Tornadoes can occur almost anywhere in the world but are most common in the central and eastern United States during spring and autumn months. They typically last only a few minutes and travel a few miles, but can persist much longer (e.g., more than 90 minutes) and track much farther (e.g., more than 100 mi), in extreme cases.

On a local scale, the tornado is the most intense of all atmospheric circulations. Its vortex is typically a few hundred yards in diameter but can range in width from less than 10 yards (yd) to over 2 mi. Wind speeds are typically estimated on the basis of wind damage using the Enhanced Fujita (EF) Scale (see Table 22-1).

Tornadoes occur with both isolated and squall line thunderstorms. However, over 80 percent of all tornadoes in the United States are produced by supercell thunderstorms. Multiple tornado occurrences associated with a particular large-scale weather system is termed a “tornado outbreak.” On rare occasions, one supercell can produce multiple tornadoes over many hours. In addition, families of tornadoes have also been observed as appendages of the main cloud extending several miles outward from the area of lightning and precipitation. Thus, any cloud connected to a severe thunderstorm may contain hidden vortices.

An aircraft entering a tornado vortex is almost certain to suffer loss of control and structural damage. Since the vortex extends well into the cloud, any pilot inadvertently caught on instruments in a thunderstorm could encounter a hidden vortex.

Turbine engines have a limit on the amount of water they can ingest. Updrafts are present in many thunderstorms, particularly those in the developing stages. If the updraft velocity in the thunderstorm approaches or exceeds the velocity of the falling raindrops, very high concentrations of water may occur. It is possible that these concentrations can be in excess of the quantity of water that turbine engines are designed to ingest. Therefore, severe thunderstorms may contain areas of high water concentration, which could result in flameout and/or structural failure of one or more engines.

Airborne weather avoidance radar is, as the name implies, for avoiding severe weather—not for penetrating it. Whether to fly into an area of radar echoes depends on echo intensity, spacing between the echoes, and the capabilities of the pilot and the aircraft. The ability of airborne weather radar to detect weather phenomena is limited in both direction and range. Some airborne radars are fitted with a turbulence display mode, which is based on the Doppler effect. These Doppler radars can detect turbulence associated with precipitation (sometimes referred to as wet precipitation), but these radars are unable to detect clear-air turbulence (CAT). The radar display also does not provide assurance of avoiding instrument weather conditions from clouds and fog. A phenomenon called attenuation (see Section 15.2.5) may exist when a cell absorbs or reflects all of the radio signals sent by the radar system (see Figure 15-5). Attenuation may prevent the radar from detecting additional cells that might lie behind the first cell. This is sometimes referred to as a radar “shadow.” For aircraft equipped with airborne weather radar, pilots are expected to be familiar with the operating techniques and limitations of the specific system.

It is important to note that while hail always gives a radar echo, it may fall several miles from the nearest visible cloud, and hazardous turbulence may extend to as much as 20 mi from the echo edge.

Never regard any thunderstorm lightly, even when radar observers report the echoes are of light intensity. Avoiding thunderstorms is the best policy. The following is guidance for avoiding thunderstorms:

If unable to avoid penetrating a thunderstorm, the following is guidance for before entering the storm: