Download Composition of the Atmosphere: Composition of the Atmosphere

Composition of the Atmosphere:
The cubic foot of typical atmosphere composed of about 78% Nitrogen and 21%
Oxygen and the remaining 1% is made up of several gases.
Water is constantly being absorbed and released in the atmosphere; this action is
responsible for major changes in the weather.
The atmosphere is a mixture of gases that exists in fairly uniform proportion up to
approximately 30 miles above the earth. (Figure 4-1)
Atmosphere Levels:
Troposphere is the one closest to the earth extending from the surface to an average
altitude of about 6 or 7 miles.
Troposphere in the mid latitudes is average about 37000 feet.
The Tropopause is a thin layer of the atmosphere at the top of the troposphere; it acts
as a lid to confine most of the water vapor.
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Above the Tropopause are three more atmospheric layers. The first is the
Stratosphere which extends to a height of approximately 19 to 22 miles. Above the
Stratosphere are the Mesosphere and the Thermosphere.
The average temperature lapse rate changes abruptly at the Tropopause, above this
point in the stratosphere, temperature continues to decrease, but at a slower rate.
The height of the Troposphere varies; it slopes upward from about 20000 ft at the
poles to 60000 ft at the equator.
It is also higher in the summer than it is in the winter.
The Troposphere has the greatest impact on weather.
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Convection:
Uneven Heating of the earth`s surface causes variations in air temperature and
density.
Convection is defined as the circular motion that results when warm air rises and is
replaced by cooler air.
If the earth did not rotate, a huge convective circulation pattern would develop as air
flowed from the poles to equator and back again. (Figure 4-3)
Atmospheric Pressure:
Variations in altimeter setting between weather reporting points are primarily caused
by the unequal heating of the earth`s surface.
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Isobars are measured in millibars and are usually drawn at four millibar intervals.
High is center of high pressure surrounded on all side by lower pressure.
Low is an area of low pressure surrounded by high pressure.
A Ridge is an elongated area of high pressure.
A Trough is an elongated area of low pressure.
A Col can designate either a neutral area between two highs and two lows, or
intersection of a ridge and a trough.
High Pressure Good weather good visibility, calm or light winds, few clouds
Ridge also normally present good weather
Low Pressure Poor weather low clouds, poor visibility, PPTN, gusty wind,
turbulence
Weather may be very violent in the area of Trough
The speed of the resulting wing depends on the strength of the Pressure Gradient.
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Coriolis force:
If the earth did not rotate, pressure gradient force would propel wind directly from
high to lows.
Coriolis is a deflective force that is created by the difference in rotational velocity
between the equator and the poles.
The velocity of the earth`s surface varies from about 900 knots at the equator to zero
at the poles.
Coriolis force deflects the air to the right in the northern hemisphere and to the left in
the southern hemisphere.
The amount of deflection produced by Coriolis force varies with latitude; it is zero at
equator and increase toward the poles. It also is proportional to the speed of the air
mass.
Wind:
Pressure gradient and Coriolis forces works in combination to create wind.
Pressure gradient force cause air to move from high pressure to low pressure areas.
As the air begins to move Coriolis forces deflect it to the right in the Northern
hemisphere.
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Winds in the northern hemisphere flow clock-wise around highs and counter clockwise around lows.
Air flow outward and downward from a high and inward and upward toward a low.
The stronger the pressure gradient, the stronger the wind.
Friction causes a wind shift near the earth`s surface.
Global Wind Patterns:
In northern hemisphere, Coriolis force produces a three-cell circulation pattern
around two semi permanent high pressure at 30˚ to 60 ˚ north latitude.
Around 30 ˚ north, the flow is eastward causing air to pile up at this latitude; this
creates a semi permanent high pressure area.
At the surface air flows back toward the equator, this low-level southerly flow is
deflected to the right again and creates the northest trade winds, Common to areas
like the Caribbean.
The low-level air flowing northward from high to pressure at 30 ˚also is deflected to
the right by Coriolis; this creates the Prevailing Westerlies, common to the mid section
of North America.
The cold polar air that flows southward is deflected to the right to create the Polar
Easterlies.
