Download Weather Info 2

Frontal Systems
2.50 Front and Frontogenesis
Front – An area of discontinuity that forms between two contrasting air masses with sufficiently different
temperature and moisture properties.
Frontogenesis – The formation of a new front or the regeneration of a decaying front that occurs
when a relatively sharp frontal zone develops between two air masses whose properties contrast more
and more over time.
2.51 Front Characteristics
Frontal cloud and precipitation patterns of most fronts are not recognizable above 15,000 feet. However,
temperature and pressure gradients often extend into the troposphere. Winds usually shift 90 from one
side of the front to the other. Speeds of warm fronts are usually 15 knots, and cold fronts move about 20
knots.
2.52 Polar Front
The polar front is the zone separating the warm tropical air masses and the cold polar air masses.
2.53 Atmospheric Discontinuities Across Fronts
The four major properties that distinguish a front are the changing temperature, varying dew points, wind
shifts and pressure gradients.
2.54 Frontal Weather Factors
The five major factors that influence weather are the slope of the front, the spread of the frontal
movement, the degree of stability of the lifted air, the amount of moisture available, and the temperature
and moisture gradient.
2.55 Cold Front Characteristics
A cold front is the leading edge of an advancing cold air mass. A cold front overtakes and uplifts the warm
air mass creating violent unstable conditions, cumulonimbus clouds, thunderstorms and severe
turbulence. Ahead of the front, the SW winds increase, the barometric pressure decreases and
altocumulus clouds appear on the horizon. The cumulonimbus clouds move in and develop rain or snow
showers, increasing in intensity as the front approaches. As the front passes, the pressure rises sharply,
the wind shifts to the NE and the clouds clear rapidly. The temperature and the dew point decrease, and
colder drier weather begins. Cold fronts generally move southeast at 20 knots.
2.56 Squall Lines
A squall line is a nonfrontal line of violent thunderstorms. They usually develop 50 to 300 miles ahead of
the cold front and run parallel to it. Squalls contain severe weather conditions such as thunderstorms,
heavy rain, lighting, icing, heavy turbulence and hail or tornadoes. Squall lines are considered to be
frontal weather.
2.57 Warm Fronts
A warm front is the leading edge of the advancing warm air mass that is overtaking and replacing a colder
air mass. A warm front typically moves slower (15 knots) in a NE direction and tends to glide over the top
of the colder air. The front is gradual and characterized by a sequence of clouds, beginning with cirrus,
cirrostratus, then altostratus, nimbostratus, and then the low-lying stratus, with rain and fog. With the
passage of a warm front, the winds change from the SE to the SW, and are followed by an increase of
pressure and temperature.
2.58 Stationary Fronts
A front that has no movement from one air mass toward the other. It is characterized by a pressure
gradient and by a 180 shift in winds. The weather patterns of a stationary front are often very similar to
the weather patterns of a warm front, but are usually less intense.
2.59 Occluded Fronts
Occluded fronts occur when three fronts meet. It occurs when a cold front overtakes a warm front that is
behind a cold front. The type of occlusion that develops depends on which front remains in contact with
the earth. Occluded fronts generally align themselves to the north and south and move to the northeast at
the speed of the front that is near the earth. The weather associated with the occlusion will be a
combination of both types of frontal-weather. A cold front is an occlusion where the colder of the two cold
fronts is the one overtaking the warm front. A warm front occlusion is an occlusion where the warmer of
the two cold fronts is overtaking the warm front, and rising over the other cold front.
2.60 Upper Fronts
An upper front is a cold front aloft that moves over an even colder air lying in the lower layers of the
atmosphere.
2.61 Inactive Fronts
Inactive fronts do not create clouds or precipitation. This occurs when the fronts are too dry to form
clouds. The front only has a shift in wind direction and pressure.
Thunderstorms
2.62 Requirements for Thunderstorm Formation
Lifting action, unstable air, high moisture content, and a cloud building through the freezing level.
2.63 Thunderstorm Life Cycle Characteristics and Pressure
Cumulus Stage – The initial stage is always a cumulus cloud. Updrafts extend the cloud upward, and
snow and ice particles begin to develop and are carried upward into the cloud. High pressure exists
throughout the cloud.
Mature Stage – The mature stage is reached when the raindrops and ice particles begin to fall.
Downdrafts occur, expelling ice and rain. Strong vertical wind shears occur from simultaneous updrafts
and downdrafts. Pressure varies throughout the cloud.
