Cloud classification can be further broken down into specific
cloud types according to the outward appearance and cloud
composition. Knowing these terms can help a pilot identify
visible clouds.
The following is a list of cloud classifications:
• Cumulus—heaped or piled clouds
• Stratus—formed in layers
• Cirrus—ringlets, fibrous clouds, also high level clouds
above 20,000 feet
• Castellanus—common base with separate vertical
development, castle-like
• Lenticularus—lens-shaped, formed over mountains
in strong winds
• Nimbus—rain-bearing clouds
• Fracto—ragged or broken
• Alto—middle level clouds existing at 5,000 to 20,000
feet
Ceiling
For aviation purposes, a ceiling is the lowest layer of clouds
reported as being broken or overcast, or the vertical visibility
into an obscuration like fog or haze. Clouds are reported
as broken when five-eighths to seven-eighths of the sky is
covered with clouds. Overcast means the entire sky is covered
with clouds. Current ceiling information is reported by the
aviation routine weather report (METAR) and automated
weather stations of various types.
Visibility
Closely related to cloud cover and reported ceilings is
visibility information. Visibility refers to the greatest
horizontal distance at which prominent objects can be
viewed with the naked eye. Current visibility is also reported
in METAR and other aviation weather reports, as well as
by automated weather systems. Visibility information, as
predicted by meteorologists, is available for a pilot during a
preflight weather briefing.
Precipitation
Precipitation refers to any type of water particles that
form in the atmosphere and fall to the ground. It has a
profound impact on flight safety. Depending on the form of
precipitation, it can reduce visibility, create icing situations,
and affect landing and takeoff performance of an aircraft.
Precipitation occurs because water or ice particles in clouds
grow in size until the atmosphere can no longer support
them. It can occur in several forms as it falls toward the
Earth, including drizzle, rain, ice pellets, hail, snow, and ice.
Drizzle is classified as very small water droplets, smaller
than 0.02 inches in diameter. Drizzle usually accompanies
fog or low stratus clouds. Water droplets of larger size are
referred to as rain. Rain that falls through the atmosphere but
evaporates prior to striking the ground is known as virga.
Freezing rain and freezing drizzle occur when the temperature
of the surface is below freezing; the rain freezes on contact
with the cooler surface.
If rain falls through a temperature inversion, it may freeze
as it passes through the underlying cold air and fall to the
ground in the form of ice pellets. Ice pellets are an indication
of a temperature inversion and that freezing rain exists at a
higher altitude. In the case of hail, freezing water droplets are
carried up and down by drafts inside cumulonimbus clouds,
growing larger in size as they come in contact with more
moisture. Once the updrafts can no longer hold the freezing
water, it falls to the Earth in the form of hail. Hail can be
pea sized, or it can grow as large as five inches in diameter,
larger than a softball.
Snow is precipitation in the form of ice crystals that falls
at a steady rate or in snow showers that begin, change in
intensity, and end rapidly. Snow also varies in size, from very
small grains to large flakes. Snow grains are the equivalent
of drizzle in size.
Precipitation in any form poses a threat to safety of flight.
Often, precipitation is accompanied by low ceilings and
reduced visibility. Aircraft that have ice, snow, or frost on
their surfaces must be carefully cleaned prior to beginning
a flight because of the possible airflow disruption and
loss of lift. Rain can contribute to water in the fuel tanks.
Precipitation can create hazards on the runway surface itself,
making takeoffs and landings difficult, if not impossible,
due to snow, ice, or pooling water and very slick surfaces.
Air Masses
Air masses are classified according to the regions where
they originate. They are large bodies of air that take on the
characteristics of the surrounding area or source region. A
source region is typically an area in which the air remains
relatively stagnant for a period of days or longer. During
this time of stagnation, the air mass takes on the temperature
and moisture characteristics of the source region. Areas of
stagnation can be found in polar regions, tropical oceans, and
dry deserts. Air masses are generally identified as polar or
tropical based on temperature characteristics and maritime
or continental based on moisture content.
