Wx Watch: Instability for Idiots

Meteorologically speaking, there are many factors that can affect the weather along your route of flight. These include jet stream positions, frontal activity, regionalized semipermanent weather features, temperature contrasts, and the flows of air around high- and low-pressure centers. Those broad-brush descriptors imply lots of other weather mechanics — especially when you consider that they're all thrown together in a chaotic stew that science has yet to fully comprehend.

For pilots, however, this stew brings forth one major implication that must be checked prior to each flight. We're talking about atmospheric instability (or, hopefully, the lack of it).

It's at this point that most pilots, be they students or seasoned ATPs, experience uncontrollable head-lolling, followed by lapses into REM sleep. The mere mention of lapse rates and stability seems to bring on this curious somniferous effect.

That's too bad, because the stability or instability of the air along your route is perhaps the most significant factor in the weather you'll encounter.

What is it?

We've all heard about dry adiabatic lapse rates from our earliest ground school sessions. This refers to the drop in temperature that occurs as air rises. The dry adiabatic lapse rate is three degrees Celsius per 1,000 feet. (I know, I know, there's a moist adiabatic lapse rate, too. It ranges from 1.1 to 2.8 degrees Celsius per 1,000 feet. But we're trying to simplify things here, remember?)

Air cools with altitude because as it rises it encounters lowering pressure. In trying to match the lower pressure, the rising air's molecules expand. This expansion requires energy, which is given up in the form of a temperature drop. That's why there's cooling with altitude, and that's why there's a lapse rate.

You can plot this lapse rate on a chart. Take a gander at the figure on this page, and you'll see the standard lapse rate mapped out with regard to temperature and altitude.

Visualizing stability and instability

You're flying in stable air whenever the actual lapse rate is less than the standard lapse rate. If the lapse rate is greater than standard — i.e., air cools at rates much higher than standard — then you've got an unstable setup. The figure on page 115 delineates these areas.

Vertical profiles

The charts you've seen are for demonstration purposes only. The truth is that the atmosphere's vertical stability and temperature profile is rarely that neat and clean. Instead, the atmosphere's vertical profile is made up of layers of air that can alternate among being stable, unstable, and neutrally stable. This profile changes with frontal passages and convective events, to be sure, but it can also change from hour to hour during a fair-weather day.

For example, when high pressure conditions prevail, the lower levels of the atmosphere are stable in the early morning. An inversion can form, in which the previous evening's cold air is trapped beneath warmer air aloft — beginning at, say, 1,000 to 2,000 feet agl or so. Above that altitude, a normal lapse rate prevails.

But as the sun's heat builds during the afternoon, the surface temperature warms up and the lower levels of the atmosphere undergo huge changes. By the late afternoon, the low-level lapse rate goes into the unstable zone. That's when rising currents of heated air can cause bothersome turbulence, or worse.

Rising parcels and thermal bubbles

Warm air rises and cool air descends, right? That statement's good as far as it goes, but how does it apply to flying weather? The answer lies in the lapse rates, of course. That, and the heat of rising air parcels relative to the temperature of the air around them.

Let's say it's a hot, muggy day and rawinsonde (weather balloon) soundings show that there's an unstable situation aloft. The air is cooling rapidly with altitude. Now let's say a 100-degree thermal bubble breaks free of a shopping mall's parking lot and rises into this relatively cooler air. The result? The thermal bubble, being much hotter than the surrounding air it encounters on the way up, keeps on rising faster and faster. This could be the beginning of a thunderstorm.

Alternatively, if rising air masses aren't so hot and enter regions with tamer lapse rates, then you shouldn't expect much in the way of convection.

In other words, the rising air and condensation that cause thunderstorms and other convective events can come about only when the rising air is warmer than the air it's traveling through.

Sailplane pilots like unstable air and thermal bubbles gone slightly haywire because it gives them the kind of lift they need to reach satisfying altitudes and remain aloft for longer periods of time. Hang around a sailplane field at dawn and you'll probably see a lone towplane — sans sailplane — make the first flight of the day. Its pilot isn't up there solely for kicks. He or she is recording temperatures at various altitudes, in order to plot a sounding specific to the area at hand. Those on the ground are hoping for conditions a tad on the unstable side.

