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Dec 30

The Basics of Skew-T/Log-P Diagrams

Posted by Brian Barnes under Forecast, Learning, weather

One of the most important tools that convective forecasters have is the Skew-T/Log-P Diagram.  It is a thermodynamic chart that allows forecasters to view real information about the state of the atmosphere from the surface level all the way to 100 millibars.

A lot of important information can be found using this diagram, such as cap strength, the amount of CAPE (Convective Available Potential Energy), as well as winds, dewpoint temperature and the air temperature through all sampled layers of the atmosphere.

Let’s look at the basic elements that make up the Skew-T/Log-P Diagram.  The name itself is self-explanatory in that “T” represents “Temperature” and “P” represents “Pressure”.  The Temperature lines are skewed while the pressure lines increase in the vertical.

ISOBARS – These are lines of equal pressure.  On a Skew-T/Log-P Diagram they run horizontally across the diagram and each line represents 100mb of pressure.  On the Diagram below, the top line represents the top of the atmosphere at 100mb and the the bottom of the diagram represents sea-level pressure.

ISOBAR Lines Represent Constant Pressure

ISOTHERMS – These are lines of equal temperature.  The are drawn diagonally from the bottom-left to the upper-right (thus, the name “Skew”).  They are drawn this way so that the diagram will easily fit on a single sheet of paper when printed.  Since the temperature aloft is much cooler than the temperature at the surface, if the temperature lines were drawn horizontally it would be a very wide diagram.  These lines are always solid and increment in 10 degrees Celsius.

ISOTHERM Lines Represent Equal Temperature

Saturation Mixing Ratio Lines -Lines of equal mixing ratio (mass of water vapor divided by mass of dry air -grams per kilogram) These lines also run diagonally but they are DASHED.

Saturated Mixing Ratio

Dry adiabatic lapse rate – The rate of cooling of a rising unsaturated parcel of air.  On the diagram these lines are represented in a slope from the bottom-right to the upper-left and are solid, the lines will gradually arc with height.

Dry adiabatic lapse rate

Moist adiabatic lapse rate – The rate of cooling of a rising saturated parcel of air, which is dependent on the moisture content of the air.  On the diagram these lines slope from the bottom towards the upper-right.  The moist adiabatic lapse rate will increase with height because cold air has less moisture content than warm air.

Moist adiabatic lapse rate

The above elements represent the lines you’ll use in order to measure the state of the atmosphere.  Here is the completed diagram:



This specific diagram is a recreation of a diagram used by The National Center of Atmospheric Research (http://www.rap.ucar.edu/weather/upper/).  The bullseye in the upper-left is unique to NCAR’s sounding diagram and will normally contain a blue dot within it.  The idea is to attempt and forecast the motion and type of supercell thunderstorms based on a climatology study of supercells.  You can find more information about this on NCAR’s help page for their upper-air information.
Now that we understand the basics of the diagram, let’s examine a basic plot that illustrates the environmental parcel and the dewpoint temperature.
Skew-T/Log-P Sounding

Skew-T/Log-P Sounding

The above diagram adds a few more elements of a completed Skew-T/Log-P Diagram.  Including the most important elements – The Date, Time and Location, which is illustrated underneath the diagram.  This plot for example states that it was a 12Z plot from Dodge City, KS on May 05, 2008.

Skew-T/Log-P Diagrams can be used to illustrate a sounding, which is created from rawinsonde data (weather balloon) or the diagram can be used to illustrate a forecast sounding which is created from numerical data imported from forecasting models such as the NAM, GFS or ETA.  In this example, we’re using actual data that was sampled.

The thick black line on the right represents the environmental sounding, or air temperature at each level throughout the atmosphere. The black line on the left represents the dewpoint temperature at each level throughout the atmosphere.  It should be noticed that these lines may vary in color depending on which system (or website) you’re using, but we’re using sold black lines here for illustrative purposes.

You may notice that the environmental sounding and dewpoint plot do not start at the bottom of the chart, but instead just slightly below the 900mb line.  This is simply because at the station where this was launched from (Dodge City, KS) the elevation is about 2,550 feet above sea-level.  So, the lines start at the level of pressure 2,550 feet above sea-level.

In the next installment of this primer to the Skew-T/Log-P Diagram we’ll examine this sounding and l determine some very important thermodynamic information such as CIN (cap strength), CAPE and LI, as well as a few other parameters that can greatly help you improve your convective forecasting abilities.

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