WX BRIEF

Updated essentials about thunderstorms that every pilot flying IFR needs to know

We often share the skies with these giant forces of nature, but complacency can be dangerous and caution is always advisable.




Wedge-shaped wall cloud descends from a thunderstorm moving onshore near Beaufort NC on Jul 31, 2012. Wall clouds often occur beneath the rain-free main updraft region of the storm. They usually indicate a strong storm and occasionally rotate as a mesocyclone forms within a severe supercell storm.

Even the most aggressive anti-ice and deicing measures may become overwhelmed by the accretion rates.

While generally unlikely to result in a fatal crash, lightning, hail and heavy precipitation are also dangers from thunderstorms. A single storm may produce dozens of lightning strokes per minute, and many of those strokes may extend a mile or more out from the storm.

Modern aircraft are designed to chan­nel lightning strikes away from sensitive electronic and fuel systems, but for a pilot in a thunderstorm, frequent strokes in close proximity can be disorienting and even cause temporary blindness and deafness, making it hard to continue flying the aircraft.

Hail encounters are the most frequent cause of aircraft damage from thunderstorms. Larger storms with a freezing level below about 12,000 ft are favored for hail, which is why hail is more common in higher-latitude storms than the more frequent tropical and subtropical storms.

Hail forms as a water droplet is carried above the freezing level by the storm's updraft and freezes. It can be ejected from the storm or fall back into a region of supercooled droplets or above-freezing temperatures, where it accretes a new layer of water that freezes on its next trip in the storm's updraft.

Since even weak updrafts can sustain a hail cycle, stronger updrafts can easily hurl hailstones many miles outside of the storm itself, so pilots should not only avoid thunderstorms, but give them a wide berth.

Lastly, heavy precipitation is not usually considered an aviation danger, but in extreme cases such as can be encountered in a strong storm, the quantity of rain ingested into an engine can be sufficient to cause engine failure. Normally, the engine can be restarted once the rain shaft is exited, but the engine failure can add to cockpit workload and result in loss of altitude or even control if not addressed.

In addition, heavy rain can reduce visibility to zero in an instant, which at low altitude could cause a pilot to lose visual on the runway. It can also temporarily flood the runway, increasing the risk of hydroplaning on takeoff or landing.

To maintain clearance from any thunderstorms that may occur along your route of flight, it is important to begin with a weather briefing. Find out if convection is forecast and, if so, where and when. If specific regions are indicated as having a high likelihood of storms, try to avoid flying those areas.

Also, while ATC should provide you with inflight weather advisories, it is up to you to ensure you ask for them if they are not forthcoming. In addition, not all of ATC's equipment is equal in terms of detecting and locating precipitation, and in some cases may be delayed by several minutes.

Similarly, radar information from subscription weather services is slightly delayed, and lightning detectors can only show you where lightning may be occurring, not the extent of the cell itself. If you are flying close to a cell, these limitations may result in choosing a path that takes you into adverse conditions instead of through a possible clear area.

The best bet is to avoid any known storm cells by at least 20 miles, and if you find yourself flying on instruments in an area of embedded cells and you cannot positively locate them in real time—such as with onboard weather radar—it may be wise to turn back to clear air and either find another route or land and wait it out. With few exceptions, most storm cells will either move on past an area or die out within an hour.

Similarly, postpone any takeoff or landing if your airport is surrounded by storms and your flightpath will take you within a few miles of an active or developing cell. Also if you must fly through a gap between storms, confirm first that a new cell is not building to fill that gap as you are approaching it. Lastly, if you do encounter adverse conditions, file a Pirep to help controllers keep other pilots out of harm's way.

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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