Fly - Turbulence & Icing (3)

3a) Identify atmospheric layers according to temperature characteristics in the standard atmosphere

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3b) Determine the static stability given temperature soundings, and describe its effects on air motion and on aviation

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3c) Describe how different types of turbulence form, and relate turbulence intensities to aircraft behavior

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3d) Describe the characteristics and causes of mountain waves, relate them to winds and stability using the Froude-number, and describe how they affect flight

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3e) Describe the characteristics and causes of clear air turbulence (CAT), relate them to winds shear & stability

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3f) Compare the characteristics and causes of boundary-layer & obstacle/mountain-wake turbulence, and describe their effects on aviation

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3g) Explain how and where supercooled water forms, and explain how ice on aircraft affects flight

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3h) Locate likely areas of turbulence, icing, and thunderstorms relative to drylines and to warm, cold & occluded fronts, and describe how these frontal hazards affect aviation

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How (natural) turbulence forms

-convective turbulence, or free convection, or thermal turbulence (buoyancy, warm air rising and cold air sinking) -wind-shear turbulence, or forced convection, or mechanical turbulence (different wind speeds or wind directions at different altitudes -obstacle turbulence (wind hitting an object and flowing around it) the wakes behind fixed objects such as mountains

Temperature change with altitude

-decreases smoothly at first with increasing altitude (troposphere to tropopause) 0-10km -stable for a bit then increases with altitude (tropopause to stratosphere to stratopause) 10km-45km -decreases with altitude 45-85km (stratopause to mesopause) -inconsistently increases 85-100+ (mesopause to thermosphere)

Types and Locations for turbulence

-mountain-wave turbulence (see Learning Goal 3d), -clear-air turbulence (Learning Goal 3e), -boundary-layer and obstacle-wake turbulence (Learning Goal 3f), and -convective turbulence including thunderstorm turbulence (Learning Goals 4b-g).

Static Stability Tool --> how to use it

1. find the temperature difference between the two altitudes and follow the brown line upwards... 2. ...until you hit the green line corresponding to the altitude difference 3. Where brown and green lines cross, follow the purple line up to read the static stability "S" in units of C/km

stratosphere

11km-47km -top of stratosphere = stratopause -temp is constant (isothermal) at first then gets warmer as you approach stratopause -warm because of "good" ozone later which gets hot when it absorbs harmful ultraviolet rays from the sun -modern commercial aircrafts fly in lower stratosphere to get above most storms and weather

CAPE Interpretation

1750 --> ordinary thunderstorm 1850 ---> marginal supercell thunderstorm 1950 --> supercell thunderstorm, no tornado 2150 ---> supercell thunderstorm with weak (EF0-EF1) tornado 2850 ---> supercell thunderstorm with strong (EF2-EF5) tornado

dry line

A boundary between dry and moist air masses of virtually same temperatures can still be dangerous dry air is denser --> so moist air will rise over dry air --> trigger thunderstorms along the dry line behaves similarly to a cold front in its ability to trigger thunderstorms

CAPE

Convective Available Potential Energy stability indices for thunderstorms --> don't tell us IF a thunderstorm will form, but DO tell is how strong the storm may be if it forms measures accumulated buoyant energy of a rising blob of warm humid air relative to the cooler surrounding environment that it is rising through larger CAPE = STRONGER storm Smaller CAPE = milder storm

What can pilots do in the face of Fronts?

Do their homework before taking off by checking the weather maps (see example below) and forecasts, and making a go/no-go decision for the flight. But regardless of what the forecast said (because forecasts can be wrong), believe what you actually see in front of you, and change your flight route if needed to stay safe. Fly around the front (not always possible because fronts can be thousands of kilometers long). Fly under the front (not always possible because sometimes the bad weather can reach the ground). Fly over the front (not always possible, even for commercial airlines, because frontal clouds can extend to 15 km altitude). If you are flying for fun, then don't fly when a low or front is near. Instead, wait for a high-pressure system to move in, which has fair skies and light winds. Land just before you get to the front, and wait for the front to pass before you take off again. You might need to stay overnight. This is my favorite, because I've accidently "discovered" some beautiful places during these unplanned overnight stops.

What happens if you don't have de-icing equipment and accidentally find yourself in cloud with ice accumulation?

