Aviation Weather Hazards Related to Thunderstorms

Out of all of the specific weather hazards, thunderstorms are perhaps the greatest threat to pilots because they can be your one-stop-shop for danger. When discussing thunderstorms, lightening and turbulence aren't your only enemy. They can produce anything from large hail to tornadoes and microbursts, all of which can be deadly.

Not all thunderstorms exude these events; three conditions must be present: medium to high moisture content, any amount of air instability and some lifting action. The lifting action can be caused by changes in terrain elevation, frontal changes or temperature increases and decreases.

There are two types of thunderstorms - airmass thunderstorms and severe thunderstorms. Airmass thunderstorms are the most common. They have a tendency to pop up on summer afternoons or during coastal evenings, can be short in duration and are usually scattered. Severe thunderstorms as are more violent and produce wind gusts starting at 50 knots or greater, produce hail ¾ inches in diameter or larger and can also turn out tornadoes.

Thunderstorms have three stages or a life cycle - the cumulus stage, mature stage and dissipating stage. These three stages are seen in each and every thunderstorm formation. The cumulus stage has continuous updrafts as great as 3,000 feet per minute (Abbott & Kailey 2004). The storm matures when the cloud becomes too dense to be supported by the drafts of the previous stage.

When you picture a fully-formed thunderstorm cloud, you are thinking of this, the most violent stage of the storm. One side of the storm still has warm updrafts while the leading edge of the storm has cool precipitation. Also at the leading edge are strong winds. In the dissipating stage, the cloud ceiling rises at least 10,000 feet and the updraft is found exclusively within the cloud. The entire cloud now sheds rain and accompanying down drafts.

A thunderstorm can present itself as single-cell, super-cell or multi-cell. We've all seen pictures of the updraft of a thunderstorm. They start out rather flatly at a low altitude and vertical updrafts cause the storm to billow up beyond 30,000 feet. A single-cell thunderstorm isn't anything to sniff at and lasts under an hour. Super-cell thunderstorms are quite severe and can last up to two hours.

Multi-cell storms are condensed groups of thunderstorms in different stages of the life cycle. This grouping causes the storms to interact with each other and therefore last much longer. Sometimes these groupings cause what's known as squall lines. One misconception is that quall lines are always equated with a front. While this is often the case, these squalls can happen anywhere. When they are indeed frontal thunderstorms, they develop 50 to 300 miles in front of a fast-moving cold front. (Abbott & Kailey 2004)

Thunderstorm-specific hazards:

Lightening - Reports submitted by PIREPs and Weather offices with live lightening trackers.

Lightening almost goes without saying. Lightening can be categorized as in-cloud, cloud-to-cloud, cloud to ground and between cloud and clear air although rare. No matter which type lightning's voltage is about 300,000 volts per foot and is an astounding 50,000°F. (Abbott & Kailey 2004) This heat causes the surrounding air to expand and a corresponding shock wave that we know as thunder.

Lightening strikes can cause external damage to the aircraft but rarely causes catastrophic damage to airplane or crew. It can temporarily blind pilots at night and it can effect the aircraft's navigational equipment.

Turbulence - METAR not applicable. Reports are submitted as a PIREP.

While turbulence isn't exclusive to thunderstorms, it is an expected and predictable phenomenon unlike unexpected clear-air turbulence. The Thunderstorm-related turbulence occurs right where the updrafts and down drafts combine in. Anyone who as flown through turbulence can relate to the jerky up and down sensation can relate to this.

Turbulence, nicknamed "chop" can be light, moderate or severe. When reporting turbulence to air traffic control from the air, there are no set rules in determining the severity. Pilots leave it up to common sense and previous experience when reporting light or moderate chop, although experts would say moderate chop can be felt when the updraft/downdraft gusts are around 30 knots. Severe isn't usually experience by pilots without their Instrument Flight Rating unless they experience clear-air turbulence, as they stay away from storms and clouds in general. IFR flights are usually channeled away from reported severe turbulence by air traffic control.

Microburst - no METAR code, reported by PIREPS and can be put in the remarks section of speci [sic] weather reports

Microbursts are perhaps one of the most feared weather occurrences out there. They are low in altitude and can occur in an area of 1 nautical mile or less horizontally and 1000 feet vertically (Abbott & Kailey 2004). Aircraft landing or taking off or low-flying aircraft are most likely to run into this phenomenon.

Microbursts are deadly because the winds can increase upwards of 100 knots but the real danger lies in the downbursts of up to 6000 feet per minute. Many commercial flights over the years have crashed due to microbursts, such as Delta Air Lines Flight 191 in 1985 and USAir Flight 1016 in 1994 - both happened while on approach to their respective runways.

Hail - METAR GR (Small hail is GS along with snow pellets)

Hail is caused by rain drops being picked up by the updrafts of a thunderstorm which are then frozen at high altitudes. Once they become too heavy to be supported by the updrafts the chunks of hail (anywhere from the size of a raindrop to a baseball or larger depending on how many friends it gathered on the way up) fall to the ground at a high velocity, over 100 MPH. Three-quarters of an inch or more is all the girth that's needed to cause structural damage to an airplane (Abbott & Kailey 2004).

Tornadoes - METAR FC for funnel clouds, tornadoes or water spouts

Tornadoes or funnel clouds can be predicted and spotted by local weather offices using Doppler Radar. The weather information is then sent to local air traffic control facilities and ARTCCs. The controllers will then do their best to control aircraft away from these violent storms. Low-flying, unaware, VFR aircraft would be in most danger as tornadoes are formed at cloud bases and VFR flights are not always speaking to a tower or arrival/approach facility, though pilots without an IFR rating wouldn't be allowed in areas with weather bad enough anyway. Tornadoes are caused by the rotation caused by thunderstorms. Funnel clouds are called tornadoes once they touch the ground. The wind speeds within a tornado can exceed 200 knots.

Severe weather can be avoided by obtaining a thorough and current weather report from an airport's weather office, 1-800-WX-BRIEF or an AFSS before taking off. Weather conditions are updated at least every hour on the airport ATIS. Air traffic controllers will advise all IFR flights of any changes as they are reported.

Source:

Mike Abbott & Liz Kailey et al. 2004. Colorado: Jeppesen-Sanderson.