Thunderstorm life cycle.

Thunderstorm cell formation needs three ingredients: sufficient water vapor, unstable air, and a lifting mechanism (see Figure 22-1).

Virtually all showers and thunderstorms form in an air mass that is classified as conditionally unstable. A conditionally unstable air mass needs a lifting mechanism strong enough to release the instability.

It undergoes three distinct stages during its life cycle (see Figure 22-2): towering cumulus, mature, and dissipating. The total life cycle is typically about 30 minutes.

It begins with the cumulus stage, in which lifting action of the air begins. If sufficient moisture and instability are present, the clouds continue to increase in vertical height. Continuous, strong updrafts prohibit moisture from falling.

The distinguishing feature of the towering cumulus stage is a strong convective updraft. The updraft is a bubble of warm, rising air concentrated near the top of the cloud, which leaves a cloudy trail in its wake. Updraft speeds can exceed 3,000 fpm.

A cumulus cloud begins to grow vertically, perhaps to a height of 20,000 feet (6 km). Air within the cloud is dominated by upwardly-moving, warm, moist air currents called updrafts with some turbulent eddies around the edges.

The cell transitions to the mature stage when precipitation reaches the surface. Precipitation descends through the cloud and drags the adjacent air downward, creating a strong downdraft alongside the updraft. The downdraft spreads out along the surface, well in advance of the parent thunderstorm cell, as a mass of cool, gusty air. The arc-shaped leading edge of downdraft air resembles a miniature cold front and is called a gust front.

Cumulonimbus tops frequently penetrate into the lower stratosphere as an overshooting top, where strong winds aloft distort the cloud top into an anvil shape. Weather hazards reach peak intensity toward the end of the mature stage.

The storm has considerable depth, often reaching 40,000 to 60,000 feet (12 to 18 km). Strong updrafts and downdrafts coexist. This is the most dangerous stage, when tornadoes, large hail, damaging winds, and flash flooding may occur.

Once the vertical motion near the top of the cloud slows down, the top of the cloud spreads out and takes on an anvil-like shape. At this point, the storm enters the dissipating stage.

The dissipating stage is marked by a strong downdraft embedded within the area of precipitation. Subsiding air replaces the updraft throughout the cloud, effectively cutting off the supply of moisture provided by the updraft. Precipitation tapers off and ends. Compression warms the subsiding air and the relative humidity drops. The convective cloud gradually vaporizes from below, leaving only a remnant anvil cloud.

Light rain and weak outflow winds may remain for a while during this stage before leaving behind just a remnant anvil top.

All thunderstorms have conditions that are hazards to aviation. These hazards occur in numerous combinations. While not every thunderstorm contains all hazards, it is not possible to visually determine which hazards a thunderstorm contains. Hazards include low ceiling and visibility, lightning, adverse winds, downbursts, turbulence, icing, hail, rapid altimeter changes, static electricity, tornadoes, and engine water ingestion.

Turbulence is present in all thunderstorms, and severe or extreme turbulence is common. A severe thunderstorm can destroy an aircraft. Gust loads can be severe enough to stall an aircraft at maneuvering speed or to cause structural damage at cruising speed. The strongest turbulence within the cloud occurs between updrafts and downdrafts.

The low-level wind-shear zone between the gust front and surrounding air is very turbulent airspace.

A downburst or microburst is dangerous to aircraft when climbing from takeoff or approaching to land. During these phases, the aircraft is operating at slow speeds. A major change of wind velocity can lead to loss of lift and a crash.

Hail competes with turbulence as the greatest thunderstorm hazard to aircraft.

Thunderstorms produce abundant supercooled liquid water above the freezing level.

The abundance of supercooled water makes clear icing very rapid between 0°C and -15°C, and encounters can be frequent in a cluster of cells.

Every thunderstorm produces lightning and thunder by definition.

Nearby lightning can blind the pilot, rendering the pilot momentarily unable to navigate either by instrument or by visual reference.

Generally, visibility is near zero within a thunderstorm cloud.

Pressure usually falls rapidly with the approach of a thunderstorm. Pressure then usually rises sharply with the onset of the first gust and arrival of the cold downdraft and heavy rain, falling back to normal as the thunderstorm passes. This cycle of pressure change may occur in 15 minutes. If the pilot does not receive a corrected altimeter setting, the altimeter may be more than 100 ft in error.

Single-cell thunderstorms are rare; almost all thunderstorms are multicell.

A multicell cluster thunderstorm (see Figure 22-3 and Figure 22-4) consists of a cluster of cells at various stages of their life cycle. With an organized multicell cluster, as the first cell matures, it is carried downwind, and a new cell forms upwind to take its place. A multicell cluster may have a lifetime of several hours (or more).

Thunderstorms are barriers to air traffic because they are usually too tall to fly over, too dangerous to fly through or under, and can be difficult to circumnavigate. Weather recognizable as a thunderstorm should be considered hazardous, as penetration of any thunderstorm can lead to an aircraft accident and fatalities to those on board.

It is impossible to fly over thunderstorms in light aircraft.

Flying under thunderstorms can subject aircraft to rain, hail, damaging lightning, and violent turbulence.