Clouds 3: Skew – T and Instability

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

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’

Sounding Data

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 make  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 can suggest where and what type of clouds are present. The altitude of clouds can be partially inferred by high relative humidity, where the dewpoint and temperature are close. The type of cloud is suggested by the stability of the atmosphere, determined by the steepness of the temperature profile compared to the adiabatic lapse rate. The underlying graph was downloaded from the University of Wyoming atmospheric soundings archive, http://weather.uwyo.edu/upperair/sounding.html. JeanBizHertzberg, 2022

Skew-T Plots

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. Another clue is that the temperature line will have a kink to the right. Condensation is ‘warming’ as thermal energy leaves the condensing water and enters the surrounding air and this will be seen in the temperature line angling closer to an isotherm. We are all familiar with the opposite effect, as evaporating sweat cools our skin.

The stability of the atmosphere can be determined by the angle of the temperature line compared to an adiabat. Let’s use the moist adiabat for comparison if we are in a cloud, or the dry adiabat if we are not. Here’s the procedure:

  1.  Take a point on the temperature line. For example, let’s start near the ground. This defines the initial condition of our test parcel.
  2. Do a virtual lift of our parcel by moving a short distance parallel to the nearest dry adiabat line. We are thus perturbing our system.
  3. Check to see if our lifted parcel is cooler – and more dense – or warmer – and less dense – than the actual temperature line at that same height.
  4. Now let go of the parcel. Where does it go? If our parcel is cooler and denser, when we let go of it, it will sink back down. This means the atmosphere is stable to perturbation; the lifted parcel drops back to where it came from. In contrast, if the parcel is warmer, less dense, it is buoyant, and will keep going up! Unstable!

In summary, if the adiabat is shallower, less steep, than the temperature line, the atmosphere is stable. If the adiabat is steeper, it’s unstable. Another way to think about this is if the upper air is relatively cold, colder than adiabatic cooling makes it, and the air underneath is relatively warmer we have an unstable situation. This happens often in summer, as the sun heats the ground faster than it heats the air, and we get thunderstorms if there’s enough moisture in the air. In winter, the ground radiates a lot of heat to outer space on clear nights, making the air near the ground very cold, colder than the air aloft; a stable situation. This may be called an inversion because it’s contrary to the normal adiabatic profile. This is a generality; weather has a lot of variety and is almost never ‘normal’!

A shortcut to checking the whole column of the troposphere is to look at the CAPE, in the middle of the calculated indices column. CAPE (convective available potential energy)  is an integral of all the area between the adiabat and the temperature line where the adiabat is steeper. If CAPE is zero, the atmosphere is all stable, all the way up (more on clouds in stable air later).  A CAPE of a few hundred is weakly unstable, and small to medium (humulis, mediocris) cumulus clouds are to be expected. A CAPE of 1000 or more is very unstable, and means that a thunderstorm is likely: cumulus congestus, cumulus castellanus and cumulonimbus will be seen.

Winds

Figure 3: Wind barb code from the National Weather Service.

There is one more type of data on the Skew T that is helpful in cloud ID, and that is the wind. Between the graph and the indices column there is a stack of feathers, called wind barbs. Figure 3 shows how to read the barbs. This is the standard code used by the National Weather Service when reporting data from weather stations and balloons. The wind speed is reported in knots which is a speed unit 15% faster than miles per hour. I usually ignore the difference. Knot is a very old nautical term which is still used for some navigation and weather data . Wind at altitudes above 300 mb (that’s millibar, not megabytes) are generally ‘westerly’; winds are named for where they are from, not where they are going. Note that the upper air winds are much faster than surface winds. The earth’s boundary layer effect slows and turns surface winds, while higher up the polar jet stream girdles the planet near the tropopause (8-12 km up, seen in the upper quarter of the Skew T) .

Wind can have a large effect on cloud development. For example, wind shear is when the speed or direction of the wind changes with altitude. This is common near the ground and up through the boundary layer to the ‘free atmosphere’ at a few hundred meters up. When there is wind shear higher up, it can prevent updrafts and downdrafts inside thunderstorms from canceling, and lead to severe thunderstorms storms with damaging hail that last for hours . Wind is also crucial in the formation of mountain wave clouds, discussed in XXXX

How to Download a Skew T

The best archive of Skew T data for the US is maintained by the University of Wyoming: http://weather.uwyo.edu/upperair/sounding.html.

References

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Clouds 2: Why Are There Clouds? Lift Mechanism 1: Instability
Boundary Techniques – Introduction