WX BRIEF

Thunderstorms

From isolated cells to squall lines, thunderstorms remain a consistent threat to pilots flying in unstable airmasses.


Lightning, icing and hail

Other hazards that occur in and around thunderstorms are lightning, icing and hail. Lightning is a requisite feature of a thunderstorm, as thunder is the shockwave from the lightning strike. Although modern aircraft skins and their electronic systems are generally well insulated from lightning damage, fiberglass radomes are the frequent recipients of lightning strikes as the stroke seeks the electronic components beneath, and can be completely blown away when hit. More likely, however, is that a nearby lightning stroke will temporarily blind a pilot who is looking out the window at the time.

Because most thunderstorms occur during warm weather, many pilots may forget that the vertical nature of thunderstorms means that a large proportion of the storm sits above the freezing level. That is a requisite of a thunderstorm, as most rain droplets begin as ice crystals that grow as they scavenge cloud droplets. Since icing can occur any time the OAT is below freezing and there is visible moisture, cumulonimbi are a prime candidate for accumulating icing. At higher levels in the storm, icing rates may exceed several cm per minute.

In addition to icing, the region of a thunderstorm above the freezing level is a birthing ground for hail. As falling rain droplets are pushed into updrafts, they can be carried upward to freeze. As they fall again, they gather another layer of water that freezes on the next trip to the upper part of the storm. In this way, several round trips can produce a sizable hailstone.

Even non-supercell storms can produce hailstones that are several centimeters in diameter – more than enough to shatter windscreens and damage fan blades. Importantly, the power of the updrafts in strong storms can launch hail many miles from the storm cloud, with common areas being beneath the storm's anvil and directly behind the storm. Overflying a storm is not recommended either, as hail is frequently ejected from the storm's top.

All of these threats are reason enough to avoid thunderstorms. AIM and the Pilots Handbook of Aeronautical Knowledge recommends at least 20 nm avoidance for severe thunderstorms or extreme or intense radar echoes. My rule of thumb is to stay 1 nm away for every 1000 ft of storm height, but always stay at least 20 nm from any storm.

These distances should be regarded as minimums, recognizing that larger storms are likely to manifest hazards to greater distances. And if possible, avoid ever overflying an active storm; if not, maintain a cruise altitude several thousand feet above the tops of any mature storm in the region. If you can't do that, avoid the region entirely.

Closing the gap

Another reason to avoid an area of active storms entirely is that gaps between storms can often close as new storms build within them. Unless you have a gap that keeps you at least 20 nm from either storm and no clouds growing beneath, it is best not to use it as a shortcut through the squall line. Furthermore, consider that avionics that provide feeds from surface-based radars only update every 5 minutes or so, meaning a clear space ahead on the screen may have filled in with a mature storm 2 minutes ago.

A final threat from thunderstorms is the downdraft outflow. Downdrafts often appear as a sudden pulse of fast-moving air that exits the base of the cloud, hits the ground and spreads out. These downbursts can extend several miles out from the storm itself and have contributed to several major aircraft accidents. It is best to avoid landing at an airport when an active thunderstorm is within about 2 nm from where you plan to touch down. In fact, many airports will suspend operations at such times.

While there is some debate about whether to turn around or fly straight through an inadvertent thunderstorm penetration, most experts believe it is better to maintain a wings-level attitude until exiting the storm.

The reason for this is that, even with airborne radar, you have no idea what has developed behind you, and any turn and bank increases wing loading, which in an extreme force environment could mean the difference between maintaining or losing positive control. In any case, do not worry about your altitude unless you're near the ground, as fighting the updrafts or downdrafts will only add to the stress on the airframe.

If you do find yourself in or around thunderstorms, fly the aircraft first and try to extract yourself from the area. In the face of an approaching line of storms, the wisest move may be to land and wait them out. Once you have cleared any danger, be sure to let ATC know what you encountered so they can alert other aircraft in the area.

Karsten Shein is a climatologist with NOAA 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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