Fly out of the cloud (laterally or vertically). Namely, make a U-turn to fly out of the cloud, or climb or descend to clear air above or below the cloud. Climb to altitudes where there might be warmer air in the cloud (no guarantees that warmer air exists at higher altitudes) Descend to altitudes where the air is warmer in the cloud or rain (doesn't work in winter when the cold air reaches the ground) after you've exited cloud, ice should evaporate (sublimate) very slowly in clear air, even if air is below freezing ... but this could take an hour or so! Would be faster in warm air

Tips when flying in icing conditions

If you are not flying in a cloud or through a rain shower, then ice will not form on the aircraft (see exception below). If you are at an altitude where the temperature is warmer than freezing, then cloud and rain drops are usually not supercooled and do not cause aircraft icing. They just give your plane a shower and wash off all the bugs. If you are at an altitude where the temperature is colder than -40°C, than ice will not form on the aircraft (see exception below). The rate of ice buildup depends on how much supercooled water is in the air. For non-precipitating clouds, the tiny cloud droplets cause a gradual accumulation of ice on the aircraft. For clouds with drizzle or light rain (in the cloud or falling below the cloud base), accumulation of ice is faster. If you are accidently flying through a thunderstorm at an altitude where heavy rain with large supercooled raindrops can occur, then ice buildup on the aircraft can be extremely rapid and dangerous (in addition to all the other thunderstorm hazards).

Severe turbulence

Large abrupt changes in altitude/attitude and airspeed Momentarily out of control forced violently against seat belts walking is impossible un-secure objects thrown about

Hazards of icing

On wings, it disturbs the airflow, so the wing gets less lift (the upward force that keeps airplanes up). Anywhere on the aircraft, it adds weight (helping gravity to pull the aircraft down). On propellors, it decreases their thrust (the ability to pull the aircraft forward). Anywhere on the aircraft, it increases drag (tending to slow the aircraft due to friction). On the empenage (the tail of the airplane), it reduces the ability to keep the aircraft flying level. On the wind screen, it prevents you from seeing where you are going. On carburated engines (not fuel injected; not turbines), ice forms in the carburator, partially or totally blocking flow of the fuel-air mixture into the cylinders of the engine (i.e., the engine loses power or dies). Ice can form on engine air intake filters and manifolds, reducing the air flow needed for the engines to properly burn their fuel. This literally chokes the engine. On some control surfaces (ailerons, elevator, rudder, flaps) it can prevent their movement so the pilot cannot control where the aircraft is flying. Ice can clog the pitot tube (which measures speed of the aircraft), causing erroneous speed readings in the cockpit. On some turbine (jet) engines, ice can form on the turbine blades, reducing thrust (see last subsection). On the space shuttle, ice that breaks off during launch can hit other parts of the shuttle that can damage the protective tiles on the wings and body, causing the shuttle to burn and explode during reentry.

Static Stability Tool --> how to interpret 'S'

Stable: S = + air becomes non-turbulent (no wind shear) Neutral: S = 0 Unstable: S = - air becomes turbulent Unstable air always wins if your calculations give you different values for the same location ==> unstable

supercooled water hazards during each season

Summer: icing to aircraft mainly in the top half of a thunderstorm and other deep convective clouds winter: everywhere Fall/spring: supercooled water associated with weather fronts like warm fronts

Statically NEUTRAL conditions: How it affects aviation weather

Typically forms in overcast conditions (day or night) when the wind is moderate or strong. Air can be nonturbulent, but the slightest wind shear can create dynamic instability and turbulence. Neutral air hitting a mountain creates obstacle wake turbulence downwind of the mountain.

Statically STABLE conditions: How it affects aviation weather

Typically forms near the ground at night under clear skies with calm-to-light winds, when the ground temperature decreases, and the ground cools the air that touches it. Air is usually nonturbulent (a smooth ride in an airplane) if there is no wind shear. But if this air is forced up over a mountain, then on the downwind side it sinks past its starting altitude (i.e., overshoots downward), and then starts to rise again and overshoots upward, and keeps oscillating as a mountain wave. Cold air near the ground can drain downhill and can pool in the valley floor, where frost or fog can form in the cold air. If there is wind shear in a statically stable region, then turbulence can form, along with breaking atmospheric waves called Kelvin Helmholtz waves (see Learning Goal 1b).