A continental polar air mass forms over a polar region and
brings cool, dry air with it. Maritime tropical air masses form
cP
A
mP
mP
mT
mT
mT
cT
Standard air mass abbreviations: arctic (A), continental polar (cP),
maritime polar (mP), continental tropical (cT), and maritime tropical
(mT).
Figure 12-23. North American air mass source regions.
over warm tropical waters like the Caribbean Sea and bring
warm, moist air. As the air mass moves from its source region
and passes over land or water, the air mass is subjected to
the varying conditions of the land or water which modify the
nature of the air mass. [Figure 12-23]
An air mass passing over a warmer surface is warmed from
below, and convective currents form, causing the air to rise.
This creates an unstable air mass with good surface visibility.
Moist, unstable air causes cumulus clouds, showers, and
turbulence to form.
Conversely, an air mass passing over a colder surface does not
form convective currents but instead creates a stable air mass
with poor surface visibility. The poor surface visibility is due
to the fact that smoke, dust, and other particles cannot rise
out of the air mass and are instead trapped near the surface.
A stable air mass can produce low stratus clouds and fog.
Fronts
As an air mass moves across bodies of water and land, it
eventually comes in contact with another air mass with
different characteristics. The boundary layer between two
types of air masses is known as a front. An approaching
front of any type always means changes to the weather
are imminent.
There are four types of fronts that are named according to the
temperature of the advancing air relative to the temperature
of the air it is replacing: [Figure 12-24]
• Warm
• Cold
• Stationary
• Occluded
Any discussion of frontal systems must be tempered with
the knowledge that no two fronts are the same. However,
generalized weather conditions are associated with a specific
type of front that helps identify the front.
Warm Front
A warm front occurs when a warm mass of air advances and
replaces a body of colder air. Warm fronts move slowly,
typically 10 to 25 miles per hour (mph). The slope of the
advancing front slides over the top of the cooler air and
gradually pushes it out of the area. Warm fronts contain
warm air that often has very high humidity. As the warm
air is lifted, the temperature drops and condensation occurs.
Generally, prior to the passage of a warm front, cirriform
or stratiform clouds, along with fog, can be expected to
form along the frontal boundary. In the summer months,
cumulonimbus clouds (thunderstorms) are likely to develop.
Symbols for surface fronts and other significant lines
shown on the surface analysis chart
* Note: Fronts may be black and white or color depending on their
source. Also, fronts shown in color code do not necessarily show
frontal symbols.
Warm front (red)*
Cold front (blue)*
Stationary front (red/blue)*
Occluded front (purple)*
Figure 12-24. Common chart symbology to depict weather front
location.
St. Louis Indianapolis
200 miles
Columbus
400 miles
Pittsburgh
600 miles
St. Louis
Indianapolis
1005
1002
999
999 1002 1005 1008 1011 1014
1008 1011 1014 1017
1017
Columbus Pittsburgh
65
1
10
65
59
3
59 56
60
6
50
53
10
3420
068
020
10
40 125
26
166
18
METAR KSTL 1950Z 21018KT 1SM –RA
0VC010 18/18 A2960
METAR KIND 1950Z 16012KT 3SM RA
BKN020 15/15 A2973
METAR KCMH 1950Z 13018KT 6SM HZ
0VC060 14/10 A2990
METAR KPIT 1950Z 13012KT 10SM
SCT150 12/01 A3002
COLD AIR
WARM AIR
NIMBOSTRATUS
ALTOSTRATUS
CIRROSTRATUS
CIRRUS
Figure 12-25. Warm front cross-section with surface weather chart depiction and associated METAR.
Light to moderate precipitation is probable, usually in the
form of rain, sleet, snow, or drizzle, accentuated by poor
visibility. The wind blows from the south-southeast, and the
outside temperature is cool or cold with an increasing dew
point. Finally, as the warm front approaches, the barometric
pressure continues to fall until the front passes completely.
During the passage of a warm front, stratiform clouds are
visible and drizzle may be falling. The visibility is generally
poor, but improves with variable winds. The temperature rises
steadily from the inflow of relatively warmer air. For the most
part, the dew point remains steady and the pressure levels off.
