Aviation Ground School: 2012

Meteorology

Knowledge of weather is one of the most important skill to be learnt by a flyer. So here I presented some important notes about Meteorology. Please take your time to read.

· 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 | Day time!

· 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, risesand converges towards the centre.

Secondary depressions are usually more intensethen primary depressions.

Troughsare 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 pressurefor 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 forwarm 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 latitudesdue 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 notdepend 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 & becomingexcellent 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) whichrelies 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 degreesand 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

Aviation Ground School

Pages

Tuesday, 18 September 2012

Sunday, 24 June 2012

Navigation Exam Preparation

Thursday, 3 May 2012

IF Test Preparation

Meteorology 101

Knowledge of weather is one of the most important skill to be learnt by a flyer. So here I presented some important notes about Meteorology. Please take your time to read.