Dissipating Stage – The Dissipating stage occurs as updrafts begin to dissipate and the entire cell
becomes an area of downdrafts with decreasing precipitation. Strong winds aloft may create the anvil
seen in many thunderstorms. Wind and pressure are still greatly varied.
2.64 Types of Thunderstorms
Frontal Thunderstorms – These are a result of the lifting of warm, moist, unstable air over a frontal
surface. They normally form in lines and may contain stratus clouds with warm fronts, Cumulus clouds
with Cold fronts, and both cumulus and stratus with occluded fronts.
Air Mass/Convective Thunderstorms – These are isolated thunderstorms that occur in warm,
moist, unstable air and are not associated with fronts. They receive the necessary lifting through, thermal,
convergence or orographic lifting.
2.65 Thunderstorm Hazards
Extreme turbulence, hail, microbursts, icing, lightning, and tornadoes.
2.66 Microburst
A microburst is an intense highly localized downward atmospheric flow that emanates from a convective
cloud, usually in the mid-afternoon of summer months.
2.67 Microburst Characteristics
Microbursts usually begin at the base of a cloud and spread out to areas over a two-mile expanse. They
usually last 5 to 10 minutes and may develop in groups. The winds can be as low as 20 knots or exceed
200 knots. They may create vortex rings, or swirling areas of winds along the ground. Dry microbursts
may occur under higher clouds that are providing no precipitation. High wind shear is associated with
these phenomena.
2.68 Tornadoes
An intense rotating column of air that protrudes from a cumulonimbus cloud in the shape of a funnel,
rotating over the earth’s surface for anywhere from a few feet to hundreds of miles, travelling from 25 to
40 mph. Tornadoes are a little known about phenomenon and are very destructive. To form, they must
have warm moist air near the earth’s surface, cold and dry air in the middle of the atmosphere, strong
upper level winds, and the presence of Cumulonimbus clouds. The possible indications for the onset of a
tornado are pronounced wind shear, rapidly moving cold front or squall line, strong convergence, marked
convective instability, an abrupt change in moisture content and marked convection.
2.69 Radar Assistance
Radar can help to detect thunderstorm activity by bouncing radar waves off of the down falling
precipitation to measure intensity.
2.70 Flying Near Thunderstorms
Avoid them if possible and do not venture closer than 20 miles of mature storm clouds or anvils. If not
possible, attempt to fly around the storm. If not possible, try to fly over the top, If not possible fly below the
storm. If not possible fly through the lower 1/3 of the storm.
Turbulence
2.71 Intensities of Turbulence
Light – Slight erratic changes in altitude.
Moderate – Greater intensity, but no loss of control.
Severe – Large abrupt changes in altitude, possible momentary loss of control of aircraft.
Extreme – Violent tossing and aircraft is nearly impossible to control. May cause structural damage.
2.72 Types of Turbulence
Thermal (Convective) – Localized vertical convective currents due to surface heating or cold air
moving over warmer ground.
Mechanical – Wind flowing over or around irregular terrain or other obstructions.
Frontal – Local lifting of warm air by cold air masses or the abrupt wind shift associated with most clod
fronts.
Wind Shear – Resulting from a relatively steep gradient in wind velocity or direction producing eddies.
2.73 Thermal Turbulence
Vertical air movements resulting from convective currents develop in air that is heated by contact with a
warm surface. This heating from below occurs when either cold air is moved over a warmer surface, or
the ground is strongly heated by solar radiation.
2.74 Cloud Formations and Thermal Turbulence
The upper limits of the convective currents are often marked by haze lines or by the tops of cumulus
clouds that form when the air is moist.
2.75 Mechanical Turbulence
Movement of wind over an obstacle creates wakes or eddies on either side of the obstacle that can
extend for miles beyond the surface that caused the turbulence.
2.76 Mountain Wave Turbulence
When the air is stable, large waves form on the lee side of the mountain peaks and extend up into the
stratosphere for up to 100 miles and downwind for up to 150 miles. Mountain wave turbulence is caused
by the oscillation of the wind as it tries to return to the level that it was before it was lifted. Rotor clouds
form at a lower level and are generally found at the same height as the mountain ridge. The cap cloud
obscures both sides of the mountain peak. The lenticular clouds are stationary and above the height of
the mountaintop.
