Thin air and its problems

High flight levels offer speed but require caution as well.

Wispy cirri occur in the upper flight levels and indicate some moisture as well as strong and turbulent winds aloft at these altitudes.

Since small water droplets can remain in supercooled liquid state to around –40° F (–40°C), this means that there is a small potential for icing at these levels. However, icing here is most often encountered when flying into towering cumuli, where water from lower altitudes has been lifted rapidly.

The thinner native clouds that tend to form at these altitudes are most commonly composed of ice crystals and pose little threat of icing. However, it is always wise to keep an eye on the OAT whenever flying through clouds or precipitation, and the atmosphere you are flying through might be somewhat warmer than ISA.

But outside of towering cumuli, icing in the flight levels tends to be relatively minor. This is because at an ISA temperature of –5°F, the air is capable of only holding about 1/10 of the water it could hold at the ISA average surface temperature of 59°F (15°C). So, even when saturated with supercooled droplets, nonconvective icing in such cold temperatures tends to be moisture limited.

Variable altitudes

Another factor to consider when flying the flight levels is again related to atmospheric pressure. Since you are flying a constant pressure surface instead of a constant altitude, flying into a low-pressure area will place you at a lower absolute altitude than your pressure altimeter indicates.

Similarly, you will be higher than your altimeter indicates when you fly into a region of high pressure. While being higher than indicated is not a problem, since all nearby traffic will be in the same situation, being lower than indicated can be an issue in the lowest flight levels.

In a region of below-normal pressure, an aircraft cruising outside the flight levels at 17,000 could actually be flying at the same altitude as an intercepting aircraft flying FL180. In addition, IFR conditions that commonly accompany low pressure would prohibit seeing any conflicting traffic. While ATC will usually take such considerations into account, it would be wise of any pilot not in the flight levels to avoid them by at least a few thousand feet, and of those flying a pressure altitude to fly FL200 or higher if capable, especially around any lower pressure.

Towering cumuli and thunderstorms are encountered relatively frequently in the flight levels. Hot spots in the lower atmosphere can cause these convective cells to launch a great deal of heat and moisture up to the top of the troposphere rapidly—within a matter of a few minutes.

Just as with flight at lower altitudes, pilots in the flight levels should exercise the same cautions regarding flight near thunderstorms. It is generally recommended to avoid any storms by at least 20 nm because of the strong turbulence that can surround them and due to the large hail that such storms can toss out of their tops and sides. The convective turbulence and icing encountered in a storm penetration at FL200 or FL300 can be at least as severe as what might be encountered at 6000 or 10,000 ft MSL.

Fortunately, the relatively good visibility at the higher altitudes means it is usually not a problem to see and avoid these storm cells. Unfortunately, a strong cold front or mesoscale convective complex of storms can obscure embedded cells, even at altitude. Flight through these regions is not recommended without onboard weather radar, set to scan the entire convective column—not just what is straight ahead.

If you are relying on a feed from surface radars, ensure that it is set to composite scan, and be aware that the images may be several minutes old—an eternity in the life of a storm cell.

By far the most common adverse weather conditions a pilot is likely to encounter are created by the wind. As you leave the surface behind, you also leave a lot of the friction generated by the surface. This means the winds are able to move a bit faster. In addition, because wind is driven by pressure differences, and pressure differences tend to be greater at higher altitudes, higher-level winds will also tend to blow faster than lower-level winds.

As winds at different altitudes blow at different speeds, they move against each other, shearing off eddies. Aircraft flying through these regions of differential winds are likely to encounter at least mild buffeting. Severe to extreme turbulence, however, is encountered frequently around the edges of the jet stream.

The jet stream occurs in the altitudes between the troposphere and the stratosphere. It is the product of a large discontinuity in temperature, and thus in density, above the polar front of the midlatitudes.

The result is a narrow ribbon of fast-moving air that meanders between the warmer subtropical and cooler polar air masses. The jet is guided by the pressure patterns at around 300 mb, and may exhibit strong ridging or troughing. Any time the jet changes direction, it will also create a shear zone with the surrounding environment, and when the wind is blowing at over 100 kts, the eddies that are sheared away can produce turbulence capable of structural damage in even the largest aircraft.

Managing upper air weather

Flying at higher altitudes means that upper air weather charts are necessary in addition to surface charts. Since you are flying along constant-pressure surfaces, it is best to reference constant-pressure charts such as the 500, 300 or 200-mb charts, rather than constant-height charts, such as the 18,000 or 30,000-ft charts.

A number of websites provide converters that will translate a desired pressure altitude into a pressure value that you can then use to select the most appropriate charts. You can also begin by looking at an atmospheric sounding chart along your route of flight.

Sounding charts are a display of information gleaned from a weather balloon. They show temperature, pressure and wind speed at all altitudes through the lower atmosphere. Altitudes at which different pressures are found are usually given in meters along the side of the chart, while wind speed and direction is given as a wind barb along the other edge.

The temperature trace shows how rapidly temperatures may be rising or falling as you go higher in the atmosphere—key to finding the top of the troposphere, which, along with wind maxima, can also give you the location of the jet stream. Overall, however, these soundings can give you a good idea of the optimal flight level at which to cruise.

Once you have picked a favorable flight level and know the corresponding pressure, the appropriate upper air chart can be retrieved from any number of aviation weather websites. While both constant-altitude and constant-pressure charts will quickly show the highs and lows at a given vertical reference, the contours of the constant-pressure chart will show you the altitude at which that pressure can be found (usually in meters). This bit of information will give a pilot a very good idea as to how much variability there will be in altitude along the route flown.

In addition, these charts will show wind speed and direction, as estimated from weather balloons, and the pressure gradient. At these altitudes, winds always flow parallel to the contour lines. Rapid changes in direction or speed are easily identified, and indicate areas where turbulence may be an issue. Finally, the upper air charts also display temperatures aloft, which can help you determine the likelihood of any icing.

Taking your aircraft to cruise well above the majority of the adverse weather that often plagues low-level flight is an important part of avoiding delays due to enroute weather deviations. It is also an opportunity to take advantage of fast tail winds or perhaps just give your passengers a smooth ride.

But it is important to remember that even the flight levels are not devoid of weather conditions that can affect flight adversely. Perhaps more importantly, aviators should not be lulled into forgetting that at some point they'll have to descend back into the soup in order to land.

Karsten Shein is a climatologist with the National Climatic Data Center in Asheville NC. He formerly served as an assistant professor at Shippensburg University. Shein holds a commercial license with instrument rating.


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