Clouds 3: Skew – T

Figure 1: The skew-T diagram for 6 AM September 22, 2022, in Grand Junction, Colorado. From the University of Wyoming soundings archive.

Actual Temperature Profile: Environmental Lapse Rate, Sounding Data and Skew-T Plots

The actual local temperature profile is formally called the Environmental Lapse Rate (ELR). It is measured twice a day at specific locations all over the world, using weather balloons . Here in Colorado, this is done at the Grand Junction Airport. It was also done in Denver until July 2022 when a helium shortage required stopping the DEN service . The weather balloon carries a small, expendable instrument package called a radiosonde that transmits data to ground receiving stations. The data stream is called a ‘sounding’

There are quite a few kinds of data transmitted by the radiosonde. For the purposes of understanding and identifying clouds, we are most interested in temperature, dewpoint and wind, all as a function of altitude (hang on, we’ll come back and define dewpoint in a bit). Pressure and altitude are closely related, as we saw above, so we’ll treat them as interchangeable independent variables, and consider temperature, dewpoint and wind to be dependent on them. Usually when plotting functions you put the independent variable on the horizontal x axis, and the dependent on the y, but because we are talking about altitude, that will be on the y axis. The units are in millibars, where one bar is one atmosphere of pressure, 14.7 psi, so sea level is 1000 millibars. The horizontal grid lines are thus isobars. The altitude in meters is shown inside the axis. Temperature and dewpoint in centigrade will be on the x axis, sort of. To keep a typical temperature profile looking good in a square format, the lines of constant temperature, isotherms, won’t be vertical, they will slant off to the right. That will makeĀ  a ‘Skew-T’ diagram, as shown in Figure 1. These diagrams are a bit complicated looking, and contain a lot of information. If you want to really get into it, you can take a free online short course in ‘Skew-T Mastery’ .

Figure 2: Skew-T, labeled.

Skew-T diagrams have two primary lines. The heavy line on the right shows the air temperature recorded as the balloon ascends. If we are thinking about atmospheric stability, and whether a parcel rising from below is more or less dense than ‘the neighbors’, this line represents the temperature of the neighbors.

The heavy line on the left is the dewpoint. This is the temperature at which the water vapor in the air at that height would condense into dew, i.e. become saturated. It represents the amount of water vapor in the air: if the dewpoint is far from the actual local temperature, the air is relatively dry. If the dewpoint is close to the temperature line, the air doesn’t need to cool much before being saturated (holding the maximum possible amount of water), and a cloud is likely to exist there.

References

[1]
H. Brinkmann, “Grounded: Denver weather service stops sending up weather balloons amid helium shortage,” FOX Weather, Jul. 25, 2022. [Online]. Available: https://www.foxweather.com/weather-news/denver-boulder-national-weather-service-balloons-helium. [Accessed: Sep. 15, 2022]
[1]
UCAR/COMET, “Skew-T Mastery.” [Online]. Available: https://www.meted.ucar.edu/mesoprim/skewt/. [Accessed: Sep. 15, 2022]
[1]
“Depth sounding,” Wikipedia. Jun. 04, 2022 [Online]. Available: https://en.wikipedia.org/w/index.php?title=Depth_sounding&oldid=1091426120. [Accessed: Jul. 14, 2022]
[1]
“Radiosonde,” Wikipedia. May 12, 2022 [Online]. Available: https://en.wikipedia.org/w/index.php?title=Radiosonde&oldid=1087363958. [Accessed: Jul. 14, 2022]
[1]
“Weather balloon,” Wikipedia. Jul. 08, 2022 [Online]. Available: https://en.wikipedia.org/w/index.php?title=Weather_balloon&oldid=1096996123. [Accessed: Jul. 14, 2022]
Photons, Wavelength and Color