Statically STABLE conditions: How it affects aviation weather

Typically occurs in sunny days over land when the average winds are relatively light. It can be caused when the sun heats the land and the land warms the bottom layer of air. Thermals of warm air rise with cooler descending air between the thermals. Cumulus clouds might form at the top of the thermals if the thermals are humid enough and the unstable air is deep enough. If the unstable air extends over most of the depth of the troposphere, then thunderstorms can form (with many flight hazards).

Frontal hazards to pilots

VFR pilots (i.e., not flying on instruments) encounter clouds, precipitation, low ceilings (= low cloud base), and poor visibility near the front that can block their intended flight (Learning Goal 1g). Strong winds can occur near fronts, including wind shifts with time and wind shear with altitude. The result could be headwinds that slow the flight and require the aircraft to consume more fuel. Strong crosswinds at small airports could prevent safe landings. See Learning Goals 2d and 2e. Thunderstorms, which have many flight hazards (Learning Goal 4b) can occur along cold fronts and can be hidden inside occluded fronts. Depending on the season, if the cold air near the ground is below freezing and the air above the front is warmer than freezing, then rain falling from warm air layers into lower cold layers can become supercooled to create aircraft icing hazards and freezing rain (Learning Goal 3g). Drylines behave as cold fronts and can trigger dangerous thunderstorms. Heavy snowfall during frontal passage could temporarily close an airport until the snowplows can clear it.

Extreme turbulence

aircraft violently tossed about and is practically impossible to control may cause structural damage not used in Canada

lenticular clouds

aka mountain wave cloud, or lee-ward cloud when there is a layer of humid air near the altitude of the mountain wave, then these clouds can form at the crests of the waves provide visible evidence of the presence of mountain waves ... but most mountain waves don't form these clouds

temps colder than -40°C

all liquid is already frozen into ice crystals, so there is not an icing hazard at these altitudes

Troposphere

almost all weather clouds occur here -temp decreases with altitude -11km top of troposphere the temp is about -56 degrees C -top of troposphere = tropopause -thinner during winter and near poles -aircrafts fly here

Rawinsondes

are weather balloons launched by national weather services, they have small sensor packages and radio transmitter hanging from the bottom as balloon rises, sensor measures temperature, humidity, and pressure eventually at high altitude the balloon bursts and sensor gently falls by small parachute to earth

mean-free path

average distance between molecules

frontogenesis

birth of a new front

Front

boundary between warmer and cooler air often have clouds, precipitation, strong winds, and turbulence --> all could be flight hazards often rotate around a low-pressure center ("low") both temperature and wind speed, wind direction, clouds, humidity, etc. can all change across the front

moderate turbulence

changes to altitude/attitude but aircraft remains in control rapid bumps or jolts (chop) Definite strain against sea belts objects dislodged difficulty walking

How to deal with ice

check forecast and avoid areas of ice VFR pilots stay out of clouds anyways, but should avoid rain showers if temp is below freezing at their flight altitude some aircrafts have de-icing equiptment on the aircraft de-icing, and anti-icing, Some jet aircraft duct some of the hot jet exhaust through the wing leading edges and other critical areas of the aircraft, to keep them too warm for ice to form. Other aircraft have electric heaters embedded in the wings or glued to the leading edge, but these heaters draw so much electricity that they are not used on smaller aircraft. Some aircraft have pneumatic boots glued to the leading edge of the wing (where ice usually has the greatest accumulation), which can be inflated and deflated with air to crack off any ice that has formed. You can recognize these as the black covering on the leading edge of the wing. Some aircraft carry a special antifreeze or other chemical that can be pumped onto the propellors or windscreen to disolve the ice. Almost all carburated aircraft have a control called "Carburator Heat" that can be activated by the pilot to draw warm air from near the hot engine into the carburator. This reduces engine power slightly (because hot air is less dense and has fewer oxygen molecules for the engine to use), but melts or prevents carburetor ice which would otherwise cause the engine to stop.

