Meteorology
· Water vapour weights 5/8 that of dry air of
comparable temperature and pressure. 0.2% in polar regions and 4.0% in
tropical regions.
· Ozone is approx. 150km deep, temperatures
tend to be maximum at approx 50km above the earths surface.
· Stevenson screen: This is used to measure humidity.
Dry and bulb and wet bulb thermometers which are housed in a structure, is
placed 4ft above the ground away from buildings and trees. Always
faces south in the SH and north in the NH! This to prevent direct sunlight.
· The greater the evaporation the more latent heat
released which means a greater temp. drop. The closer 2 readings means the
air is saturated, mist can be forecast when both readings are within 3 degrees!
· Temperature decreases at approximately 2% per
3,000ft
· Density decreases at approximately 10% per
3,000ft
Radiation
· Earth is heater via solar/short wave radiation or insolation | VIOLET OR ULTRA VIOLET Daytime!
· Earth is cooler via long wave radiation or terrestrial
radiation | RED OR INFRARED |
Night time!
· Heating of the earths surface depends on: Daily
rotation, yearly rotation, surface type and shape of the earth
· The suns rays heat up the atmosphere by reflecting
off the earths surface | terrestrial radiation |
· Convection: Heat transfer in vertical motion, can be associated with the
formation of thunderstorms, CB ect.
· Advection: Heat transfer in the horizontal motion, common in the tropics and
associated with formation of certain fog
· Solar radiation penetrates deeper into water
because it is more transparent then land. The specific heat capacity of land
is 0.3 calories per cm3 and water
is 1.0 calories per cm3. One calorie is the amount of energy required to raise one gram of water
through 1 degree Celsius.
· Diurnal variation: gain and loss of heat through day and night periods. the DV of water
0.2 deg - 0.5 deg and the DV of land is 10 deg - 25 deg. | 50 times
greater for land! | CLOUD
COVER WILL DECREASE DUIRINAL VARIATION
· When insolation is greater then terrestrial radiation
the surface temperature will rise. when these 2 are equal the surface temp.
will start to fall. Daily maximum temp at 14:00, minimum just after dawn!
·
Inversion: This is a reversed lapse rate where temp can increase with height or
temp and decrease with decrease in height. This is common in warm fronts and is
associated with the formation of ice pellets rime ice ect.
· Temp inversion is common on a clear night when
terrestrial radiation is at its maximum, this force in lower layers of air to
become cooler then the higher layers. This is destroyed by the presence of sun
rays. Cloud cover and fog maintains temp. inversions
· Inversion during the day may form when warm
air mass passes over a very cold sea or land surface.
· ELR: The actual temp. of the air measured at different heights through
and air mass at one place and time. When ELR greater the DALR means Absolute
instability, if ELR less than SALR or negative means absolute stability. If ELR
between DALR & SALR means conditional stability depending on where the
dewpoint lies. The SALR can me as low as 1 deg / 1000ft at the equator and
as high as 2.4 deg / 1000ft at the poles.
·
Isothermal: When the ELR is said to be 0 and the temperature remains
constant! E.G Tropopause -56.5 degrees
· 1 Hpa = 27.3 ft at sea level, 1Hpa = 47ft at
20,000ft, 1 Hpa = 98.91ft at 40,000ft and 1 Hpa = 160ft at 60,000ft
· This is why flight levels above FL29 have 2000ft separation,
to ensure safe separation
· QFE: The barometric pressure at aerodrome level. The altimeter will read 0 at
the aerodrome level. Can be used to check altimeter
·
QNH: The barometric pressure at aerodrome level computed to mean sea level
using the ISA formula. This height is called altitude. Height computed
to ISA is called pressure altitude.
· Spot QNH: The valid QNH where the reading took place
· Regional QNH: The Lowest forecast QNH for a region which is used so the altimeter
will indicate lower. This is also safer!
· Isallobars: Lines joining places of equal pressure rising or decreasing tendencies,
not usually drawn on charts
·
Isobars: Lines joining places of equal pressure at mean sea level
· Low pressure system: Associated with cold and warm fronts, air flows anti clockwise, rises
and converges towards the centre.
