St. Louis Indianapolis
200 miles
Columbus
400 miles
Pittsburgh
600 miles
METAR KSTL 1950Z 31023G40KT 8SM
SCT035 05/M03 A2976
METAR KIND 1950Z 29028G45KT 1/2SM TSRAGR
VV005 18/16 A2970
METAR KCMH 1950Z 16017KT 2SM BR
OVC080 11/10 A2970
METAR KPIT 1950Z 13012KT 75SM
BKN130 08/04 A3012
St. Louis Indianapolis Columbus Pittsburgh
1023
1020101710141011100610051002999999100210051006
1011 1014 1017 1020 1023
42
8 32
076
26
52
2
51
47
7
40
2
058 1428 34 200
2066
62
12
WARM AIR
CUMULONIMBUS
COLD AIR
NIMBOSTRATUS
ALTOSTRATUS
CIRROSTRATUS
CIRRUS
COLD AIR
Figure 12-27. Occluded front cross-section with a weather chart depiction and associated METAR.
warm front is colder than the air of the cold front. When this
is the case, the cold front rides up and over the warm front. If
the air forced aloft by the warm front occlusion is unstable,
the weather is more severe than the weather found in a cold
front occlusion. Embedded thunderstorms, rain, and fog are
likely to occur.
Figure 12-27 depicts a cross-section of a typical cold
front occlusion. The warm front slopes over the prevailing
cooler air and produces the warm front type weather. Prior
to the passage of the typical occluded front, cirriform and
stratiform clouds prevail, light to heavy precipitation falls,
visibility is poor, dew point is steady, and barometric pressure
drops. During the passage of the front, nimbostratus and
cumulonimbus clouds predominate, and towering cumulus
clouds may also form. Light to heavy precipitation falls,
visibility is poor, winds are variable, and the barometric
pressure levels off. After the passage of the front, nimbostratus
and altostratus clouds are visible, precipitation decreases, and
visibility improves.
Thunderstorms
A thunderstorm makes its way through three distinct stages
before dissipating. 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. Within approximately 15 minutes,
the thunderstorm reaches the mature stage, which is the most
violent time period of the thunderstorm’s life cycle. At this
point, drops of moisture, whether rain or ice, are too heavy
for the cloud to support and begin falling in the form of rain
or hail. This creates a downward motion of the air. Warm,
rising air; cool, precipitation-induced descending air; and
violent turbulence all exist within and near the cloud. Below
the cloud, the down-rushing air increases surface winds and
decreases the temperature. 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. This is when the downdrafts
spread out and replace the updrafts needed to sustain the
storm. [Figure 12-28]
It is impossible to fly over thunderstorms in light aircraft.
Severe thunderstorms can punch through the tropopause and
reach staggering heights of 50,000 to 60,000 feet depending
on latitude. Flying under thunderstorms can subject aircraft
to rain, hail, damaging lightning, and violent turbulence.
A good rule of thumb is to circumnavigate thunderstorms
identified as severe or giving an extreme radar echo by at
Cumulus Stage (3–5 mile height) Mature Stage (5–10 mile height) Dissipating Stage (5–7 mile height)
32 °F
0 °C
Equilibrium level
40,000 ft.
30,000 ft.
20,000 ft.
10,000 ft.
5,000 ft.
Figure 12-28. Life cycle of a thunderstorm.
least 20 nautical miles (NM) since hail may fall for miles
outside of the clouds. If flying around a thunderstorm is not
an option, stay on the ground until it passes.
For a thunderstorm to form, the air must have sufficient water
vapor, an unstable lapse rate, and an initial lifting action to
start the storm process. Some storms occur at random in
unstable air, last for only an hour or two, and produce only
moderate wind gusts and rainfall. These are known as air
mass thunderstorms and are generally a result of surface
heating. Steady-state thunderstorms are associated with
weather systems. Fronts, converging winds, and troughs
aloft force upward motion spawning these storms that often
form into squall lines. In the mature stage, updrafts become
stronger and last much longer than in air mass storms, hence
the name steady state. [Figure 12-29]
Knowledge of thunderstorms and the hazards associated with
them is critical to the safety of flight.
Hazards
All thunderstorms have conditions that are a hazard 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.
Squall Line
A squall line is a narrow band of active thunderstorms. 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. The line may be too long to detour easily and too
wide and severe to penetrate. It often contains steady-state
thunderstorms and presents the single most intense weather
hazard to aircraft. It usually forms rapidly, generally reaching
maximum intensity during the late afternoon and the first
few hours of darkness.
