In this lab, we will learn
about upper air plots and upper air maps. We will learn how to decode upper air
plots and how to breifly analyze upper air maps.
"Upper Air" is a term
in meteorology which refers to any type of reading that is not on the surface.
Rawindsonde balloons launched twice a day across the nation record
meteorological information such as temperature, pressure, humidity and wind
speed and direction, as they ascend into the atmosphere.
An upper level map is a map of all of the station's readings that were recorded
at a given constant pressure. The most common upper level maps are the 850mb,
700mb, 500mb, 250mb, and 200mb. On the 850mb map, every station is reading a
pressure of 850 millibars.
(850 mb)
Temperature:
The value located in the upper left corner is the temperature in degrees
Celsius. In this example, the reported temperature is 12 degrees.
Dew Point / Dew Point Depression:
The value located in the lower left corner is either the dew point or the dew
point depression in degrees Celsius. The depression is the difference between
the temperature and the dew point. If the dew point depression is being
plotted, an "X" represents a dew point depression greater than 30
degrees Celcius.
Some maps, such as Unisys, plot the actual dew point temperature, not the
depression on the map. Use a little common sense by looking at all of the plots
to determine what is being plotted on your particluar map. If there are plots
where the lower left number is higher than the upper left number (as in our
example), then dew point depression is being plotted. (Because the dew point
temperature can not be higher than the actual temperature.) We will use maps
from the Oklahoma Weather Lab. They
plot the dew point depression on their maps.
In this example, the reported dew point depression is 14 degrees Celcius. This
means that the dew point temperature is 14 degrees less than the temperature
(dew point temperature = -2 degrees Celsius, in our case).
Geopotential Height:
The value located in the upper right corner represents the geopotential height
in meters (m). This is how high above the Earth (approximately) the instrument
is when it records the given pressure. On some maps, this number is truncated
for ease of use. Use the following table to decode geopotential heights.
(Unfortunately, not all map makers use the same truncation. Figure out what
system they are using before decoding)
Pressure |
Rule |
Example |
850mb |
Add 1000m |
533 = 1,533m |
500mb |
Multiply by 10 |
591 = 5,910m |
250mb |
Multiply by 10 and |
098 = 10,980m |
In this example, the geopotential height is 1533 m. (Note: 1m = 3.28ft)
Wind Barb:
The wind barb, as we learned before, indicates wind direction and wind speed.
The same rules apply to upper air wind plot readings as they do for surface
plot readings. In this example, there is a 50 knot wind coming from the East.
The following two maps, from
the Oklahoma Weather Lab, are upper air
maps of 500mb and 250mb, respectively. On the 500mb map, notice how the winds
parallel the lines of constant heights. On reason this is happening is because
there is no longer any influence from the friction of the ground.
250 mb: The jet stream is the area of the highest winds (>50
knots). See if you can find the jet stream (there may be more than one) on the
map.