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Skew -T school

Deep down, you know you love ’em

What if you could look up through the atmosphere and see the entire vertical temperature and dew point profiles, from the surface to 40,000 feet?
Skew-T Log-P charts come in many forms but most show the elements identified at left. One glance can tell a lot. The close temperature-dew point spread from the surface to 600 mb/ about 14,000 feet indicates cloudiness between those altitudes. A rising air parcel is colder than temperatures aloft, so convection shouldn’t happen. The freezing level is just above 700 mb/about 10,000 feet. Looks like a quiet, if IMC, situation.
Skew-T Log-P charts come in many forms but most show the elements identified at left. One glance can tell a lot. The close temperature-dew point spread from the surface to 600 mb/ about 14,000 feet indicates cloudiness between those altitudes. A rising air parcel is colder than temperatures aloft, so convection shouldn’t happen. The freezing level is just above 700 mb/about 10,000 feet. Looks like a quiet, if IMC, situation.

Or be able to identify near-exact freezing levels? Or see where cloud layers and clear air exist? Or see where and what type of precipitation—along with icing conditions (including freezing rain)—could occur? Or see temperature inversions? Or even be able to tell if thunderstorms are likely? Sure, you can learn all these weather conditions by visiting the websites we commonly use in preflight briefings. But those sources mainly rely on textual information or one-dimensional maps.

But by using Skew-T, Log-P charts (“Skew-Ts” for short) you can answer all those questions—and see them in easy-to-interpret plots. Skew-Ts might have earned bad raps from pilots bewildered by their dense network of angled lines and zig-zag plots. Or maybe it’s their use of millibars and Celsius to define altitudes and temperatures, respectively. However, with a little bit of effort the Skew-T’s value becomes quickly apparent—and even intuitive. We’ll go slow here to give you a grounding in Skew-T basics, and neglect for now the hodographs that usually accompany them. (Hodographs show the turning of wind direction with altitude and are used to predict tornados.)

Chart symbology

First things first. The chart’s Y-axis (the north-south line) has the scale for altitude, and its X-axis (east-west line) has the scale for temperature. Instead of defining altitude in terms of height above mean sea level, it’s defined with levels of constant pressure—expressed in millibars. And instead of using Fahrenheit for temperature, Celsius is the rule. The lines for temperature are “skewed” in a northeast-southwest direction, hence the chart’s name.

Also, in a northeast-southwest orientation are dashed lines, which represent the saturation mixing ratio—the amount of water vapor in the atmosphere compared to a mass of dry air. Some Skew-Ts don’t show these, but they’re used to determine where dew points and temperatures merge.

Running in northwest-southeast directions are the lines representing the lapse rates. The dry adiabatic lapse rate (DALR—5.5 degrees F/3 degrees C per 1,000 feet of lift) is fairly straight; the moist adiabatic lapse rate (MALR—3.3 degrees F/about 2 degrees C per 1,000 feet of lift) is slightly curved. Over on the right side of some Skew-T charts is another Y-axis that gives wind barbs showing the wind speeds and general directions with altitude.

Click on image for captions

The profile at right is the classic signature for sleet/ice pellets at low altitude. Clouds and snow aloft—just below 700 mb/10,000 feet—falls into a wedge, or “warm nose,” of above-freezing temperatures, turns into rain, then refreezes after it falls through 850 mb/about 5,000 feet. There’s enough subfreezing air below to allow the frozen droplets to reach the surface. The “SHARPy” analysis above can be found on the Pivotal Weather website by clicking on a GFS model’s map. The Skew-T chart shows that warmer parcels (white dashed line) break through the temperature inversion—or “cap”—just above 850 mb/about 5,000 feet and rise into colder air. This is the LFC, or level of free convection. CAPE levels are high (see lower left box) and the hodograph at top right shows winds turning with altitude, so there’s a marginal tornado risk. This chart was captured during this year’s EAA AirVenture, when severe storms came to the Oshkosh, Wisconsin, area. Welcome to Phoenix in July. Talk about a wide temperature-dew point spread! It’s 58 degrees’ worth at the surface, and doesn’t narrow until nearly 500 mb/18,000 feet. This Skew-T has “dry” written all over it. And with zero CAPE, light, and straight line winds up through 300 mb/30,000 feet, it’s a desert profile through and through. This bare-bones Skew-T (left) shows what can happen in the early morning hours when nighttime cooling leaves a layer of cool air hugging the surface. Meanwhile, warmer air remains atop it. This is a classic nocturnal inversion. Surface temperatures will rise with the heat of the day, moving surface temperature plots to the right. As thermals rise, they’ll mix with the air aloft and generate low-level turbulence and gusty surface winds. One rule of thumb holds that surface gusts will be as strong as the winds at 850 mb/5,000 feet, which right now are posted as 15 knots.

Temperature, dew point, and air parcel traces

Superimposed on this clutter of diagonal lines are the traces for the observed, or environmental, temperature and dew point temperatures. All those lapse rates and ratios are standardized measures—but the temperature and dew point traces are from weather balloons with radiosonde transmitters, temperature profilers, and satellites with remote-sensing equipment. They’re the actual, ambient—or derived, in the case of satellite sensors—conditions at the time of observation. Temperature traces are shown in red, and dew point temperatures are in green or blue. But not all Skew-Ts are alike. Some charts use black lines for both temperature and dew point.

