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