Mention Skew T Log P charts, and the temptation is to head for the tall grass. Even die-hard weather freaks can find them confusing, and they're certainly difficult for neophytes to plot and interpret. I often think that many would-be meteorologists switched to an art history major when they were first made to plot their own Skew T Log P charts in Meteorology 101. But have no fear, dear reader; this explanation will be short and sweet.
Balloons make 'em
Weather balloons are launched twice a day at many locations around the world—most of them on or near airports. In the United States there are 98 balloon launch sites. Once aloft, their small, dangling package of instruments begins transmitting information on temperature, pressure, dew point, relative humidity, wind strength and direction, and other variables as the balloons make their way to bursting altitude (as high as 80,000 feet). These readings are recorded back at home base, then sent on to the rest of the meteorological community. Other ground-based equipment—vertical wind profilers and Doppler weather radar—also contribute temperature information. It's the ambient temperature and dew point temperature that are plotted on the Skew T Log P charts.
Chart lowdown
Take a look at the Skew T Log P chart in Figure 1 on the previous page. Hey, come back here! Now sit down and give me 10 minutes—okay, five. This will be painless, I promise.
Right away, you'll notice that the temperature scale is printed slantwise (hence the name Skew T—the T is for temperature). Lines representing temperatures from minus 100 to plus 50 degrees Celsius are printed from top left to bottom right. The Fahrenheit equivalent is printed along the bottom of the chart.
Lines representing the dry adiabatic lapse rate slant upward diagonally from right to left, and curved lines representing the moist adiabatic lapse rate also slant upwards in that direction. A solid brown line represents the standard atmospheric temperature.
Uh, oh. A few people have left the room. I think the word adiabatic probably scared them off. Better speed this thing up.
Now then, those of you who are still with me will notice that the horizontal lines running from top to bottom are labeled in millibars. Down at the bottom you'll see a "1000" figure, representing 1,000 millibars of pressure, and also representing an altitude very close to sea level, because we all know that standard sea-level atmospheric pressure is 29.92 inches of mercury, or 1013.2 millibars. Going up the scale, you'll see these numbers progressing through 850 millibars (approximately 5,000 feet msl), 500 millibars (about 18,000 feet msl), 200 millibars (about 39,000 feet), and so on.
A wind scale may appear to the right of the chart, running vertically and indicating via wind barbs the wind direction and strength at altitude.
The plots
Look again at Figure 1. Here I've plotted temperature (the line on the right) and dew point temperature (the line on the left). So what we're looking at is a vertical slice of the atmosphere's temperature/dew point spread. Where the lines converge, you can expect clouds and/or precipitation. It's easy to see that just above the surface—between around 940 millibars and 920 millibars. Translated, that means a cloud layer beginning at 2,000 feet, with tops around 2,500 feet or so.
Then the lines diverge, indicating drier air, and converge again between 700 and 640 millibars (between about 7,000 and 12,000 feet). This represents a likelihood of another cloud layer at those altitudes. So the convergence and divergence of these lines are good graphic indicators of the cloud or moisture situation aloft.
Cold and ice
Skew T charts can also indicate the probability of icing. You'll notice that the 0-degree-Celsius line runs right through that low layer of clouds, and that the 0-to-minus-9-degree Celsius range falls in the high cloud layer. Those are prime icing temperatures, so I'd expect icing to be a factor in both cloud layers. Conversely, the air seems dry enough between layers (and certainly above 12,000 feet) that the risk of icing is likely to be less in those regions.
Rack up another use for the Skew T charts: icing predictors.
Trend lines
The slant of the temperature/dew point plots tells still another important tale: the stability or instability of the atmosphere. Colder-than-standard data points will be to the left of the fat brown standard temperature line; warmer-than-standard temperatures are to the right.
If a rising parcel of heated air encounters colder temperatures aloft, the parcel will continue rising and could develop into a thunderstorm. So if you see plotted lines slanting sharply to the left (say, slanting off in a 10 o'clock to 11 o'clock direction) this is a surefire indicator of very unstable air. It would be reasonable to expect storms to develop.
A good discussion of this topic can be found in Peter F. Lester's excellent book, Aviation Weather, published by Jeppesen-Sanderson in 1995. It's on pages 5-8 through 5-14.
Temperature plots that run in the 11 o'clock to 2 o'clock sector of the chart indicate stable air. And plots that follow the standard atmosphere line indicate a perfectly standard atmosphere—something that rarely appears in nature. If temperatures are to the right of the fat brown line, expect diminished airplane performance as higher density altitudes rob engine power.
Temperature plots that run straight up and down indicate isothermal regions. Those are vertical sections of the atmosphere where the temperatures are the same.
You can see the type of temperature signature that depicts a temperature inversion—a rise in temperature with altitude. At the surface, the temperature is a hair below freezing. Temperatures fall to minus 3 Celsius at 2,000 feet (940 millibars), then rise in an inversion layer that runs from about 2,500 feet (920 millibars or so) to about 6,000 feet (the 800-millibar level). Inversions can cause reduced visibility beneath them, and can signal a zone of turbulence at their upper boundaries.
That makes three more uses of the Skew T Log P chart: as an indicator of stability or instability, as a performance predictor, and as an inversion detector.
The horrible truth
You don't really need to know how to plot and interpret Skew T Log P charts. I know that comes as a comfort to those few readers who were polite enough to stick with me. Instead, the nation's forecasters and computer models do all the number crunching for you. All you have to do is obtain a thorough preflight briefing, and pay attention to the convective outlooks (they rely heavily on sounding information), the freezing levels, and the lifted and K index charts, where areas of unstable air are circled for easy identification.
For my fellow weather geeks, I encourage you to check out the following Web site, so you can see scads of Skew T Log P charts and their soundings ( www-das.uwyo.edu/upperair/sounding.html).
Another Web site ( www.nemas.net/edu/soundings/soundings.htm) explains about soundings in detail. To all you entertainment-starved meteo-fanatics, I say happy hunting!
To the rest I say: You are released! Call when airborne!
Links to additional information about weather can be found on AOPA Online ( www.aopa.org/pilot/links/links0009.shtml). E-mail the author at [email protected].
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