Thunderstorm
and Turbulence
Avoidance
REFERENCES
• TC 1-218, Aircrew Training Manual Utility Airplane 2 March
1993
• AR 95-1, Flight Regulations, 1 September 1997
• Aeronautical Information Manual (AIM)
• FM 1-230, Meteorology for Army Aviators, 30 September 1982
• “Thunderstorms - Up (Too) Close and Personal”, Flying Safety,
June 1998
OUTLINE
• Thunderstorm Formation
• Weather Radar
• Thunderstorm Avoidance
• Turbulence/Turbulence Avoidance
THUNDERSTORMS
Thunderstorm Facts
• Can occur anywhere in the world and at any time of the day.
• Nearly 1,800 thunderstorms are occurring at any moment.
• Over 40,000 form every day around the world
• All thunderstorms produce lightning and thunder, with the potential
to produce damaging straight-line winds, large hail, heavy rain and
flooding, and tornadoes.
A thunderstorm is classified as severe when it contains one or more
of the following phenomena:
• Hail 3/4” or greater
• Winds gusting in excess of 50 Knots (57.5 mph)
• A Tornado
Thunderstorm Types
•Frontal:
• Warm-front Thunderstorms
• Cold-front Thunderstorms
• Prefrontal Squall-line Thunderstorms
• Stationary-front Thunderstorms
• Occlude-front Thunderstorms
•Air Mass Thunderstorms
• Convective Thunderstorms
• Orographic Thunderstorms
Convective Thunderstorm
Thunderstorm Formation
Three Stages of Thunderstorm Development
• Cumulus (Building) Stage
• Mature Stage
• Dissipating Stage
Cumulus Stage
• Rising of warm moist air during
the day will form a cumulus cloud.
• The cloud will continue to grow
as long as the warm moist air
continues to rise.
• Formation of a cumulus cloud
could take as little as a few minutes.
Mature Stage
•Raindrops start to fall through the cloud
when the rising air can no longer hold
them up.
•Cool dry air starts to enter the cloud and
because it is heavier than warm air, it starts
to descend in the cloud (known as a
downdraft). The downdraft pulls the heavy
water downward, making rain.
• This cloud has become a cumulonimbus
cloud because it has an updraft, a
downdraft, and rain.
• Lightning and thunder start to occur, as
well as heavy rain. The cumulonimbus is
now a thunderstorm cell
Dissipating Stage
• After about 30 minutes, the
thunderstorm begins to dissipate.
• This occurs when the downdrafts
begin to dominate over the updraft.
• Since warm moist air can no
longer rise, cloud droplets can no
longer form.
• The storm dies out with light rain
as the cloud disappears from
bottom to top.
Weather Radar
1. Aircraft radar set generates an electromagnetic pulse
2. Pulse is directed toward a reflecting body
3. Pulse is reflected back to the source
4. Radar Set receives the signal and displays the results
Weather Radar
4
1
2
3
Weather Radar Returns
Weather Radar
Scalloped Edge, Fingers and U-shaped echoes are all
indicative of potential Hail. Hail has been associated
with these types of returns
Weather Radar
Although they may occur in any Thunderstorm,
Tornadoes have been associated with hook, bow and
pendant (figure 6) shaped returns.
Tornadoes (cont.)
Notches, Fingers and Hooks
Pendant Shape
Weather Radar
Targets with asymmetric
coloring and shapes. Severe
storms created by windshears aloft
will tilt to one side. This gives
shapes and colorings that are not
even or concentric.
Targets with an "arrow shape."
Again, this is indicative of a storm
with tilt and the possibility of
severe hazardous weather.
Weather Radar
Potential Downwind Hail
Hail falls many times outside of
the thunderstorm. Checking the
winds at altitude and correlating
it to the side of the storm that
hail will fall should help identify
that potential hazard.
Rain Gradient
Steep Gradient
Heavy
Moderate
Moderate
Light
Light
Heavy
Rain
Clear
Shallow Gradient
Clear
Weather Radar Images
The steeper the gradient, the more
sever the turbulence
Circle/Oval Shapes
A gradient is considered steep when
the distance from the edge of the
echo to the edge of the red display is
3 NM or less.
Steep Gradient
Thunderstorm Planning Considerations
•Surface dew point and temperature are a good indicator of storm
severity. Thunderstorms forming over an area where the dew point is 50
F or higher with more than a 30 degree spread between temperature and
dew point indicate a potential for extremely strong storms.
• Use the antenna tilt feature effectively to identify tops of the moisture
and to determine if rain is so heavy that it is attenuating the radar beam.
• If winds at 18,000 feet (500mb level) are southwesterly, you can
expect storms to form.
• Consider flying in the morning before the afternoon heat can trigger
storms.
• Consider delaying takeoff when a cell is within 20 miles of the
departure airport.
AR 95-1 Flight Restrictions
Thunderstorms
Aircraft will not be intentionally flown into
Thunderstorms
Thunderstorm Do’s and Don’ts
DON’TS
 Don’t take off or land in the face of an approaching T-storm
 Don’t attempt to fly under a T-storm
 Don’t fly into a cloud mass containing embedded T-storms without
weather radar
 Don’t trust visual appearances to be an indicator of turbulence
inside a T-storm
Thunderstorm Do’s and Don’ts
DO’S
 Do avoid by at least 20 NM all T-storms identified as severe.