Secondary depressions are usually more intense
then primary depressions.
Troughs
are associated with low pressures
· Anticyclone: Region of high pressure, where isobars are further apart then low
pressures. In SH airflow is anti clockwise and air flows down and outwards. Anti
cyclones build up/intensify, give way, weaken or collapse.
Ridge
of high pressure
· Pressure Gradient
Force: Acts at 90 deg to isobars, always acts from high
pressure to low pressure.
· Earth surface is heated unequally causing
differential pressure.
Humidity
· The atmosphere contains moisture in the form of
invisible gas called water vapour. The hotter the air the more it can hold, so
the amount of water vapour a parcel of air can hold
depends on the temperature.
· Dry air has a very small concentration of moisture
in it, while moist air has a large amount of moisture in it
· Absolute Humidity: The mass of water vapour contained in air expressed in grams per m3
· Relative Humidity: The amount of water vapour a mass of air contains relative to what it
can contain. When temp. increases the relative humidity will drop. 100% relative
humidity is termed saturated
· Humidity mixing ratio: This is the mass of water vapour in a mass of air (g/kg) expressed as a
ratio. The ratio will remain the same (if lifted, cooled heated or
compressed) as long as no moisture was added or removed from the air.
· Vapour pressure: Atmospheric pressure which is due entirely to the pressure of water
vapour a given temperature. Vapour pressure is at its maximum when the air is
saturated i.e. 100% humidity
· Dewpoint: The temperature at which air must be cooled at a constant pressure
for saturation to take place. The Dew point spread means the difference between surface temperature
and the dew point temperature.
· A small spread means that low cloud of
fog formation is likely. It has little bearing on precipitation.
· FOR PARTICIPATION TO OCCUR, AIR MUST BE SATURATED
THROUGH THICK LAYERS ALOFT
· Water vapour pressure is maximum when air is
saturated i.e. relative humidity 100%
Density
· Air density is calculated and not measured, it is affected
by temperature, pressure, height/altitude and humidity/water vapour present in
the air. It is inversely proportional to all of these factors except pressure,
as pressure increases so does density
· Density altitude: This type of altitude is primarily concerned/related to aircraft
performance, 118.8 ft is added for every degree correction of pressure
altitude. It is pressure altitude corrected for temperature
Stability
· The formation of clouds is primarily
depended on the stability of the air before it is forced to rise
· Adiabatic process: The decrease and increase in temperature due to expansion
& compression. When air rises it is cooled and expands, when air
descends it is compressed and heated.
· ELR: This is the environmental/ambient/actual lapse rate and is variable,
for ISA it is 1.98 degrees/1,000 ft
· SALR: The saturated adiabatic lapse rate is the lapse rate of air containing
moisture, 1.5 degrees/1,000 ft. This is variable as in poles it
would be as high as 2.5 degrees/1,000 ft and at the equator it can be as low as
1.0 degrees/1,000ft. This is due to the release of latent heat when air
is condensated/saturation takes place.
· DALR: This is the only value which is not variable and remains constant at 3
degrees/1,000ft
· At - 40 degrees SALR and DALR have the same
values, this is due to the fact that the air is unable to hold any vapour
· Absolute Stability: When actual lapse rate is less than DALR and SALR,
an inversion or isothermal indicates stability
· Absolute Instability: When actual lapse rate is greater then DALR and SALR
·
Conditional stability: When actual lapse lies in-between DALR and SALR, this
is dependent on where the dewpoint lies, after air is saturated it will cool
slower and tend to become unstable (warmer than surrounding air)
· Neutral stability: Where ELR = DLR or ELR = SALR
Winds
· The primary cause of wind variation is dependent on
the air density which is dependent on temperature. Gravity is the cause for
warm air to rise and cold air to sink (warm air weighs less than cold
air because it holds more water vapour)
· Pressure gradient: The caused when pressure flows from a high to a low pressure zone
· Corolis force: The apparent deflecting force acting at 90 degrees to isobars/direction
of movement, minimum at the equator and maximum at the poles. It is directly
proportional to wind velocity
· Geostrophic wind: The resultant wind from pressure gradient force & corolis force, these
two forces must be balanced for wind to follow the isobars. As distance
between isobars decreases the wind speed increases. Therefore for a constant
pressure gradient the wind speed towards the poles will be less than towards
the equator
· Buys-Ballots-Law: Look Low Left or back to the wind Low pressure is on the right
· Gradient wind: Where the isobars are curved because centrifugal force
has to be taken into account, this force always moves away from the
centre of the radius
· High pressure: Centrifugal force + Pressure gradient force = Corolis force.
