Temperatures

For a Skew-T chart, the name comes from the fact that the isotherms do not run vertically on the chart; rather, they are drawn "skewed" at a 45 degree angle from bottom left to top right. The temperature isotherms appear as fairly thick, solid blue lines, with the value of the isotherms plotted along the bottom of the chart. Zero degrees Celsius is dead center in the middle of the chart. Temperature values provide the other part of the foundation for the diagram. Given the plottings of temperature, dewpoint temperature, and parcel profile, you can use the foundation markings (pressure and temperature) to determine various parameters of the sounding, and, subsequently, determine a great deal about the condition of the atmosphere.

Several temperature values are plotted on a chart, such as the temperature of the atmosphere (ambient temperature), the temperature of a parcel of air rising dry adiabatically, and the dewpoint temperature. The ambient temperature is always plotted on the right. On some web pages versions, such as NOAA's GOES soundings chart, colors are used to help identify the temperature of the environment, the dewpoint temperature, and the temperature of the parcel of air. In essence, different thermodynamic products from different institutions look different. Stated another way, the same sounding data plotted on two different thermodynamic diagrams look different! Let's do a side-by-side comparison with two diagrams for Pittsburgh, Pennsylvania, both taken on December 4 2000 at 1200 GMT. We'll return to this comparison later, but for now we'll only look at temperature values.

On the Wyoming Skew-T plot, you will see two dark solid lines. The line on the right represents the temperature of the environment. You should be able to determine from this plot that the value at a pressure of 500mb is -25 C. You will have to interpolate the 25 degrees since the black temperature plot hits the blue 500mb isobar between -20 and -30. Temperature isotherms for the Wyoming are plotted along the bottom of the graph in dark blue. The line on the left represents the dewpoint temperature. At 500mb, the dewpoint temperature is roughly -38 C.

Inversions

The temperature portion of a thermodynamic diagram can also be used to determine if their is a temperature inversion on a given day in a given location. A temperature inversion exists when a warm layer of air lies on top of a colder layer of air. Inversions are particularly problematic in air quality issues, in that they serve to trap emissions and cause an increase in the overall unhealthiness of the air.

On a thermodynamic diagram, we can determine if there is an inversion if we see a shape resembling the one at the right. You should note that this is probably easier to see on a Stuve diagram, since the isotherms (temperature lines) go vertically on that diagram. On the Skew-T, recall that the temperature isotherms are skewed, so, while it may look like the temperature is increasing, it may actually be staying at the same temperature. Regardless, what you are looking for here is a decrease in the temperature (as one would expect), followed by an increase in temperature for some distance, then a return to a decreasing temperature as height increases. If you click on the diagram of the inversion above, note that they are showing the "z"-shape temperature line where the inversion is.