AC 00-6B Aviation Weather: Thunderstorms
A thunderstorm is a local storm, invariably produced by a cumulonimbus cloud, and always accompanied by lightning and thunder, usually with strong gusts of wind, heavy rain, and sometimes with hail.
Thunderstorm cell formation requires three ingredients: sufficient water vapor, unstable air, and a lifting mechanism (see Figure 19-1).Sufficient water vapor (commonly measured using dewpoint) must be present to produce unstable air. Virtually all showers and thunderstorms form in an air mass that is classified as conditionally unstable.
A conditionally unstable air mass requires a lifting mechanism strong enough to release the instability. Lifting mechanisms include: converging winds around surface lows and troughs, fronts, upslope flow, drylines, outflow boundaries generated by prior storms, and local winds, such as sea breeze, lake breeze, land breeze, and valley breeze circulations.
Thunderstorm Cell Life Cycle
A thunderstorm cell is the convective cell of a cumulonimbus cloud having lightning and thunder. It undergoes three distinct stages during its life cycle: towering cumulus, mature, and dissipating. The total life cycle is typically about 30 minutes.
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 feet per minute.
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 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.
Thunderstorm Types
There are three principal thunderstorm types: single cell, multicell (cluster and line), and supercell.
A single cell (also called ordinary cell) thunderstorm consists of only one cell. It is easily circumnavigated by pilots, except at night or when embedded in other clouds. Single cell thunderstorms are rare; almost all thunderstorms are multicell.
A multicell cluster thunderstorm 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).
Sometimes thunderstorms will form a squall line that can extend laterally for hundreds of miles. New cells continually re-form at the leading edge of the system with rain, and sometimes hail, following behind. Sometimes storms which comprise the line can be supercells. The line can persist for many hours (or more) as long as the three necessary ingredients continue to exist.
A supercell thunderstorm is an often dangerous convective storm that consists primarily of a single, quasi-steady rotating updraft that persists for an extended period of time. It has a very organized internal structure that enables it to produce especially dangerous weather for pilots who encounter them. Updraft speeds may reach 9,000 feet per minute (100 knots). This allows hazards to be magnified to an even greater degree. Nearly all supercells produce severe weather (e.g., large hail or damaging wind) and about 25 percent produce a tornado. A supercell may persist for many hours (or longer).
Hazards
A thunderstorm can pack just about every aviation weather hazard into one vicious bundle. These hazards include: lightning, adverse winds, downbursts, turbulence, icing, hail, rapid altimeter changes, static electricity, and tornadoes.
A microburst is particularly dangerous during landing if the pilot has reduced power and lowered the nose in response to the headwind shear. This leaves the aircraft in a nose-low, power-low configuration when the tailwind shear occurs, which makes recovery more difficult. It can cause the airplane to stall or land short of the runway.
Rapid Altimeter Changes
Pressure usually falls rapidly with the approach of a thunderstorm, then rises sharply with gust frontal passage and arrival of heavy rain showers in the cold downdraft, falling back to normal as the storm moves away. This cycle of pressure change may occur in as little as 15 minutes.
Test you knowledge of thunderstorms.