|
Constant pressure chart basics
Constant pressure charts like the 500 mb chart
shown to the right may seem obvious to interpret (click here to view a larger
image). It has many features similar to what you might see on
the surface analysis chart.
There are H's and L's that represent highs and lows and there
are lines that resemble lines of constant pressure or isobars.
However, those H's and L's are not marking the position of
high and low pressure centers and the lines are not lines of constant
pressure or isobars. So why is this different from the
surface analysis chart?
The surface analysis chart like the one
shown below (click here to view a larger image)
depicts, in part, the pressure that is recorded by barometers located
at the surface. However, if we plotted the surface pressure
strictly from these observations of pressure it would look a lot like a
 surface
relief map or topographic map. That is, higher elevation
terrain generally has a lower pressure. So you would simply
be plotting the difference in pressure due to the height of the terrain
and not due to the relative differences in the pressure.
Consequently, meteorologists adjust to
the observed pressure at the surface to a height we know as mean sea
level. This depicts the surface pressure as if there were no
terrain features higher or lower than sea level. In other
words, the surface analysis chart depicts how pressure changes while
holding the height constant. This is formally described as
pressure depicted at a constant geo-potential height of zero.
A constant pressure chart is just
that...a chart where the pressure is held constant. Unlike
the surface analysis chart, the pressure does not vary. What
varies on this chart is the height
of that specific pressure surface. For example, the solid lines on the
constant pressure chart above depict the height of the 500 mb surface.
So, why use constant pressure instead of "constant altitude"
to depict the weather aloft? It would seem more intuitive to
depict how the pressure varies at 10,000 feet instead of the how the
height varies at 700 mb.
For the
last 50+ years, meteorologists have drawn weather maps describing upper
air conditions using constant pressure surfaces. These
charts are prepared twice a day at 0000 UTC and
1200 UTC for
several mandatory pressure levels (925 mb, 850 mb, 700 mb, etc.) from
the temperature, humidity and wind data provided by the
operational radiosonde network (weather balloons), supplemented with
data from other sources such as aircraft and satellites.
Meteorologists use these constant pressure charts rather
than constant altitude charts for several reasons.
1. Since most aircraft of the time used
pressure altimeters, most "constant altitude" flights were actually
flown on constant pressure surfaces. Aircraft today still fly
constant pressure surfaces at altitudes at or above 18,000 feet (also
known as the flight levels). This is done by using
a constant altimeter setting of 29.92. As the pressure
changes while en route, the pilot does not adjust the altimeter
setting. Instead, they adjust their true altitude up or down
in order to fly a constant pressure surface.
2. The radiosonde telemetry data from
which the charts are prepared are reported in terms of pressure.
As the balloon ascends, temperature, dew point temperature
and wind are measured and reported as a function of pressure, not
altitude.
3. Use of pressure as the vertical
coordinate simplifies many of the thermodynamic equations and
computations used in numerical weather prediction models. In
other words, when you hold pressure constant it makes these equations
easier to solve. And that was real important when computing
power wasn't as good as it is today. Therefore, in addition
to the charts made from the radiosonde data, forecast data is also
depicted on constant pressure charts.
Meanwhile, we're stuck seeing these charts since most
forecasters have adapted to them over the years. In addition
to the height of the pressure surface, many constant pressure charts
available online will also depict the actual or forecast wind speed and
direction. Therefore, many pilots do examine these charts for
the winds. The Aviation
Digital Data Service (ADDS) provides the winds on constant
pressure surfaces without displaying the height contours as shown to
the left (click here to view a larger
image). Even though the chart is labeled with a specific mean
sea level height (such as 18,000 feet MSL), please be aware that this
is only an approximation. This is still a constant pressure
chart (note the pressure in parentheses following the altitude).
As a result, the true altitude of the winds can vary by 2,000
feet or more. This ADDS rendition is best used for aircraft
flying at 18,000 feet and higher since they are truly flying a constant
pressure surface.
|