 Do clear the tops of a known or suspected severe T-storm by 1000’
for every 10 knots of wind at the cloud tops.
 Do circumnavigate the entire area if there is 6/10 T-storm coverage.
 Do remember that vivid and frequent lightning indicates the
probability of a strong thunderstorm.
 Do regard as extremely hazardous any storm with tops 35,000 feet
or higher.
 Provide extra distance from cells moving at 20 knots or greater and
the cell at the south end of a line of storms. This cell does not have to
compete for moisture with other cells so it has an abundant "fuel"
supply to generate turbulence.
Enroute Avoidance Procedures
• Avoid flight under a CB overhang if possible
• Above 23,000 feet avoid all echoes by 20 NM
• Below 23,000 feet:
• Avoid echoes having steep gradients by at least 5 NM when
OAT is 0oC or warmer
• Avoid echoes having steep gradients by 10 NM when OAT is
less than 0oC.
• Increase distances by 5 NM for rapidly changing echoes or
those that exhibit hooks, figure-6’s, U-shapes, scalloped edges,
or other forms of protrusions
• Weak echoes may be flown through, or near, if avoidance is
impossible or impractical.
TURBULENCE
Fatal Turbulence Events Since
1980
10May80; Indian Airlines 737-200; near Rampur Hat, India: Aircraft experienced severe en route
turbulence. Two of the 132 passengers were killed.
7Oct81; NLM CityHopper F28-4000; Moerdijk, Netherlands: Shortly after takeoff, the aircraft entered an
area of severe thunderstorm activity. The aircraft apparently had a catastrophic in flight structural failure due
to severe turbulence because it was seen to emerge from the clouds with one of its wings broken away. All
four crew members and 13 passengers were killed.
16Aug82; China Airlines 747; near Hong Kong: The aircraft encountered severe in flight turbulence. Two
of the 292 passengers were killed.
3Oct90; Eastern DC9-31; Florida, USA: The aircraft encountered clear air turbulence at 31,000 feet (9450
m). One injured passenger died three weeks later.
5Sept96; Air France 747-400; near Ouagadougou, Burkina Faso: Severe turbulence associated with a
weather front seriously injured three of the 206 passengers. One of the three passengers later died of injuries
caused by an in flight entertainment screen.
28Dec97; UA 747-100; over Pacific Ocean near Japan: Aircraft encountered severe turbulence during
cruise about two hours after departing Japan. One of the 346 passengers was killed. 3 of the 23 crew
members sustained serious injuries.
AR 95-1 Flight Restrictions
Turbulence
Turbulence
• Aircraft will not be intentionally flown into known or forecast
extreme turbulence or into known severe turbulence.
• Aircraft will not be flown into forecast severe turbulence unless
MACOM Commander has established clearance procedures and -
Flight into Severe Turbulence
Clearance Procedures
• Weather info is based on area forecast
• Flight will be made into areas where severe turbulence is unlikely
• Flights are for essential training or essential missions only
• Flight approval authorities are specified
• Flights are terminated or depart turbulence if severe turbulence is
encountered
Turbulence Intensity Levels
•
•
•
•
Light
Moderate / Moderate Chop
Severe
Extreme
Light Turbulence
Aircraft Reaction
• Momentary changes in aircraft altitude and attitude.
Reaction Inside the Aircraft
• Occupants may feel a slight strain against seat belts.
• Unsecured objects may be displaced slightly.
Moderate Turbulence
Aircraft Reaction
• Changes in the aircraft’s altitude and attitude
• The aircraft remains in positive control at all times
• Usually causes variations in indicated airspeed
Moderate Chop - causes rapid bumps or jolts without appreciable
changes in altitude or attitude
Reaction Inside the Aircraft
• Occupants feel a definite slight strain against seat belts.
• Unsecured objects are dislodged.
Severe Turbulence
Aircraft Reaction
• Causes large abrupt changes in altitude and/or attitude.
• Causes large variations in indicated airspeed
• Aircraft may be temporarily out of control
Reaction Inside the Aircraft
• Occupants are forced violently against seat belts or shoulder straps.
• Unsecured objects are tossed about.
Extreme Turbulence
Aircraft Reaction
•The aircraft is violently tossed about and is practically impossible to
control
•May cause structural damage
Clear Air Turbulence (CAT)
High level turbulence (normally above 15,000) not associated with
cumuliform clouds should be reported as CAT
Turbulence Frequency
Occasional - Less than 1/3 of the time
Intermittent - 1/3 to 2/3 of the time
Continuous - More than 2/3 of the time
Summary
 Three Stages of Thunderstorms:
 Cumulus, Mature, and Dissipating Stages
 Weather Radar Returns
 Thunderstorm Avoidance Criteria
 Above FL230 - 20NM
 Below FL230 - 5 NM above 0oC; 10 NM below 0oC
 Turbulence