern hemispheres.

The contour values get smaller as you move toward colder air.  The cold air is in the north in the northern hemisphere and in the south in the southern hemisphere (the pattern is effectively flipped in the southern hemisphere compared to the northern hemisphere).  The winds blow parallel to the contour lines and from west to east in both hemispheres. 

We'll finish this lecture by looking, in a little more detail, at how upper level winds can affect the development or intensification of a surface storm.  This material might be a little difficult and confusing at this point.  Don't worry if that is the case.

Surface and upper level maps are superimposed in the figure above.  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; 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.

This figure 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 with time (the low would be "filling" ).

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

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.

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

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.

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. 

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.

Here are the answers to the "test your understanding" question found earlier in this lecture.

1. This is a constant pressure chart.  The pressures at Points A, B, C, and D are all the same - 500 mb. 

2. Point A is found at the lowest altitude - 5400 meters.  Point D is found at the highest altitude - 5640 meters.

3. The coldest air is found below Point A, the warmest air is below Point D.

4. The winds blow parallel to the contours from west to east as shown on the map above.  The winds at Point C are blowing from the west.