Chapter 22, Thunderstorms 22-1
22 Thunderstorms
22.1 Introduction
A thunderstorm is a local storm, invariably produced by a cumulonimbus (CB) cloud, and always
accompanied by lightning and thunder, usually with strong gusts of wind, heavy rain, and sometimes hail.
There are as many as 40,000 thunderstorm occurrences each day worldwide, and the United States certainly
experiences its share.
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.
Chapter 22, Thunderstorms 22-2
22.2 Necessary Ingredients for Thunderstorm Cell Formation
Thunderstorm cell formation needs three ingredients: sufficient water vapor, unstable air, and a lifting
mechanism (see Figure 22-1). 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 needs 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.
Figure 22-1. Necessary Ingredients for Thunderstorm Cell Formation
22.3 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 (see Figure 22-2): 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 fpm.
Chapter 22, Thunderstorms 22-3
Figure 22-2. Thunderstorm Cell Life Cycle
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. Uplift along the gust front may trigger the formation of new cells, sometimes well ahead of the
parent cell. Cumulonimbus tops frequently penetrate into the lower stratosphere as an overshooting top,
where strong winds aloft distort th e cloud top into an anvil shape. Weather hazards reach peak intensity
toward the end of the mature 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.
22.4 Thunderstorm Types
There are three princip al thunderstorm types: single -cell, multicell (cluster and line), and supercell. All
thunderstorms are hazardous to aircraft.
A single-cell or common (also called ordinary -cell) thunderstorm consists of only one cell. This type of
thunderstorm often develops on warm and humid summer days. These cells may be severe and produce hail
and microburst winds . Its life cycle was covered in the previous section. 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 (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). New cells will continue to form as long as the three necessary ingredients exist (see Section 22.2).
Individual cells within the cluster may move in one direction while the whole system moves in another . It
can cover large areas and its persistence make s it a bit tougher to circumnavigate than a single -cell
thunderstorm. An area of multicell cluster thunderstorms can be like a minefield for air traffic.
Chapter 22, Thunderstorms 22-4
Figure 22-3. Multicell Cluster Thunderstorm
Sometimes thunderstorms will form in a narrow band or squall line that can extend laterally for hundreds
of miles. Often it develops on or ahead of a cold front in moist, unstable air, but it may develop in unstable
air far removed from any front. 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
(see Section 22.2). These squall lines are the thunderstorm type which presents the most effective barrier
to air traffic , because the line is us ually too tall to fly over, too dangerous to fly through or under, and
difficult to circ umnavigate. About 25 percent of all tornadoes in the United States are spawned by
squall lines.
Figure 22-4. Multicell Line Thunderstorm
Chapter 22, Thunderstorms 22-5
A supercell thunderstorm (see Figure 22-5) is an often dangerous, long-lived 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 fpm (100 kt). 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). New cells will continue
to form as long as the three necessary ingredients exist (see Section 22.2).
A supercell’s size and persistence make it a bit tougher to circumnavigate than a single-cell thunderstorm.
Also, multicell clusters and lines may have supercells incorporated as part of the system as well.
Figure 22-5. Supercell Thunderstorm
22.5 Factors that Influence Thunderstorm Motion
A thunderstorm is a process, not a solid object or block of wood. Storm motion equals the combined effects
of both advection and propagation (see Figure 22-6). Advection is the component of storm motion due to
individual cells moving with the average wind throughout the vertical depth of the cumulonimbus cloud.
The wind at FL180 (500 mb) usually provides a good approximation. Propagation is the component of
storm motion due to old cell dissipation and the new cell development. Storm motion may deviate
substantially from the motion of the individual cells, which comprise the storm.
Chapter 22, Thunderstorms 22-6
Figure 22-6. Factors that Influence Thunderstorm Motion
Individual cells that comprise the storm move northeast (advection ) but dissipate and are replaced by
new cells (propagation). Storm motion equals the combined effects of both advection and propagation.
22.6 Thunderstorm Terminology
Anvil. The flat, spreading top of a cumulonimbus cloud, often shaped like an anvil. Thunderstorm anvils
may spread hundreds of miles downwind from the thunderstorm itself and sometimes may spread upwind.
Bow Echo. A radar echo that is linear but bent outward in a bow shape.
Derecho. A widespread, long-lived, straight-line windstorm that is associated with a fast -moving band of
severe thunderstorms.