2.77 Flight in the Vicinity of Mountain Waves
1. Avoid turbulence by flying around the turbulence, or fly 50% higher than the height of the highest
mountain range.
2. Avoid rotor, lenticular and the cap clouds.
3. Approach the mountain range at a 45 angle so that a quick turn can be made if a downdraft
occurs.
4. Do not trust the altimeter; it may indicate altitudes 2,500 feet higher than your true altitude.
5. Penetrate turbulent areas at air speeds recommended for your aircraft.
2.78 Frontal Turbulence
Frontal turbulence is caused by lifting of warm air by a frontal surface leading to instability and/or wind
shear between the warm and cold air masses.
2.79 Large Scale Wind Shear
Turbulent wind shear flight conditions are frequently found in the vicinity of the jet stream where large
shears in both the horizontal and vertical planes are found as well as in association with land and sea
breezes, fronts, inversions and thunderstorms.
2.80 Turbulence Avoidance
Choose a cruising altitude away from the jet stream, avoid areas of rapidly shifting wind direction, and
observe pilot and weather reports prior to flight.
Icing
2.81 Effects and hazards of Aircraft Icing
It increases weight and drag, decreases lift and thrust and decreases performance. It disrupts the smooth
flow of air over airfoils, decreasing lift and thrust and increasing drag and stall speed.
2.82 Supercooled water
Liquid found at air temperatures below freezing. Usually found in clouds between 0  C an –15  C.
2.83 Wet Snow avoidance
Climb to colder temperatures until encountering dry snow or descend to an altitude where temperatures
are well above freezing.
2.84 Requirements for the Formation of Structural Icing
Outside air temperature below freezing, aircraft skin temperature below freezing and visible moisture.
2.85 Temperature Range for Structural Icing
0  C or colder. The most sever icing is encountered between 0  C and -10  C.
2.86 Factors for Accumulation of Structural Icing
The size and number of water drops in a given volume of air, airfoil thickness, and airspeed. Larger
airfoils, smaller drops and decreased airspeed reduce accumulation.
2.87 Anti-icing/Deicing equipment
Deicing Boots and rubber bladders are installed on the leading edges of lift producing surfaces. They
inflate/deflate in cycles, allowing ice to crack and peel off into the air stream. Anti-icing fluids are pumped
through holes in the wing’s leading edge. Ground crews also use de-icing fluids.
2.88 Types of Structural Icing
Clear Ice – A transparent form of structural ice that forms through the slow freezing of large, supercooled water droplets. It usually occurs between 0  C and -10  C.
Rime Ice – A milky white, opaque and granular deposit of ice formed through the rapid freezing of small
super-cooled water droplets. It usually occurs between -10  C and -20  C.
Mixed Ice – A combination of rime and clear ice. It is the most common type.
Frost – A thin layer of crystalline ice that forms on exposed surfaces when the temperature of the
exposed surface is below freezing and the dew point is below freezing. It can occur in flight as well as on
the ground.
2.89 Induction Icing, Compressor Icing and Fuel System Icing
Induction Icing – Ice forming in the intake ducts during flight at temperatures above freezing, due to
low pressure inside the ducts.
Compressor Icing – Ice forming on compressor inlet screens and compressor inlet guide vanes that
restrict the flow of inlet air.
Fuel System Icing – Water mixed with jet fuel that freezes in the fuel lines.
2.90 Ground Icing Hazards
Frozen precipitation or frost, wet aircraft coming out of hangers, water or slush splashed onto the aircraft,
and ice or snow on the runways.
2.91 Conditions Associated with Air Masses, Fronts and Thunderstorms
Air Mass Icing – Stable air masses produce stratus clouds with rime icing conditions. Unstable air
masses produce cumulus clouds with clear icing conditions
Frontal Icing – Cold Fronts and squall lines generally have a narrow weather and icing band with
cumuliform clouds and clear icing conditions. The most critical area is where water is falling from warm air
above to a freezing temperature below causing clear icing conditions. Occluded fronts are the most
erratic, causing clear mixed and rime ice.
Thunderstorm Icing – The worst icing conditions are encountered at and just above the freezing
level, but can be encountered throughout the cloud at different stages of the thunderstorm.
2.92 Procedures to Minimize the Effects of Icing
1. Do not fly parallel to a front while encountering icing
2. Avoid the area below 4,000 feet around ridges in temperatures less than 0  C.
3. Do not make steep turns and avoid high angles of attack with ice on the airplane due to increased
stall speeds.
