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Skew-T log (p) 101
A temperature sounding is plotted on a
thermodynamic chart. There are three (3) basic thermodynamic
charts used by forecasters to include the Stuve, tephigram and the
Skew-T log (p) diagram. These three thermodynamic diagrams
are designed to present the environmental temperature, dewpoint
temperature and wind as a function of pressure or altitude.
The Skew-T log (p) diagram is by far the most popular of the
three.
Starting with the isotherms (lines of
constant temperature) and isobars (lines of constant pressure), the
temperature and dewpoint temperature can be plotted as a function of
pressure. Lines of constant height can also be included for
convenience since standard atmospheric pressure has a distinct
relationship to height above mean sea level (MSL). The result
is a partial base thermodynamic diagram that looks similar to the image
above to include isobars (black), isotherms (red) and lines of constant
height (blue). Click here to view a larger image
of this base diagram.
A thermodynamic diagram is also considered a nomogram.
That is, they are diagrams that contain lines
showing the solutions to a set of different equations, in this case,
thermodynamic equations. The remaining three lines on the
complete base diagram are the dry adiabatic lapse rate (DALR), moist  adiabatic
lapse rate (MALR) and the saturation mixing ratio. These last
three are
constructed by solving three different thermodynamic equations as
a function
of temperature and pressure.
The
result
is a complete base thermodynamic diagram that looks similar to the
image on the left. Click here to view a larger image
of this Skew-T log (p) base diagram.
You can
think the base diagram as a sophisticated form of graph paper on which
the environmental temperature, dewpoint temperature and wind are
plotted.
Together with the plotted data the lines on the base diagram
provide a means for meteorologists to calculate dozens of important
thermodynamic indicies and provides a visual reference that has a high
glance value. This allows them to predict convective
outbreaks, icing potential, cloud heights and turbulence just to name a
few.
When looking at a Skew-T log (p)
diagram, it is important to understand the origin of
the environmental temperature, dewpoint temperature and wind
plotted on the chart. Such data can come from two sources.
The first is from
a radiosonde launch (weather balloon) depicting actual
environmental
conditions - this is referred to as a RAOB (or radiosonde observation)
like the one shown to the right for Amarillo, Texas. Click here to view a larger image.
The data
can also be plotted from the output from a numerical weather prediction
model depicting an analysis or forecast. RAOBs are usually
only available twice
at day at 0000 UTC
and 1200 UTC at specific launch sites around the world. On
the other hand, forecast model output is available at a much higher
spatial and temporal resolution.
The RUC
Soundings Java tool provides an interactive utility to view
the data from RAOBs and several different forecast models.
The data from the forecast model is influenced by the
radiosonde launches as well as a plethora of other environmental data
to include surface observations (METARs), aircraft reports, satellite
and ground-based radar just to name a few. To learn more
about this diagram from a pilot's perspective, please consider
purchasing the Introduction to the Skew-T log (p) Diagram
premium workshop.
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