Sea & Land Breezes:
Land is warmer than water during the day; this creates the Sea Breeze, a wind that
blows from the cool water to the warmer land.
At night, land cools faster than water and a land breeze blows from the cooler land to
the warmer water.
Valley & Mountain Winds:
A valley wind is created as the cooler air over the valley sinks and air close to the
mountain flow upward.
At night, the surface of slopes cools rapidly lowering the temperature of the air close
to the surface. The cooler air flows down the slope and displaces the air in the valley,
this is called mountain wind.
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Katabatic Winds:
A katabatic wind is the name given to any wind blowing down an incline.
A mountain wind is one type of katabatic wind.
A warm descending wind is called Foehn wind but may also be referred to locally by
other names such as the Santa Ana in California and the Chinnook in the Rockies.
Moisture:
If the air is dry, the weather usually will be good, if the air is very moist, poor or even
severe weather can occur.
The amount of moisture the air can hold depends on air temperature.
Change of State:
Every physical process of weather is accompanied by a heat exchange.
Changes in state occur through the processes of Evaporation, Condensation,
Sublimation, Melting and Freezing.
Evaporation is the changing of liquid water to invisible water vapor. As water vapor
forms, heat is absorbed from the nearest available source.
Condensation occurs when water vapor changes to a liquid. When condensation
takes place, the heat absorbed by water vapor during evaporation is released.
The heat released is referred to as the latent heat of condensation; it is an important
factor in cloud development.
Sublimation is the changing of ice directly to water vapor or of water vapor to ice.
Melting is the change of ice to water.
Freezing is the reverse; the heat exchange in each of these process is small and has
relatively little affect on water.
Water vapor is added to the atmosphere by evaporation and sublimation.
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Humidity:
Dew point is the temperature at which air reaches a state where it can hold no more
water.
The amount of moisture in the air depends on air temperature.
Dew point is the temperature at which air becomes saturated.
Visible Moisture:
Clouds, fog or dew always form when water vapor condenses.
As air cools to its saturation point, the processes of condensation and sublimation
change invisible water vapor into state that is readily seen.
Clouds and fog:
A small and decreasing temperature/dewpoint spread indicates conditions are
favorable for the formation of fog.
When clouds form near the surface they are referred to as fog.
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Clouds and fog usually form as soon as air becomes saturated.
When the temperature/dewpoint spread reaches 4˚F (2˚C) and continue to decrease,
the air is nearing the saturation point and the probability of fog and low clouds
forming increases.
Precipitation:
Drizzle, rain, snow, hail, ice crystals are all forms of precipitation.
Water droplets that remain liquid fall as drizzle or rain.
Rain that remains liquid even though its temperature is below freezing is considered
to be supercooled, and is referred to as freezing rain.
Rain may also freeze as it falls, striking the ground as ice pallets.
Ice pallets usually are evidence of freezing rain and a warmer layer of air at higher
altitude.
If wet snow is encountered at your flight altitude, the temperature is above freezing.
With low relative humidity, rain may evaporate before it reaches the surface, when
this occurs, it is called Virga.
Dew & Frost:
Frost forms on aircraft surfaces when the surface is at or below the dewpoint of the
surrounding air and the dewpoint is below freezing.
On cool, still night, surface features and objects may cool to a temperature below the
dewpoint of the surrounding air, moisture then condenses out of the air in the form
of dew.
Atmospheric Stability:
Stability is the atmosphere`s resistance to vertical motion.
Stable air resists vertical motion and tends to inhibit cloud formation, precipitation
and severe weather.
Unstable air has a tendency to move vertically this can lead to significant cloud
development, turbulence and hazardous weather.
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Temperature & Moisture:
The variations of density produce convective currents in which warm air rises and is
replaced by cooler, denser air.
Moisture is another factor affecting stability. Water vapor is lighter than air therefore,
as moisture added to air; its density decrease and it tend to rise.
You can see that the greatest instability occurs when the air is both warm and moist.
Air that is both cool and dry resists vertical movement and is very stable.
Rising & Decreasing Air:
The temperature change is caused by a process known as adiabatic heating or
cooling which is a change in the temperature during expansion or compression when
no energy added to or removed from the air.