Gradient
wind > Geostrophic wind
· Low pressure: Corolis force + Centrifugal force = Pressure gradient force.
Gradient
wind <Geostrophic wind
· Cyclostrophic wind: Circular motion of winds where there is no corolis force present, it is
common at the equator and the PGF is balanced by the centrifugal
force instead of the coriolis force
· Surface friction: When winds do not flow parallel to isobars, this is due to a reduction in wind which reduces
the coriolis force which no longer can balance the PFG. Therefore the wind will
cut isobars at an angle, generally this will occur below 2,000ft
·
During the day the wind will back &
increase, while at night it will veer & decrease
· Thermal winds: The change in direction of the geostrophic wind with
height (variation
in temperature with height). These winds always blow with colder
air to its right in SH.
· Isobaric surfaces: Points in the atmosphere which have equal pressure values,
isobaric surfaces will have their greatest slopes at mid latitudes
due to the great temperature differences. The mid latitudes near the top of the
troposphere is where westerly winds are strongest
·
During the summer seasons the hottest air is not found at the equator
but in the summer hemisphere and this causes a zone between the equator and 20 degrees North/South Latitude where the
temperature will increase instead of decreasing. This causes isobaric
surfaces to slope towards the poles which results in easterlies
(predominate bwtn equator and 15 degrees lat)
· Jet streams: A narrow frontal zone caused by differences in temperature which result
in strong westerlies. They are long, narrow and have wind speeds of at least 60kts
and rarely above 200kts. They are common at mid latitudes and lie wholly
on the boundary of warm air masses. Jet streams generally exist between 20,000ft
& 60,000ft, and CAT exist in the in the polar side of a jet
stream within 1,000ft of the tropopause. a change of 2,000ft to
4,000ft is usually sufficient to avoid CAT
· Air will usually try and flow around a mountain
rather then flow over as less energy is required, therefore it flows along
mountains
· The forcing of air through valleys is called the funnel
effect
· The Fohn/Chinook: Hot + dry winds from the interior flowing down the leeward side of the
mountain due to loss of moisture
· Berg Wind: Very hot and dry wind from the interior unlike the fohn wind it does not
depend on the height of the mountain but rather the length of
the decent from inland to sea level
· Anabatic: Daytime winds flowing upslope
· Katabatic : Nightime winds flowing downslope, related to valley fog or valley
mist, may also continue during the day if surface is covered with snow
· Sea breeze: During the day airflows from the sea to the land, 15-20 kts in
tropics, less than 10kts in temperate latitudes
·
Land breeze: At night airflows from the land to the sea as the land cools and heats
faster than the sea
·
Monsoon: Associated with land masses in tropical and subtropical areas, the winds
vary depending on the time of the year. Reaches 2 peaks. The first just after
the start of the season and the other just before the end of the season.
· Trade winds: Dominate the tropic areas, the winds here are warmer than the poles.
Therefore the air rises and colder heavier air flows from the poles. Due to Coriolis
effect these winds are deflected to the right in the NH & left in the
SH resulting in N East (NH) & S East (SH) trade winds.
· ITCZ: There region where trade winds converge is called the intertropical
convergence zone. The ITCZ follows the sun during the summer months moving well
north or well south depending on where it is summer. It is in this region that thunder
storm activity is significant.