Tornadoes
The most violent thunderstorms draw air into their cloud
bases with great vigor. If the incoming air has any initial
rotating motion, it often forms an extremely concentrated
vortex from the surface well into the cloud. Meteorologists
have estimated that wind in such a vortex can exceed 200
knots with pressure inside the vortex quite low. The strong
winds gather dust and debris and the low pressure generates
a funnel-shaped cloud extending downward from the
cumulonimbus base. If the cloud does not reach the surface,
it is a funnel cloud; if it touches a land surface, it is a tornado;
and if it touches water, it is a “waterspout.”
Turbulence
Anvil
Storm movement
First gust
Roll cloud
Wind shear turbulence
Dust
Wind shear turbulance
Figure 12-29. Movement and turbulence of a maturing thunderstorm.
Tornadoes occur with both isolated and squall line
thunderstorms. Reports for forecasts of tornadoes indicate
that atmospheric conditions are favorable for violent
turbulence. 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 severe thunderstorm could
encounter a hidden vortex.
Families of tornadoes have 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 carries a threat of violence.
Turbulence
Potentially hazardous turbulence is present in all
thunderstorms, and a severe thunderstorm can destroy an
aircraft. Strongest turbulence within the cloud occurs with
shear between updrafts and downdrafts. Outside the cloud,
shear turbulence has been encountered several thousand feet
above and 20 miles laterally from a severe storm. A low-level
turbulent area is the shear zone associated with the gust front.
Often, a “roll cloud” on the leading edge of a storm marks the
top of the eddies in this shear, and it signifies an extremely
turbulent zone. Gust fronts often move far ahead (up to 15
miles) of associated precipitation. The gust front causes a
rapid, and sometimes drastic, change in surface wind ahead
of an approaching storm. Advisory Circular (AC) 00-54, Pilot
Windshear Guide, explains gust front hazards associated with
thunderstorms. Figure 2 in the AC shows a cross section of a
mature stage thunderstorm with a gust front area where very
serious turbulence may be encountered.
Icing
Updrafts in a thunderstorm support abundant liquid water
with relatively large droplet sizes. When carried above
the freezing level, the water becomes supercooled. When
temperature in the upward current cools to about –15 °C,
much of the remaining water vapor sublimates as ice crystals.
Above this level, at lower temperatures, the amount of
supercooled water decreases.
Supercooled water freezes on impact with an aircraft. Clear
icing can occur at any altitude above the freezing level, but at
high levels, icing from smaller droplets may be rime or mixed
rime and clear ice. The abundance of large, supercooled
water droplets makes clear icing very rapid between 0 °C and
–15 °C and encounters can be frequent in a cluster of cells.
Thunderstorm icing can be extremely hazardous.
Thunderstorms are not the only area where pilots could
encounter icing conditions. Pilots should be alert for icing
anytime the temperature approaches 0 °C and visible moisture
is present.
Hail
Hail competes with turbulence as the greatest thunderstorm
hazard to aircraft. Supercooled drops above the freezing level
begin to freeze. Once a drop has frozen, other drops latch on
and freeze to it, so the hailstone grows—sometimes into a
huge ice ball. Large hail occurs with severe thunderstorms
with strong updrafts that have built to great heights.
Eventually, the hailstones fall, possibly some distance from
the storm core. Hail may be encountered in clear air several
miles from thunderstorm clouds.
As hailstones fall through air whose temperature is above 0
°C, they begin to melt and precipitation may reach the ground
as either hail or rain. Rain at the surface does not mean the
absence of hail aloft. Possible hail should be anticipated
with any thunderstorm, especially beneath the anvil of a
large cumulonimbus. Hailstones larger than one-half inch
in diameter can significantly damage an aircraft in a few
seconds.
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 restrictions; but the hazards are multiplied when
associated with the other thunderstorm hazards of turbulence,
hail, and lightning.
Effect on Altimeters
Pressure usually falls rapidly with the approach of a
thunderstorm, rises sharply with the onset of the first gust
and arrival of the cold downdraft and heavy rain showers,
and then falls back to normal as the storm moves on. 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 feet in error.
Lightning
A lightning strike can puncture the skin of an aircraft
and damage communications and electronic navigational
equipment. Although lightning has been suspected of igniting
fuel vapors and causing an explosion, serious accidents due
to lightning strikes are rare. Nearby lightning can blind the
pilot, rendering him or her 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.
Engine Water Ingestion
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
terminal 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
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.
Chapter Summary
Knowledge of the atmosphere and the forces acting within
it to create weather is essential to understand how weather
affects a flight. By understanding basic weather theories, a
pilot can make sound decisions during flight planning after
receiving weather briefings. For additional information on the
topics discussed in this chapter, see the following publications
as amended: AC 00-6, Aviation Weather For Pilots and Flight
Operations Personnel; AC 00-24, Thunderstorms; AC 00-45,
Aviation Weather Services; AC 91-74, Pilot Guide: Flight in
Icing Conditions; and chapter 7, section 2 of the Aeronautical
Information Manual (AIM).