After the passage of a warm front, stratocumulus clouds
predominate and rain showers are possible. The visibility
eventually improves, but hazy conditions may exist for a
short period after passage. The wind blows from the south-
southwest. With warming temperatures, the dew point
rises and then levels off. There is generally a slight rise in
barometric pressure, followed by a decrease of barometric
pressure.
Flight Toward an Approaching Warm Front
By studying a typical warm front, much can be learned
about the general patterns and atmospheric conditions
that exist when a warm front is encountered in flight.
Figure 12-25 depicts a warm front advancing eastward from
St. Louis, Missouri, toward Pittsburgh, Pennsylvania during
a flight from Pittsburgh to St. Louis.
At the time of departure from Pittsburgh, the weather is good
VFR with a scattered layer of cirrus clouds at 15,000 feet.
As the flight progresses westward to Columbus and closer
to the oncoming warm front, the clouds deepen and become
increasingly stratiform in appearance with a ceiling of 6,000
feet. The visibility decreases to six miles in haze with a falling
barometric pressure. Approaching Indianapolis, the weather
deteriorates to broken clouds at 2,000 feet with three miles
visibility and rain. With the temperature and dew point the
same, fog is likely to develop. At St. Louis, the sky is overcast
with low clouds and drizzle and the visibility is one mile.
Beyond Indianapolis, the ceiling and visibility are too low
to continue VFR. Therefore, it would be wise to remain in
Indianapolis until the warm front passes, which may take
up to two days.
Cold Front
A cold front occurs when a mass of cold, dense, and stable
air advances and replaces a body of warmer air.
Cold fronts move more rapidly than warm fronts, progressing
at a rate of 25 to 30 mph. However, extreme cold fronts
have been recorded moving at speeds of up to 60 mph.
A typical cold front moves in a manner opposite that of a
warm front. It is so dense, it stays close to the ground and
acts like a snowplow, sliding under the warmer air and
forcing the less dense air aloft. The rapidly ascending air
causes the temperature to decrease suddenly, forcing the
creation of clouds. The type of clouds that form depends
on the stability of the warmer air mass. A cold front in the
Northern Hemisphere is normally oriented in a northeast to
southwest manner and can be several hundred miles long,
encompassing a large area of land.
Prior to the passage of a typical cold front, cirriform or
towering cumulus clouds are present, and cumulonimbus
clouds may develop. Rain showers may also develop due
to the rapid development of clouds. A high dew point and
falling barometric pressure are indicative of imminent cold
front passage.
As the cold front passes, towering cumulus or cumulonimbus
clouds continue to dominate the sky. Depending on the
intensity of the cold front, heavy rain showers form and may
be accompanied by lightning, thunder, and/or hail. More
severe cold fronts can also produce tornadoes. During cold
front passage, the visibility is poor with winds variable and
gusty, and the temperature and dew point drop rapidly. A
quickly falling barometric pressure bottoms out during frontal
passage, then begins a gradual increase.
After frontal passage, the towering cumulus and
cumulonimbus clouds begin to dissipate to cumulus clouds
with a corresponding decrease in the precipitation. Good
visibility eventually prevails with the winds from the west-
northwest. Temperatures remain cooler and the barometric
pressure continues to rise.
Fast-Moving Cold Front
Fast-moving cold fronts are pushed by intense pressure
systems far behind the actual front. The friction between
the ground and the cold front retards the movement of the
front and creates a steeper frontal surface. This results in a
very narrow band of weather, concentrated along the leading
edge of the front. If the warm air being overtaken by the
cold front is relatively stable, overcast skies and rain may
occur for some distance behind the front. If the warm air
is unstable, scattered thunderstorms and rain showers may
form. A continuous line of thunderstorms, or squall line,
may form along or ahead of the front. Squall lines present
a serious hazard to pilots as squall-type thunderstorms are
intense and move quickly. Behind a fast-moving cold front,
the skies usually clear rapidly, and the front leaves behind
gusty, turbulent winds and colder temperatures.
Flight Toward an Approaching Cold Front
Like warm fronts, not all cold fronts are the same. Examining
a flight toward an approaching cold front, pilots can get a
better understanding of the type of conditions that can be
encountered in flight. Figure 12-26 depicts a flight from
Pittsburgh, Pennsylvania, toward St. Louis, Missouri.