You can use the actual lapse ratio to determine the stability of the atmosphere.
The dry adiabatic lapse rate 3˚C/1000'
The moist adiabatic lapse rate 1.1˚C to 2.8˚C /1000'
The lapse rate and temperature/dewpoint spread can be used to calculate cloud
bases.
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Calculate Cumulus cloud base = Temperature – Dewpoint / 4.5
Inversions:
A stable layer of air and temperature increase with altitude are features of
temperature inversion.
Visibility and fog, haze, smoke, low clouds
Temperature inversion occurs in stable air with little or no wind and turbulence.
On clear, cool night as the ground cools, its lowers the temperature on the adjacent
air, if this process continues the air within a few hundred feet of the surface may
become cooler than the air above it. That called frontal inversion.
Observing stable & unstable air:
Moist, unstable air causes the formation of cumuliform clouds and showers.
Stable air generally smooth with layered or stratiform clouds. Visibility is usually
restricted with widespread areas of clouds and steady rain or drizzle.
Unstable air is usually bumpy with good surface visibility outside of scattered rain
showers.
(Figure 6-4 IRM)
Clouds:
Clouds are visible moisture that has condensed or sublimated onto condensation
nuclei such as dust, salt, or combustion particles.
Type of Clouds:
Low from surface to 6500 ft AGL Cumulus – Stratocumulus – Stratus
Middle from 6500 ft AGL to 23000 ft AGL Altocumulus – Altostratus –
Nimbostratus
High from 16500 ft AGL to 45000 ft AGL Cirrus – cirrocumulus – cirrostratus
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Cloud with extensive vertical development from 1000 ft or less to 10000 ft or more,
tops sometime exceed 60000 ft MSL Towering cumulus – Cumulonimbus
Stratus Clouds are layered clouds that form in stable air near the surface due to
cooling from below, low turbulence, restricted visual flying, icing condition are
possible if temperature are at or near freezing, gray appearance and cover a wide
area.
Stratocumulus Clouds are white, puffy clouds that form as stable air is lifted; they
often form as stratus layer breaks up or as cumulus clouds spread out.
Cumulus clouds resulting from the heating of the earth`s surface – flat bottoms
and dumb shaped tops – indicate a shallow layer of instability – expect turbulence –
little icing and precipitation.
Turbulence can be expected at and below the bases of fair weather cumulus.
Altostratus clouds are flat – dense clouds that cover a wide area – uniform gray or
gray/white – minimal turbulence – moderate icing
Altocumulus clouds gray or white – patchy clouds of uniform appearance – often
form when altostratus clouds start to break up – light turbulence and icing – extend
over wide area – may contain highly supercooled water droplets.
Nimbostratus clouds gray or black – more than several thousand feet thick –
contain large quantities of moisture – create heavy icing if temperatures are near or
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below freezing – produce widespread area of rain or snow – may merge into low
stratus or stratocumulus.
Cirrus clouds thin – wispy clouds – form above 30000 ft – white or light gray – exist
in patches or narrow bands – sometimes blown from the tops of thunderstorms or
towering cumulus clouds – form in stable air – in some cases they are an advance
warning of approaching bad weather.
Cirrostratus clouds thin – white – form in long bands or sheets against deep blue
background – several thousand feet thick – low moisture – no icing.
Cirrocumulus clouds white patchy clouds – look like cotton – form as a result of
shallow convective currents at high altitude – light turbulence.
Towering Cumulus clouds similar to cumulus clouds except they have more vertical
development – look like large mounds of cotton with billowing cauliflower tops –
brilliant white at the top to gray near bottom – indicate a fairly deep layer of unstable
air – moderate to heavy turbulence with icing – often develop into thunderstorm.
They contain moderate to heavy convective turbulence with icing and often develop
into thunderstorms.
Embedded thunderstorms are frequently obscured by massive cloud layers and
usually con not be seen.
Cumulonimbus clouds commonly called thunderstorms – vertical developed clouds
– form in very unstable air – gray/white to black – contain large amount of moisture
– many flying hazards – form when moist, unstable air is lifted.
