Dec 30

The Basics of Skew-T/Log-P Diagrams

Posted by Brian Barnes under topic Articles, Meteorology

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.

The Skew-T diagram contains all the information needed to understand the atmosphere at a particular point. From these data, forecasters can determine wind speed and direction, temperature and dewpoint throughout all atmospheric layers and more.

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 skewed lines on a Skew-T Diagram give the diagram its name. These lines, called Isotherms, represent temperature.

Lines of pressure increase horizontally from 1000 millibar (mb) at the surface to 100 mb at the top of the troposphere.

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 bottom of the diagram represents sea-level pressure.

isobar skew-t diagram

ISOTHERMS – These are lines of equal temperature. Isotherms are always solid and usually increment in 10 degrees Celsius. The reason they are drawn at an angle is to get all the data to fit on a single printed sheet of paper. The temperature aloft is colder than the surface temperature.

isotherms skew-t diagram

Saturation Mixing Ratio Lines -Lines of equal mixing ratio (mass of water vapor divided by mass of dry air -grams per kilogram). Usually represented by dashed lines, also drawn diagonally.

saturated mixing ratio skew-t diagram

Dry adiabatic lapse rate – The rate of cooling of a rising unsaturated parcel of air.  Drawn so they gradually arc with height and typically solid.

dry adiabatic lapse rate skew-t diagram

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 skew-t diagram

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

skew-t diagram

Skew-T Diagrams Can Look Different

Skew-T Diagrams often look different depending on the source but contain the same information. The diagram shown here is a recreation of The National Center of Atmospheric Research‘s diagram.

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.

The above diagram adds a few more elements of a completed Skew-T/Log-P Diagram.  The most important piece of information on any Skew-T Diagram is the date, time and location. This plot, for example, states that it was a 12Z plot from Dodge City, KS on May 05, 2008.

How Skew-T Diagrams Are Used

The most common use for Skew-T/Log-P Diagrams is to illustrate data from a sounding (weather balloon). Another use for a Skew-T diagram is to illustrate data derived from numerical models used in forecasting.

The last two lines on the diagram that we need to talk about represent temperature and dewpoint temperature at various heights. The line to the right is always temperature. The line to the left is always dewpoint temperature. While we’re just using black lines, the color of these lines does vary depending on who creates the diagram.

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 diagram represents data from Dodge City, Kansas, which is about 2,550 feet above sea-level. As a result, the lines of pressure start from that elevation.

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.