Downdraft. A small-scale column of air that rapidly sinks toward the ground, usually accompanied by
precipitation as in a shower or thunderstorm. A microburst is the result of a strong downdraft.
Gust Front. The leading edge of gusty surface winds from thunderstorm downdrafts that is sometimes
associated with a shelf cloud or roll cloud. May also be referred to as a gustnado or outflow boundary.
Mesoscale Convective System (MCS). A complex of multiple thunderstorms that becomes organized on
a scale larger than the individual thunderstorms but smaller than extratropical cyclones and normally
persists for several hours or more.
Roll Cloud. A low, horizontal tube -shaped arcus cloud associated with a thunderstorm gust front. Roll
clouds are relatively rare; they are completely detached from the thunderstorm base or other cloud features,
thus, differentiating them from the more familiar shelf clouds.
Severe Thunderstorm. A thunderstorm that produces hail with a diameter of one inch (U.S. quarter size)
or larger, convective winds of 50 kt (58 mph) or greater, and/or tornadoes.
Shelf Cloud. A low, horizontal wedge -shaped cloud associated with a thunderstorm gust front. Unlike a
roll cloud, a shelf cloud is attached to the base of the parent cloud above it, which is usually a thunderstorm.
Updraft. A small-scale current of rising air. If the air is sufficiently moist, then the moisture condenses to
become a cumulus cloud or an individual tower of a towering cumulus or cumulonimbus.
Chapter 22, Thunderstorms 22-7
22.7 Hazards
All thunderstorms have conditions that are hazard s 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.
22.7.1 Low Ceiling and Visibility
Generally, visibility is near zero within a thunderstorm cloud. Ceiling and visibility also may be restricted
in precipitation and dust between the cloud base and the ground. The restrictions create the same problem
as all ceiling and visibility restrictio ns, but the hazards are increased when associated with the other
thunderstorm hazards of turbulence, hail, and lightning that make precision instrument flying virtually
impossible.
22.7.2 Lightning
Every thunderstorm produces lightning and thunder by definition. Lightning is a visible electrical discharge
produced by a thunderstorm. The discharge may occur within or between clouds, between a cloud and air,
between a cloud and the ground, or between the ground and a cloud.
Lightning can damage or disable an aircraft. It can puncture the skin of an aircraft , and it can damage
communications and electronic navigational equipment. Lightning has been suspected of igniting fuel
vapors causing an explosion; however, serious accidents due to lightning strikes are extremely rare. Nearby
lightning can blind the pilot, rendering the pilot momentarily unable to navigate either by instrument or by
visual reference. Nearby lightning can also induce permanent errors in the magnetic compass . Lightning
discharges, even distant ones, can disrupt radio communications on low and medium frequencies. Though
lightning intensity and frequency have no simple relationship to other storm parameters, severe storms, as
a rule, have a high frequency of lightning.
22.7.3 Downburst and Microburst
The downward moving column of air in a typical thunderstorm is large. Convective clouds, shower cells,
and thunderstorm cells sometimes produce intense downdrafts called downbursts that create strong, often
damaging winds and wind shear. Downbursts (see Figure 22-7) can create hazardous conditions for pilots
and have been responsible for many LLWS accidents. Smaller, shorter -lived downbursts are called
microbursts.
Chapter 22, Thunderstorms 22-8
Figure 22-7. Downburst Life Cycle
A microburst (see Figure 22-8) is a small-scale, intense downdraft that, when reaching the surface, spreads
outward symmetrically (see Figure 22-9) or asymmetrically (see Figure 22-10), in all directions from the
downdraft center. It is the most severe type of wind shear. Microburst activity may be indicated by an
intense rain shaft at the surface, but virga (i.e., streaks of precipitation falling from a thunderstorm cloud
but not reaching the ground) at the cloud base and/or a ring of blowing dust is sometimes the only visible
clue (see Figure 22-11).
A typical microburst has a horizontal diameter of less than 2.5 mi and a nominal depth of 1,000 ft. The
lifespan of a microburst is about 5 –15 minutes , during which time it can produce downdrafts of up to
6,000 fpm; increasing headwind and headwind losses of 30–90 kt, seriously degrading performance. It can
also produce strong turbulence and hazardous wind direction changes.
Figure 22-8. Illustration of the Evolution of a Microburst