Airmasses:
An airmass is large body of air with fairly uniform temperature and moisture content.
It may be several hundred miles across and usually form where air remains stationary
or nearly stationary for at least several days.
Moist, unstable air typically has cumulus clouds.
Source
Source Region:
The area where an airmass acquires the properties of temperature and moisture that
determine its stability is called its source region.
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A source region is usually located where air tends to stagnate.
Some excellent areas are snow and ice covered Polar Regions, tropical oceans, and
large deserts.
The middle latitudes are poor source region because of the strong westerly winds and
the continual mixing of tropical and polar airmasses.
Classification:
The airmasses are generally divided into polar or tropical to identify their temperature
characteristics and continental or maritime to identify their moisture content.
Maritime Polar Cool, moist
Continental Polar cool, dry
Maritime Tropical Warm, moist
Continental Tropical Hot, dry
Modification:
Modification:
As an airmass moves out of its source region, it is modified by the temperature and
moisture of the area over which it moves.
The degree to which an airmass is changed depends on several factors:
1. Its speed
2. Nature of the region it moves over
3. Temperature difference between the airmass and the new surface
4. Depth of the airmass
Warming from below:
Warming from below decrease airmass stability
As an airmass moves over a warmer surface, its lower layer are heated and vertical
movement of the air develops depending on temperature and moisture levels, this
can result in extreme instability.
Cooling from below:
Cooling from below may cause smooth air and poor visibility.
Cooling from below increase stability.
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When an airmass flows over a cooler surface, its lower layers are cooled and vertical
movement is inhibited. As a result, the stability of the air is increased and low clouds
or fog may form.
This cooling from below creates a temperature inversion and may result in low ceiling
and visibility for long periods of time.
Fronts:
Fronts are boundaries between airmasses.
Since the weather along a front often presents a serious hazards to flying.
Type of Fronts:
Fronts are named according to the temperature of the advancing air relative to the
temperature of the air it is replacing.
A cold front is one where cold air is moving to displace warmer air.
In a warmer front, warm air is replacing cold air.
A stationary front has no movement.
Cold fronts are usually fast moving and often catch up to and merge with a slower
moving warm front, when cold and warm fronts merge, they create an occluded
front.
Frontal discontinuities:
The change between two airmasses may be very abrupt, indicating a narrow frontal
zone. On other hand, the changes may occur gradually, indicating a wide and
perhaps, diffused frontal zone.
1. Temperature:
A change in the temperature is one of the easiest ways to recognize the passage of a
front.
The changes may be less abrupt at middle and high altitudes than it is at the surface.
2. Wind:
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When you are flying across a front, you will notice a change in wind direction. Wind
speed also may change.
The wind always shifts to the right in the northern hemisphere.
3. Pressure:
As a front approaches, atmospheric pressure usually decreases, with the area of
lowest pressure lying directly over the front.
Pressure changes on the warm side of the front generally occur more slowly than on
the cold side.
Frontal Weather:
The type and intensity of frontal weather depend on several factors:
1. Availability of moisture
2. The stability of the air begin lifted
3. The speed of the frontal movement
4. Slope of the front
5. The moisture and temperature variations between two fronts
Cold Front:
The speed of a cold front usually dictates the type of weather associated with the
front.
Some general weather characteristics that are found in most cold front:
1. Cumulus clouds
2. Turbulence
3. Showery precipitation
4. Strong, gusty winds
5. Clearing sky and good visibility after the front-passage
FastFast-moving cold fronts:
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Fast-moving cold fronts are pushed along by intense high pressure systems located
well behind the front.
These fronts are particularly hazardous because of the steep slope and wide
differences in moisture and temperature between two airmasses.
When a cold front pushes into extremely moist, unstable air, a squall line often forms
a head of front itself.
The squall line is an area of severe weather characterized by extensive vertical clouds
development and turbulence. It often develops 50 to 300 miles ahead of front.
A squall line often forms 50 to 200 miles ahead of fast-moving cold front.
The weather usually clears quickly behind a cold front. You will notice reduced cloud
cover, improved visibility, lower temperature, and gusty surface winds following the
passage of fast-moving cold front.