·
Doldrums/horse
latitudes: Sub-tropical 30 deg S/N well defined areas
of high pressure of sinking, can cause vast areas of no wind
·
Polar air flow: North East in the NH and SE in the SH
·
Anemometer: Measures wind speed
Air
Masses
· Polar maritime: Low water vapour and stable at source, becoming unstable as it moves
over warmer water forming Cb, flying conditions are bumpy
· Polar continental: Very dry and very cold making it stable, becomes moderately unstable
away from source
· Tropical maritime: Exceptionally unstable at source, away from source very low clouds, fog
and moist conditions
· Tropical continental: Unstable with little cloud (as there is no moisture available over
land). Away from its source exceptionally fine weather
· Modification of Air
masses: Depends on 3 factors; The speed which
determines the time it has to acquire the surface characteristics, the diurnal
variation which cools/heats lower layers of the air mass and determines
stability, mechanical influences for
example the type of terrain it travels over i.e. mountains
Clouds
·
High: Base > 20,000ft Cirro (made from ice crystals)
·
Medium: Base between 8,000ft & 20,000ft Alto
·
Low: Base < 8,000ft Stratoform
·
Vertical: Cumuliform type cloud
·
Cirrus: Resemble mares tales
·
Cirrocumulus: Resemble fish scales
·
Cirrostratus: Forms a halo
·
Nimbus: Suffix refers to low rain clouds, usually dark/grey
·
Lenticular: Related to low level turbulence¸ shaped like almonds or lenses
·
Capillatus: Anvil shape at the top of a Cb
·
Formation: Caused by the cooling of air at a constant pressure which takes
place in 4 ways;
· Loss of heat by radiation from air, mixing of 2 air
masses loss of heat through conduction of cold surface adiabatic cooling by lifting
·
Lifting: Brought about by; Orographically, turbulence, convergence, fronts and
convection
· The more moist in a mass of air the more cloud is
developed, the more unstable the air is, the more vertically developed it
becomes
· Col area: Situated between 2 highs and 2 lows where there are variable winds
and forming cumulus cloud due to the convergence
· Turbulence cloud: Low clouds up to 4,000ft requires strong winds and stable lapse
rate and high humidity. Common in the evening
or early morning.
·
Frontal cloud: Due to adiabatic cooling when one air mass rises over the other
· Ceiling: When more than half the sky is covered by
cloud below 20,000ft (measured by ceilometers)
Fronts
· Major phenomenon associated with a front is
the sudden change in wind
· Warm front: When warm air overtakes colder air, it is slower moving that CF,
associated with bad visibility and has a shallow slope of 1 in 150
·
Characteristics of a
WF: Temperature: Rises
& remains steady
Dewpoint: Rises & remains steady
Visibility: Bad visibility remaining poor
Pressure: Falls
steadily then continues to fall steady
Wind: Backs from NNW-NW
Clouds: Cirrus,
Cirrostratus, Altostratus, Nimbostratus, Cb
· Flying Conditions: Icing occurs between 0
to -10 degrees & 0 to -20 degrees in Cb embedded in Ns
clouds. Freezing rain when rain falls from cloud above 0 degrees. Temperature
inversions, fog, mist, haze and wind-shear are common.
· Cold front:
When cold air overtakes warm air having a steep
slope of 1 in 40/80, it is a narrow (20-50 miles wide) band of weather moving
at a fast speed.
·
Characteristics of a
CF: Temperature: Sudden drop & remains low and steady
Dewpoint:
Marked drop
Visibility: Decreases & becoming
excellent after the passage
Pressure: Marked drop &
remains steady/rising slightly
Wind: Backs from NW- SW
Clouds: Cb, Altocumulus, Nimbostratus & cumulus
· Flying conditions: Frequent showers &
thunderstorms, hail, turbulence, lightning and squall lines
· Occluded fronts: When a
cold/warm front catches up to another cold/warm front
· Cold front occlusion: Cold
air --- Warm dry air --- Cool air
· Warm front occlusion: Cool air
--- Warm moist air --- Cold air
· Factors determining weather intensity any fronts; Slope, stability of warm mass, moisture content, speed of overtaking
mass, temperature diff between air masses
· Quasi-stationary front: slow moving
sideways and may just be a dividing surface of cold and warm air masses
Visibility
· Haze: Visibility < 5,000M
· Mist: 1,000M < Visibility < 3,000M
· Fog: Visibility < 1,000M
· Radiation fog: Clear skies, low to the
ground light winds btwn 2-8kts, hight relative humidity, mostly occurs in
winter just after dawn, heavy wind lifts fog transforming it to low stratus before
dissipating
· Advection fog: Warm moist air over a cold surface usually occurs over the sea but may
also drift 20-30 miles onto land. Generally deep fog (as deep as 2,000ft) which
relies on wind and lasts longer over the sea due to the cold ocean
currents
· Upslope fog: Inland fog on slopes that persists with stronger
winds, is formed through adiabatic cooling more frequently at night due to
terrestrial radiation.
