There is an Optional
Assignment that accompanies this 3-part series on Upper
Level Charts.
Here's a little more in-depth look at upper-level charts.
Hopefully you still remember the trough (u-shape)
and ridge (n-shape) features introduced in
Pt. 1, the fact that cold air is found under an upper level trough
and warm air below a ridge, and that upper level winds blow
parallel to the contour lines and from west to east.
1. After you've finished reading this
section you should better understand what the title "850 mb Chart"
on the upper level map above refers to.
2. 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 have a better idea of where the names trough
and ridge come from.
3. Note that the values on the contours
decrease as you move from the equator toward higher
latitude. You should understand why that happens
(temperature also decreases as you move toward higher latitude,
maybe that is the explanation). You should understand why
troughs and ridges are associated with cold and warm air,
respectively.
You really only need to remember two things from earlier in the
semester: (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.
On the left side of the figure above (a crossection) pressure
drops from 1000 mb to 800 mb, a 200 mb change, when moving upward
1500 meters in the cold air. It decreases from 1000 mb to
900 mb, only 100 mb, in the same vertical 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. This approach is shown above (a 3-dimensional
view). Pressure dropped from 1000 mb to 800 mb at 1500 m
altitude in the cold dense air at left. There was only a 100
mb drop (from 1000 mb to 900 mb) in the warm at the right side of
the figure. 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.
This would seem like a straight forward 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, meteorologists do things
differently. Rather than plotting conditions at
a constant altitude above the ground, meteorologists 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 higher, to 1800 m.
Every point on the sloping surface above has the same pressure,
850 mb. The altitude at which that pressure value is found
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 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.
It 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. Click here when you think you have it
figured out.
The chart for the figure above would look like:
In the next figure we will keep the same warm in the west and cold
in the east temperature change. Then we will add south to
north temperature change, temperatures in the north will be colder
than in the south.
Here's what the temperature pattern will look like.
What will the wavy 850 mb constant pressure surface look like
now? Basically the northern edge of the wavy figure above
will tilt downward into the colder air at higher latitude.
It's a little hard to draw but the result is shown below:
The chart for this tilted wavy surface has the familiar ridge and
trough features
Now let's go back and look at the map we started with and see if
we are able to answers the questions listed at the start of this
section.
1. The title tells you this is a map depicting 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.
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.
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?
Finally
we will compare upper level charts in the northern and southern
hemisphere
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 winds blow
parallel to the contour lines and from west to east in both
hemispheres.