4. Do not land with reduced power when the potential for ice on the wings and other exposed
surfaces exists.
5. Do not forget that fuel consumption is greater under icing conditions, due to increased drag and
power required.
6. Avoid icing conditions in the terminal phase of flight sue to reduced airspeeds.
7. Always remove ice and frost before attempting takeoff.
8. In stratiform clouds, you can often alleviate icing by changing altitude.
2.93 Types and Intensities of Icing
Trace – Perceptible ice formation, but not hazardous. No icing/deicing equipment is used.
Light – The rate of accumulation may create a problem if flight is prolonged in this environment. Use of
icing/deicing equipment reduces/prevents accumulation.
Moderate – The rate of accumulation is potentially hazardous and use of anti-icing/deicing equipment
is necessary for flight.
Severe – The rate of accumulation is such that deicing/anti-icing equipment fails to reduce or control
the hazard.
2.94 PIREPS
Pilot reports are necessary because weather personnel often can not observe icing.
Ceilings and Visibility
2.95 Visibility Definitions
Visibility – The ability to see and identify prominent unlighted objects by day and prominent lighted
objects by night. It is expressed in statute miles, hundreds of feet, or meters.
Flight Visibility – The average horizontal distance of visibility, as seen from the cockpit of an airplane
in flight. It is measured in statute miles.
Prevailing Visibility – The greatest forward horizontal visibility equaled or exceeded throughout at
least half the horizon circle. It is measured in statute miles.
Slant Range Visibility – The distance on final approach when you can see the runway.
Runway Visual Range (RVR) – The horizontal distance a pilot will see by looking down the
runway from the approach end. It is expressed in hundreds of feet or meters.
2.96 Sky Coverage
Classification
SKC
Meaning
Sky Clear
Sky Coverage
0/8ths
FEW
Trace
0/8-2/8ths
SCT
Scattered
3/8-4/8ths
BKN
Broken
5/8-7/8ths
OVC
Overcast
8/8ths
VV
Vertical Visibility
8/8ths
2.97 Obscuring Phenomena
Any collection of particles, such as haze, fog and smoke which reduce horizontal visibility to less than
seven miles.
2.98 Ceiling and Vertical Visibility
Ceiling – The height above the ground (AGL) with the lowest broken or overcast layer; or the vertical
visibility into an obscuring phenomena.
Vertical Visibility – The distance that can be seen directly upward into an obscuring phenomena.
2.99 Relationship between Vertical Visibility, Obscuring Phenomena and a Ceiling
If the sky is totally hidden from view by a surface based phenomenon, the reported ceiling is determined
by the vertical visibility upward as seen from the ground. When surface based obscuring phenomena
present a slant range visibility problem they may also be classified as a ceiling.
2.100 Fog
Fog is a visible mix of small water particles that resides within 50 feet of the surface, is greater than 20
feet in depth and reduces vertical and horizontal visibility to less than 5/8 of a statute mile.
2.101 Four Requirements for Fog Formation
1.
2.
3.
4.
The air must have a high water content.
The temperature and dew point temperature must be equal.
Condensation nuclei must be present in the air.
Light surface winds must be present.
2.102 Wind and Fog
The horizontal motion of air next to the earth’s surface produces friction; friction causes the air near the
ground to tumble, resulting in eddy currents. These currents mix the air sufficiently to cause a layer of air
near the ground to reach a saturated state and produce fog or low clouds.
103.
Types of Fog and
2.104 How They Dissipate
Ice Fog – A type of radiation fog that forms in moist air during extremely cold, calm conditions.
Steam Fog – An evaporation fog forming when cold air moves over warmer water. Steam Fog
dissipates by heating the air through conduction.
Upslope Fog – Forms when orographic lifting causes enough adiabatic cooling to reduce the air
temperature to the dew point temperature. Dissipation occurs when a wind shift pushes the air down the
slope and causes adiabatic warming.
Radiation Fog – Fog formed due to the reduction of air temperature from nocturnal cooling. Radiation
Fog is dissipated through winds of greater than ten knots or through heating of the earth and the
consequent conduction.
Frontal/Precipitation Fog – Fog forming through the addition of moisture to air through the
evaporation of rain or drizzle. Dissipation occurs with the passage of the frontal system of through
cessation of precipitation.
Advection Fog – Fog forming as warm, moist air travels over a cold surface and the air cools below
its dew point. It is dissipated through wind shift.