Fast-moving cold fronts force warmer air to rise, this causes wide spread vertical cloud
development along a narrow frontal zone.
SlowSlow-moving cold fronts:
As a slow- moving cold front meets unstable air, cumulonimbus and nimbostratus
clouds may develop near the surface front, creating hazards icing and turbulence.
A slow-moving cold front meeting stable air usually causes a broad area of Stratus
clouds to form behind the front.
Warm fronts:
Steady precipitation with little turbulence usually precedes warm fronts.
Warm fronts occur when warm air overtakes and replaces cooler air. They usually
move at much slower speed than cold fronts.
Some of the common weather patterns found in a typical warm front include:
1. Stratus clouds, if the air is moist and stable
2. Little turbulence, except in an unstable airmass.
3. Precipitation ahead of the front.
4. Poor visibility with haze or fog.
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5. Wide area of precipitation.
If the air is warm, moist and stable, stratus clouds will develop.
If the air is warm, moist and unstable, cumulus clouds will develop.
Stationary Fronts:
Stationary fronts have qualities of both warm and cold fronts.
When the opposing forces of two airmasses are relatively balanced, the front that
separates then may remain stationary.
The weather in a stationary front is usually a mixture of that found in both warm and
cold fronts.
Frontal Occlusions:
A frontal occlusion when a fast-moving cold front catches up to a slow-moving warm
front.
The difference in temperature within each frontal system is a major factor that
influence which type of front and weather are created.
Thunderstorms:
Thunderstorm formation requires unstable conditions, a lifting force, and high
moisture levels.
The lifting action may be provided by several factors, such as rising terrain
(orographic lifting) fronts, or heating of earth`s surface (convection).
Life cycle:
The three stages of thunderstorm are:
Cumulus – Mature – Dissipating
The cumulus stage is characterized by continues updrafts.
In the cumulus stage, a lifting action initiates vertical movement of air.
Thunderstorm reaches the greatest intensity during the mature stage, which is
signaled by the beginning of precipitation.
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The resulting downdraft may reach a velocity of 2500 fpm; the down-rushing air
spreads outward at the surface, producing to sharp drop in temperature, a rise in
pressure, and strong, gusty surface winds. The leading edge of this wind is referred to
as a gust front or the first gust.
As the thunderstorm advances, a rolling, turbulent circular-shaped cloud may form at
the lower leading edge of the cloud; this is called the roll cloud.
A dissipating thunderstorm is characterized by downdrafts, during this stage; the
upper level winds often blow the top of the cloud downwind, creating the familiar
anvil shape.
Type of Thunderstorms:
Thunderstorms are generally classified as airmass or frontal storms.
Airmass Storms:
Airmass storms are usually caused by convection or orographic lifting.
1. They are usually caused by solar heating of the land, which result in convection
currents that lift unstable air, and are most common during summer afternoons
or in coastal areas at night.
2. Orographic lifting mountain peak
3. Nocturnal thunderstorms in late spring and summer during late night or
early morning hours.
Frontal Storms:
Frontal thunderstorms can be associated with any type of front.
1. Warm front stratiform clouds showery precipitation
2. Cold front cumulonimbus clouds
3. Occlusions can also spawn storms
A squall line is a non-frontal band of thunderstorm that contains the most severe
types of weather related hazards.
Thunderstorm Hazardous:
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Thunderstorms may include lightening, hail, turbulence, gusty surface wind, or even
tornadoes.
The cumulonimbus is the type of cloud that produces the severe turbulence.
Lightening is always associated with thunderstorms.
Hail can occur at all altitude within or outside a storm.
Severe turbulence often exists in a thunderstorm.
Thunderstorms turbulence develop when air currents changes direction or velocity
rapidly over a short distance.
The strongest turbulence occurs in the shear between the updrafts and downdrafts.
Funnel clouds are violent, spinning columns of air which descend from the base of a
cloud.
If a funnel cloud reaches the earth`s surface it is referred to as a tornado.
If it touches down over water, it is called a water spout.
Turbulence:
There are many causes of turbulence, including wind shear, convective currents,
obstruction to wind flow, clear air turbulence, and wake turbulence.
