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Wx Watch: Skew-T Basics

Charting clouds and stability

Pilots looking for some valuable extra preflight information about the state of the atmosphere can find it on Skew-T Log-P (Skew-T, for short) charts. Although these charts have been known to cause fear, loathing, and confusion, it’s worthwhile to understand their many messages.

These charts plot temperature and pressure, but the lines representing temperature (in Celsius) run diagonally. They’re plotted along southwest-northeast axes, and so they’re skewed to the right. The “T” stands for temperature, so there you have the “Skew-T” portion of the name. “P” stands for pressure, which is also plotted on these diagrams—but pressure levels (which roughly correspond to altitudes) are presented in consecutive order, or logarithmically, with horizontal lines. That’s the “Log-P” part. As you might expect, pressure levels decrease, but altitudes increase as you move up the diagram.

Lapse rates and winds aloft

You’ll find more than temperature and pressure lines on Skew-T diagrams. Two of them represent standard lapse rates: the dry and moist adiabatic. These lines run from southeast to northwest.

The dry adiabatic lapse rate is the drop (or rise) in temperature of a dry air parcel as it ascends (or descends) with altitude. On color charts, these lines are brown.

The moist adiabatic lapse rate is the change in temperature of a saturated air parcel. Color charts show these saturation adiabats, as these lines are also called, in green. Saturation adiabats are slightly curved in the lower levels of the atmosphere, but become parallel to the dry adiabats at the top of the charts because the air is so dry at very high altitudes.

Look along the bottom of a Skew-T chart and you can see that their temperature ranges are from -45 degrees to +50 degrees Celsius. The vertical axis of the chart runs from the surface (represented as 1013.2 mb) to 150 millibars—or the approximate standard pressure level of the atmosphere at 45,000 feet msl.

Now for an exercise: Follow a dry adiabat upwards until it intersects a pressure level—say, 850 millibars (about 5,000 feet msl). Let’s start with zero degrees Celsius at the surface, then parallel the dry adiabat slope until you reach 850 millibars. Interpolate the temperature at that altitude, and you’ll see that it’s minus 15 degrees Celsius at 5,000 feet. Put another way, the temperature has dropped three degrees Celsius per 1,000 feet—the dry adiabatic lapse rate! So in a glance, the Skew-T diagram can tell you temperatures at any altitude. (The standard lapse rate of two degrees Celsius/3.5 degrees Fahrenheit is based on an average rate of cooling among many sites in the world.)

Winds aloft are also plotted on Skew-T charts, using wind barbs. (The wind strength is in meters per second; as a crude translation, multiply meters per second by two to get miles per hour.) They’re over on the right side and, like the pressure levels on the left side, correspond to the winds at a range of altitudes.

Environmental lapse rates

Up to now, we’ve been talking about lapse rates in the abstract, following their “standard” behavior. But what interests us more are the environmental lapse rates—how the actual temperatures and dew points are behaving with altitude. These, too, are plotted on Skew-Ts. Temperatures aloft are drawn in red; dew points are most often represented by a green or blue line. Both these lines are fatter than the lapse rate lines we just talked about, so they’re easy to pick out.

That’s a good thing, because the traces these lines make tell us a lot about the probability of cloud layers, and stability of the atmosphere. Know what to look for, and you can make educated guesses as to the chance of cloud layers aloft, or understand why a certain temperature profile may lend itself to thunderstorms, morning fog, or freezing rain.

By the way, Skew-Ts get their temperature and dew point data from instrument packages aboard radiosonde weather balloons—the same balloons that give us our winds aloft reports.

Temperature-dewpoint spreads

You can see temperature-dew point spreads on Skew-Ts quite easily. Just watch for the temperature and dew point traces to narrow (indicating saturated air, and a high chance of cloudiness) or widen (indicating drier, cloud-free air). Then note the pressure level/altitude at which these spreads occur. Presto—you’ve made a cloud density and depth forecast. Of course, precipitation will also cause close temperature-dew point spreads.

The environmental temperature soundings also indicate stability. We know the orientation of the dry adiabatic lapse rate lines, so if environmental temperatures are colder than standard—especially in the warmer months of the year—then this is an indication of potential instability. Why? Because any warm air parcels rising from heated surfaces or incoming warm frontal air will rise rapidly if the air surrounding them is colder. Warm air rises, true, but only if the air around it is colder!

The alignment of the temperature sounding is a good way to quickly determine instability—or stability. If the sounding tends to run steeper than the orientation of the dry adiabats, then the actual lapse rate is cold enough to promote further lifting of heated air parcels. In other words, the air is absolutely unstable. On the other hand, if the temperature sounding follows the orientation of the dry adiabats, it’s a marker for neutral stability. Any heated air parcel rising into a neutrally stable layer, if displaced vertically, will return to its original position.

Finally, if the temperature sounding increases with altitude, this is a sign of very stable conditions. This is called an inversion. It’s a stable situation because any rising air parcels will stop dead in their tracks as the parcel’s temperature matches the temperature within the inversion.

The parcel may even sink—if its temperature is relatively colder than the ambient temperature within the inversion layer. This is why inversions trap pollutants and reduce visibilities in haze, mist and fog.

Morning fog

Radiation fog typically occurs when high pressure, clear nights, and very light winds conspire to radiate heat away from the surface. The result can be a dense morning fog that burns off later in the day. Skew-T diagrams show a characteristic temperature signature for the radiation fog scenario. The temperature sounding shows a layer of colder air very near the surface, followed by a rise in temperature to the top of the inversion layer. Temperatures resume a cooling trend with altitude.

Another Skew-T temperature signature shows the profile associated with freezing rain. It shows a very thin layer of subfreezing temperatures close to the surface, followed by warmer air aloft.

Finding Skew-Ts

There are plenty of Web sites that delve into Skew-T Log-P arcana, but here’s one of my favorite sources. Under the “Type of Plot” menu, select “GIF: Skew T”, and then select the sounding date and time that interests you. Forecast Skew-Ts aren’t available on this site—but you can examine past sounding dates and times, and can even look at Skew-Ts from other parts of the world.

Forecast Skew-Ts—generated by computer models— are online, but only for the eastern half of the nation. Click on the “SkewT East” sounding times, then click on the scroll bar header, and then click on a site identifier. Up pops a 72-hour sequence of Skew-Ts. Another site gives forecast Skew-Ts for the United States.

Skew-T diagrams can be useful elements. To be sure, there are shortcomings. Balloon launch stations are few and far between, leaving big gaps in data. The balloons are only launched twice a day, at 0000Z and 1200Z, and are quickly blown downwind of the launch site. This means data can be old, and irrelevant to conditions aloft at the launch location. And the government has indicated that balloon soundings might be reduced in the future. If that happens, we’ll lose a trusty old forecasting tool.

E-mail the author at [email protected].

Thomas A. Horne

Thomas A. Horne

AOPA Pilot Editor at Large
AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.

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