The bad news

That's all fine for sailplane pilots flying on thermally active fair-weather days. (At least, they may start out as fair-weather days; by 4 or 5 p.m. those thermals can morph into spark-belching supercells.) But for the rest of us, unstable air is generally a bad thing.

Instability's worst effects strike in the form of moderate to severe turbulence, which can cause structural damage or which, in the form of embedded or isolated thunderstorms, can also break an airplane. These events can show up whenever there's saturated air and lifting activity, be it from strong thermal contrasts during frontal passages, upslope flows of air, or isolated thermal cells. In other words, unstable air is present in quite a number of the adverse weather situations that pilots can encounter.

How to find it

Luckily for us, finding areas of unstable air is not difficult. From a flight service briefer, a weather-by-fax service, DUATS, or Internet sources, you can easily locate areas experiencing unstable conditions. Here are a few of the weather products you can ask for:

Severe weather outlook chart (AC). This is a two-panel chart that warns of the possibilities of general and severe (i.e., 50-knot surface winds, three-quarter-inch hail, or tornadoes) thunderstorms and identifies the areas of coverage. Such severe weather outlook areas are designated with hatched lines. Since thunderstorms imply instability, the AC will show you where the most threatening areas are.

Composite moisture stability chart. This really comprises four charts, presented together. The chart with stability information is called the "Lifted Index" panel. (The others show freezing levels, precipitable water, and average relative humidity.) On this panel you'll see station models that consist of one number atop another — sort of like a fraction. The top number is the lifted index. If it's less than zero, then thunderstorms are possible. If it's minus 4 or less, then severe thunderstorms are possible. Areas with lifted indices higher than 10 mean stable air and clear skies. Areas with similar values are connected by contour lines.

Average relative humidity panel. This can also provide useful hints about the weather to come. Relative humidities less than 60 percent usually mean clear or scattered clouds. Between 60- and 80-percent relative humidity, expect an overcast. Above 80 percent, that overcast will likely generate rain, especially as the percentage trends toward the 100-percent mark.

Area forecasts and significant weather prognosis (prog) charts. The text that accompanies these reports will mention any significant instability, along with reasons why.

Winds and temperatures aloft (FDs). By checking the temperatures aloft, you can develop a crude idea of the atmosphere's vertical temperature profile. A quick shortcut for determining standard temperatures (in Celsius) is to double the altitude, subtract 15, and change the resultant number into a negative figure (use positive numbers for lower altitudes during warmer months). For example, let's say you want to know the standard temperature for 10,000 feet. Two times 10 is 20. Subtract 15 and you come up with five degrees Celsius. Change to a negative number and you come up with a standard temperature of minus five degrees Celsius. (The actual standard temperature for that altitude is minus 4.812 degrees Celsius, so minus five degrees is close enough.) With colder-than-standard temperatures aloft and a source of moisture and lifting, you have a strong hint that the air is ripe for fireworks.

Analyzing on the fly

Forecasts change, so by the time you fly your briefing information may be dated. How can you tell that you're flying into a dangerously unstable air mass? The first clue will be turbulence. The second will be cumulus clouds, rising higher and higher. A third might be rainshowers. You say that you haven't yet experienced these phenomena but would like to make an educated guess about the stability situation? Then look around for cumulus clouds and check the ATIS, AWOS, and ASOS frequencies along your route, noting the ambient air and dew point temperatures. Then check your OAT gauge. If cumulus clouds are forming, surface temperatures are on the rise, dew points are above 60 degrees Fahrenheit, and your OAT shows colder-than-standard temperatures aloft, then you've got the moisture and temperature ingredients necessary for convection. Depending on your experience and qualifications, your airplane's capabilities, and your tolerance for risk, it may well be time to plan for a diversion and landing at an alternate airport.

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