If you encounter turbulence, establish maneuvering speed and try to maintain a level
flight attitude.
If you encounter turbulence, reduce power to slow the airplane to maneuvering
speed or less.
Wind Shear:
Wind shear can often exist near the surface when there is frontal system,
thunderstorm or temperature inversion with strong upper winds in the area.
A microburst is an intense downburst which covers only a small area and has a very
short lift cycle, however a downburst covers a larger area and may last up to 30
minutes.
Convective Currents:
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When sufficient moisture is present, cumulus clouds indicate the presence of
turbulence.
The strongest convective currents are found on hot summer afternoon.
Convective turbulence is caused by currents which develop in air heated by contact
with the warm surface below.
Obstruction to wind flow:
Mechanical turbulence when obstacles like building or rough terrain interfere with
the normal wind flow turbulence develops.
Mountain wave turbulence can be anticipated when the wind across a ridge are 40
knots or more, and the air is stable.
The crests of the waves may be marked by lens-shaped or lenticular clouds. Rotor
clouds may also form in the rotors, and cap clouds may obscure the mountain peaks.
Clear
Clear Air Turbulence:
Clear air turbulence can take place at any altitude and is often present with no visual
warning.
CAT may be caused by wind shear, convective currents, or obstruction to normal
wind flow.
It often develops in or near the jet stream, which is a narrow band of high altitudes
wind near Tropopause.
Wake Turbulence:
Wingtip vortices are created when an airplane generates lift, the intensity of the
turbulence depends on aircraft weight, speed and configuration.
The greatest vortex strength occurs when the generating aircraft is heavy, slow and
in a clean configuration.
Wingtip vortices tends to sink below the flight path of the aircraft which generated
them and are most hazardous during light, quartering tailwind conditions.
To avoid turbulence when landing behind a large aircraft, stay above the large
airplane`s glide path and touch down beyond its touchdown point.
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If a large airplane has just taken off as you approach to land, touch down well before
the large aircraft`s lift off point.
When taking off behind a large aircraft, lift off before the large airplane`s rotation
point and climb out above or upwind of its flight path.
When departing after a large aircraft has landed, lift off beyond its touchdown point.
Wake turbulence can be extremely hazardous, particularly to pilot of small aircraft
taking off or landing behind large aircraft.
Jet engine blast is a related hazard. It can damage or even overturn a small airplane if
it is encountered at close range.
Reporting Turbulence:
Light Slight erratic changes in altitude or attitude. Light chop is slight
Moderate change in IAS, moderate chop is rapid bumps or jolts without appreciate
changes in altitude or attitude.
Severe Abrupt changes in altitude or attitude, large variation in IAS
Extreme Aircraft practically impossible to control, may cause structural damage.
Icing:
Visible moisture is necessary for structural icing to form, and freezing rain usually
produces the highest accumulation rate.
When the temperature of the aircraft surface is 0˚C or colder, ice can build up on any
exposed surface of an aircraft.
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There are two general types of ice: Rime , Clear
Mixed icing is a combination of the two rime and clear ice.
RimeIce Stratus clouds + tiny water droplet + instantaneous/fast freezing + opaque
appearance
Clear Ice Cumulus clouds + large water droplet + slow freezing + large
accumulation + difficult removal
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If the frost is not removed from the wings before flight, it may decrease lift and
increase drag, preventing the aircraft from becoming airborne.
Restriction to visibility:
1. Radiation fog / Ground fog over fairly level surfaces on clear, calm, humid
nights + stable air + high pressure systems
2. Advection fog low layer of warm, moist air moves over a cooler surface +
cloudy skies along coastline + wind up to 15 knots will intensify the fog.
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3. Upslope fog moist, stable air is forced up a sloping land mass + form in
moderate to strong winds + under cloudy skies.
*.
Advection and upslope fog requires a wind for formation.
4. Precipitation-induced fog warm rain or drizzle falls through a layer of cooler
air near the surface + very dense
5. Steam fog low level turbulence and icing are associated with steam fog +
called sea smoke
Haze, smoke, smog, blowing dust or snow restrict visibility
Haze concentration very fine salt or dust + stable + sometimes extend to 15000 ft
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