· Valley fog: Katabatic wind in mountainous regions with a
continuous down slope of colder air (funnel effect)
· Steam fog: Also know as artic sea smoke, occurs in the polar
regions when a cold air moves over warmer water. Forms only over
water or wet surfaces a short distance just above it usually at dawn
· Frontal fog: Usually occurs at the slope of the fog when cloud
comes down to surface, or saturation due to continuous fall of rain. This type
of fog indicates that rain will end and good weather is on its way
· Tropical fog: When warm moist air is cooled below dewpoint,
common during winter months and found over the sea
· Smog: Combination of smoke and fog occurring at night
when large amounts of industry smoke is omitted to the atmosphere
· RVR: Used when visibility is less than 1500M, measured
by Transmissometer
· Oblique: Used to describe visibility from the air, looking forward
and down. When AC above haze climbing will increase
visibility, when AC in haze climbing will decrease
visibility
Precipitation
· Ice partial theory: Also know as the
Bergeron process, when ice crystals form from super cooled water droplets and
become too large to be held in suspension by up-currents eventually melting as
they fall
· Coalescence theory: When temperatures are
above 0 degrees and the up-drafts cause water particles to collide with
each other increasing in size until it falls through the cloud. Both
theories compliment each other
· Intensity: Slight: < 2.5mm/hr, Moderate: > 2,5mm <
12.5mm/hr, Heavy: > 12.5mm/hr with intermittent or continuous durations
· Sleet: Where both snow and rain fall together or snow
melts into rain while falling, snow creates extremely bad visibility
· Drizzle, does not make a splash when hitting the
surface (0.5mm) while Rain does make a splash (max 5.5mm), rain
can fall up to 30ft/second
· Freezing rain: Usually falls from
stratocumulus or stratus clouds and become super-cooled when falling through
air below 0 degrees freezing on impact when in contact with the AC
· Ice pellets: When super-cooled rain falls through colder air
and freezes, usually occurs in a temperature inversion in warm fronts
· Hail: Associated with Cb as a result of sublimation where
crystals can grow up to 1kg!
· Cloudbursts: Very heavy showers of short duration because
rain is held in suspension due to up-draughts, when these draughts cease the
rain is suddenly released usually 30ft/second 5.5mm rain drops in Cu or Cb
· Virga: Water droplets or ice crystals falling from the
ground but evaporating before they reach the surface
Thunderstorms
· Require: Sufficient moisture, Instability of at least 10,000ft above the
condensation level and a trigger action
· Trigger actions
include: Convective currents, frontal movement, Convergence
of air masses, Orographic lifting (storms remain stationary over the type of
feature)
· Nocturnal
thunderstorms: When mid level cloud formation creates a blanket
effect and re-radiation from the surface remains constant
· Cellular structure: Cbs are made up of several cells each having a life of 2-3 hours
& a diameter of 1-8km, cloud filed lanes up to 2km are called Saddlebacks
· Developing stage: Merging of cumulus cells, marked by up-draughts of 50ft-300ft/second (3000ft-6000ft/minute),
governed by DALR and vertical development stops when the could air is equal
temperature to surrounding air
· Mature stage: Marked by onset of rain & is the most intense period,
water accumulates and eventually overcomes up-draughts. Descending air warms at
SALR and reaches velocities up to 2,400ft/minute causing squall and
change in wind direction (first
gust). The formation of cloud is referred to as the storm collar and the lower ragged cloud is the scud
· Dissipating stage: Drown draughts gain over up-draughts, light rain, anvil formation
composed of ice crystals
· Wind shifts occurring with thunder storms may be as
much as a 180 Degree shift in direction and pressure normally drops
· Hazards: Hail (the greatest hazard), wind shear, icing (0 to -20
degrees), visible discharge known as saint Elmo’s fire
· Penetration: Maintain Attitude, disengage auto-pilot, reduce speed to
recommended penetration speed or Va, secure items. Do not attempt to turn,
ignore fluctuations of altimeter and air speed readings, turn off radio
equipment effected by static attempt to penetrate below 10,000ft & avoid
by 1nm for every 2,000ft of vertical development
Tropical cyclones
· Usually
occurs over warm water in the tropics, the warmer the water the more intense
the cyclone reaching speeds over 300km/hr
· Formation
occurs in a band from 5 – 25 degrees N/S latitudes. The eye (varies
15-20 miles) of the cyclone is usually relatively calm conditions with clear
skies surrounded by walls extending to above 50,000ft
· Water
spout: When tornadoes form
over water, tornados are prevalent over mid latitudes i.e. USA.