At the time of departure from Pittsburgh, the weather is VFR
with three miles visibility in smoke and a scattered layer of
clouds at 3,500 feet. As the flight progresses westward to
Columbus and closer to the oncoming cold front, the clouds
show signs of vertical development with a broken layer at
2,500 feet. The visibility is six miles in haze with a falling
barometric pressure. Approaching Indianapolis, the weather
has deteriorated to overcast clouds at 1,000 feet and three
miles visibility with thunderstorms and heavy rain showers.
At St. Louis, the weather gets better with scattered clouds at
1,000 feet and a ten mile visibility.
A pilot using sound judgment based on the knowledge of
frontal conditions will likely remain in Indianapolis until the
front has passed. Trying to fly below a line of thunderstorms
or a squall line is hazardous, and flight over the top of or
around the storm is not an option. Thunderstorms can extend
up to well over the capability of small airplanes and can
extend in a line for 300 to 500 miles.
Comparison of Cold and Warm Fronts
Warm fronts and cold fronts are very different in nature as are
the hazards associated with each front. They vary in speed,
composition, weather phenomenon, and prediction. Cold fronts,
which move at 20 to 35 mph, travel faster than warm fronts,
which move at only 10 to 25 mph. Cold fronts also possess a
St. Louis Indianapolis
200 miles
Columbus
400 miles
Pittsburgh
600 miles
METAR KSTL 1950Z 30018KT 10SM
SCT010 08/02 A2979
METAR KIND 1950Z 20024KT 3SM +TSRA
OVC010 24/23 A2974
METAR KCMH 1950Z 20012KT 6SM HZ
BKN025 25/24 A2983
METAR KPIT 1950Z 20012KT 3SM FU
SCT035 24/22 A2989
1011
1008100510051008
1011
1011 1011 1014
1014
St. Louis Indianapolis Columbus Pittsburgh
46
42
10
06610
33
74 071
3
71 77
6
73
75
3
70
35
4
8 12
102 122
10
25
WARM AIR
COLD AIR
CUMULONIMBUS
Figure 12-26. Cold front cross-section with surface weather chart depiction and associated METAR.
steeper frontal slope. Violent weather activity is associated with
cold fronts, and the weather usually occurs along the frontal
boundary, not in advance. However, squall lines can form
during the summer months as far as 200 miles in advance of
a strong cold front. Whereas warm fronts bring low ceilings,
poor visibility, and rain, cold fronts bring sudden storms, gusty
winds, turbulence, and sometimes hail or tornadoes.
Cold fronts are fast approaching with little or no warning,
and they bring about a complete weather change in just a
few hours. The weather clears rapidly after passage and drier
air with unlimited visibilities prevail. Warm fronts, on the
other hand, provide advance warning of their approach and
can take days to pass through a region.
Wind Shifts
Wind around a high-pressure system rotates clockwise, while
low-pressure winds rotate counter-clockwise. When two
high pressure systems are adjacent, the winds are almost in
direct opposition to each other at the point of contact. Fronts
are the boundaries between two areas of high pressure, and
therefore, wind shifts are continually occurring within a front.
Shifting wind direction is most pronounced in conjunction
with cold fronts.
Stationary Front
When the forces of two air masses are relatively equal, the
boundary or front that separates them remains stationary and
influences the local weather for days. This front is called a
stationary front. The weather associated with a stationary
front is typically a mixture that can be found in both warm
and cold fronts.
Occluded Front
An occluded front occurs when a fast-moving cold front
catches up with a slow-moving warm front. As the occluded
front approaches, warm front weather prevails but is
immediately followed by cold front weather. There are two
types of occluded fronts that can occur, and the temperatures
of the colliding frontal systems play a large part in defining
the type of front and the resulting weather. A cold front
occlusion occurs when a fast moving cold front is colder
than the air ahead of the slow moving warm front. When
this occurs, the cold air replaces the cool air and forces the
warm front aloft into the atmosphere. Typically, the cold
front occlusion creates a mixture of weather found in both
warm and cold fronts, providing the air is relatively stable.
A warm front occlusion occurs when the air ahead of the