Air parcel temperature lets us compare the temperature of a parcel of air with the temperature of the air surrounding it. What’s an air parcel? Meteorologists use the term to describe a volume of air—think of a large balloon—that’s separate from the general environment and behaves independently. A blob of hot air rising from a parking lot, say, or localized updrafts or clouds. Many Skew-T charts plot parcel lapse rates, so you can compare them with the temperature aloft.

Air parcel analysis becomes very important in determining the chance, and severity, of thunderstorms. We all know that warm air rises—but only if the air around it is cooler. How much cooler? The gap between parcel and observed temperatures tells the tale. A wide gap means a lot of what’s called convective available potential energy (CAPE). CAPE is frequently posted with the chart. Values of 1,000 to 2,500 mean thunderstorms are likely; 2,500-4,000 indicates widespread severe storms; and above 4,000, extreme storms with large hail.

Stable and unstable

Skew-Ts make it easy to see if an air mass is trending unstable, meaning when colder air overlies warmer conditions. A quick glance at the orientation of the temperature trace gives a clue: The farther left the traces, the colder the air aloft. Any warmer air below could rapidly rise through it. If there’s enough moisture, and support from upper-air circulations, then the stage is set for thunderstorms in the warmer months of the year.

If the temperature line is running straight up and down, then you’ve got an isothermal situation. If it’s leaning to the right, there’s an inversion. There won’t be much vertical movement in cases like these. Inversions, for example, suppress cloud buildups because their warmth can exceed the temperature of any rising air from below, preventing it from climbing—unless it has so much energy that it can zoom up through it!

When there’s strong high pressure and fair weather, low-level inversions near the surface are common. In the morning surface temperatures are cool from nighttime radiation but above, say, 1,000 to 2,000 feet agl there’s warm air. As the Earth heats up during the day, you can see the inversion’s S-shaped temperature trace move to the right and watch the inversion disappear—along with its fog, frost, or low cloud levels.

Convective lifting, the ‘loaded gun,’and ‘busting the cap’

Let’s look at a snapshot (right) of how thunderstorm initiation can look on a Skew-T. We can see how the surface temperature and dew points begin to follow their lapse rate profiles, then converge—at about 5,000 feet in our example. Once they’ve merged, condensation occurs, the air becomes saturated, and a cumulus cloud forms. The level at which all this happens is called the “lifted condensation level,” or LCL for short. (An LCL callout—in meters—is often posted, sparing you the work of finding it yourself.)

Now this moist parcel of air rises, following the moist adiabatic lapse rate. But it must overcome the inhibiting force within any inversion—early morning or otherwise. Small wonder that this type of Skew-T profile is called a “loaded gun.” There’s a lot of potential energy building within our raring-to-go parcel.

If it succeeds in popping out the top of the inversion—“busting the cap,” in weather slang—and into colder air, our parcel is free to climb as it morphs into cumulonimbus clouds. It’s reached the “level of free convection,” or LFC in meteo-speak. A convective sigmet could soon be in the offing. Callouts for LFCs—yep, in meters—are also common.

click on image for captions

A “loaded gun” profile showing how you “break the cap.” The parcel analysis shows surface temperature cooling dry adiabatically, and dew point following saturation mixing ratio lines. They close at the LCL, creating clouds. Then temperature rises moist adiabatically until it pushes through the “cap” at the LFC, and goes on to rise in colder air to create thunderstorms. Here’s a screenshot from the SkewTLogPro app, showing two likely cloud layers at the points where the temperature and dew point traces narrow. The text block shows a very low CAPE value (20), a high lifted index (+8), and a parcel trace showing its temperature much lower than the surrounding air. Winds aloft are shown using both wind barbs and a wind velocity speed scale. All indications point to dense clouds up to the 700 mb/10,000-foot level, and freezing temperatures from the surface to the tropopause. This is a profile of a snow situation. Temperatures aloft fall to as low as -62 degrees Celsius at the 20 mb/40,000-foot level. Here’s a freezing rain setup. Rain droplets falling through the above-freezing “warm nose” quickly reach the freezing point at approximately 1,000 feet agl. There’s not enough altitude or time for the droplets to freeze solid as sleet/ice pellets, so the droplets remain supercooled—below freezing, yet still liquid. Result: The droplets freeze solid in a clear icing event.

2110p_SkewT.inddSkew-T sources

If you’ve made it this far, I think it’s safe to assume you have true weather-geek tendencies. Welcome to the club. The next step is to check out some Skew-Ts on your own. The National Weather Service releases weather balloons at 81 stations twice a day (00 and 12Z). The balloons take about two hours before they reach their maximum altitude of 115,000 feet, at which point they break and fall to Earth. You can see their soundings at weather.uwyo.edu/upperair/sounding.html or spc.noaa.gov/exper/soundings. However, several weather websites let you click on any point, then call up future Skew-Ts. Pivotal Weather’s GFS models (home.pivotalweather.com) can show Skew-Ts 384 hours into the future. Tropical Tidbits (tropicaltidbits.com/analysis/models), and TwisterData (twisterdata.com) also provide future Skew-Ts with a mouse-click, as do others. There’s even a Skew-T app—SkewTLogPro—that lets you view current and future Skew-Ts along your route of flight.

That’s a lot of Skew-Ts, but if it’s not enough to quench your inner geek, rest assured that there’s plenty more to them than the relatively breezy review I’ve portrayed here. Still, knowing these basics puts you ahead of most pilots. You never know—being able to scope out a Skew-T Log-P chart may come in handy someday, and can’t help but boost your overall weather savvy.

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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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