· Mechanical
turbulence: Low level
turbulence (3,000ft) caused by wind blowing over surface stirred into contact
with building hills ect
· Wake
turbulence: Are
present with any AC but more noticeable with large aircraft, creates
sinking layers 0f 400 – 500fpm. AC must always fly ABOVE flight path of
other AC.
· Separation: Taking off behind Heavy AC: 2 min, 3 min
if TO from RWY intersection,
· After heavy
AC has taken off: Get airborne before its lift off point or land
before its lift off point. Landing behind Heavy AC: 2 min medium, 3 min light
· After heavy AC has landed: Get airborne after touch
down point or land after touch down point
· Wind- shear: Often encountered in a temperature
inversion and may occur at any level in the atmosphere. Defined as a
sudden change in wind speed or direction in a short distance
· Mountain waves: Characterized by lenticular clouds,
wind must blow within 30 degrees of mountain range 15kts for small
mountains & 30kts for larger ranges.
· Head Wind Reduction: AC will undershoot runway due to less lift being created requires
lower thrust setting
· Head Wind Increase: AC will overshoot runway due to increased lift being created (higher
indicated airspeed) requires higher thrust setting
· Micro bursts: Associated with thunderstorms and extreme wind shear where head wind
can change to a tail wind. Often found when approach over water or
aerodromes situated near mountains. Approach techniques for wind shear include;
use less flap, flatter approach, use higher power setting, aim for long touch
down and be ready to overshoot
Icing
· There are 2 types of icing: engine icing and
airframe icing
· Carburetor icing: Most important factor is humidity, often occurs between 5 – 27
degrees, in pressure in the venture, loss of RPM, loss of manifold
pressure or rough running. Initial application of carb heat will result in
further drop in RPM /pressure but will improve as soon as ice is removed
· Throttle ice: Ice may form on the manifold, venture or the throttle depending on the
location of the butterfly valve
· Fuel injection: eliminates fuel evaporation icing
· Impact icing: Ice that freezes onto surfaces of the plane i.e. airframe icing, air
intake icing, carburetor and impact icing could occur simultaneously, ranges
from 0 to -18 degrees
· Super-cooled water
droplets: Could stay in liquid for up to – 40 degrees
and crystallization of the droplets is prevented due to surface tension. Larger
particles (Cb, Cu, Ns) are found between - 20 to 0 degrees, while smaller
particles are found between - 40 to 0 degrees. Smaller drops (St, As) have less
surface tension
· Hoare frost: White semi-crystalline coating the windscreen and wings, occurs
in the air when descending from freezing temperatures to warmer moist air. On
the ground occurs below 0 degrees but extra weight is not significant
· Rime ice: 0 to - 40
degrees SMALL cloud particles (super-cooled water
droplets), significantly alters the shape of the aerofoil freezes on
impact
· Glazed ice: 0 to -
20(or – 25) degrees LARGE cloud particles(super-cooled water droplets),
significantly increases aircraft weight and freezes on impact flowing to the
back of the wing
· Rain ice: Also known as freezing rain and is the most dangerous type
of icing. Occurs at temperatures just below freezing and usually
associated with a warm front although it may also be present in cold
fronts
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