frontolysis

death of an old front

Froude Number --> how to find it

defined only for positive stabilities 1. start with wind speed M at mountain top, and follow the grey line horizontally to the right until you hit the diagonal blue static stability line 2. next, follow the green lines straight down into the bottom graph, until you hit the purple diagonal line for the width of the mountain 3. finally, follow the grey line horizontally right to read the Froude Number

static stability

depends only on the temperature layering in the atmosphere, not on the wind related to the fact that cooler air is denser than warmer air if cool heavy layer of air is under the warmer lighter layer, then that part of the atmosphere is statically stable -no wind shear means a statically stable region is non-turbulent, because the air is ''happy" where it is (colder air doesn't need to sink, because it's already at the bottom)

cloud shield

extensive deck of stratiform clouds that can occur ahead of surface warm front -cirrostratus clouds at leading edge of this shield

frost

forms when water vapour deposits on the aircraft and forms a "fuzz" of small ice crystals on the aircraft

wake turbulence

human caused aircrafts leave a train of turbulence and vortices behind them as they fly

billow clouds / Fluctus

if CAT forms at an altitude where air is humid (and almost cloudy) then the updraft portion of the K-H wave can cause enough lifting to create a thin cloud along the wave crest --> if many adjacent waves have these clouds --> bollow clouds / Fluctus still CAT even if air isn't perfectly clear

stationary front

if boundary doesn't move much

cold front

if cold air advances warm air -cumuliform clouds (ie. thunderstorms) are often found at cold fronts in North America: winds ahead of cold front have southerly component, and can form strong low-level jets at night warm, humid hazy air advects from the south -squall line may form in advance of front in the warm air --> can be triggered by wind shear and advection near fronts, and can also consist of thunderstorms that formed on the cold front but progressed faster than the front -narrow bands of towering cumuliform clouds with possible thunderstorms and scattered showers -stronger gusty winds along front, and pressure reaches relative minimum -thunderstorm anvils spread hundreds of km ahead of surface front -winds shift to northerly direction behind front, advecting colder air from the north -if sufficient moisture, scattered cumulus or broken stratocululus clouds can form within cold airmass -as sirmass consists of cold air advecting over warmer ground --> statically UNSTABLE, convective, and VERY turbulent

occluded front

if cold air catches up to and merges with warm front Cold occlusions warm occlusions ^both occur when a cold front catches up to a warm front COLD occlusion = advancing cold front has colder air that is retreating ahead of the warm front otherwise = WARM front Characteristics important to pilots: The resulting clouds and weather are a combination of widespread stratiform drizzle and focused intense thunderstorms. Namely, an IFR pilot could encounter dangerous thunderstorms embedded in gentle stratus clouds. Or a VFR pilot could be flying between stratiform layers and have part of the route blocked by a thunderstorm updraft tower (see figures below). The warmest air between the cold and warm fronts is pushed upward above the collision between the cold and cool air, causing fronts aloft.

warm front

if cold air retreats from warm air -stratiform clouds often found at (and ahead of) warm fronts in N america: -southeasterly winds ahead of front bring in cool humid air from atlantic ocean, or mild humid air from Gulf of mexico -cloud shield can occur hundreds of km ahead of surface front -cirrostratus clouds at leading edge of this shield -along frontal zone can be extensive areas of low clouds and fog, creating hazardous conditions -nimbostratus clouds cause large areas of drizzle and light continuous rain -pressure reaches a minimum at front -wind shifts to more southerly direction behind the warm front, advecting warm, humid, hazy air

Pilot reports (PIREPS) of Ice

if pilot experiences ice accumulation on their aircraft, they can help pilots by reporting it to air traffic trace: ice becomes perceptible. rate of accumulation slightly greater than sublimation. De-/anti- icing not used unless encountered over long period of time light: rate of accumulation may create a problem if flight is prolonged in this environment. occasional use of icing equipment to prevent accumulation Moderate: rate of accumulation is such that even short encounters become potentially hazardous. use of icing equiptment or flight diversions is necessary severe: rate of accumulation is such that icing equiptment fails to reduce or control hazard. Immediate flight diversion is necessary

Statically unstable

if the warm air is under the dense cool air, then atmosphere structure is "unhappy" --> cool air starts to sink and warm air starts to rise the atmosphere 'turnover' where cool and warm air change places is a complex turbulent dance of eddies

Icing hazard exceptions

if very cold aircraft descends into warmer humid air then frost can form on aircraft frost isn't heavy but decreases lift and increases drag, and can block your windscreen view some nigh-bypass turbofan jet engines can develop ice on the turbines even when the aircrfat is flying at high altitude where the temp is colder than -40 this is because the ice crystals melt while flowing into the rurbine and then refreeze on the turbine blades --> turbine develops less power, or maybe even stops running

at air temperatures between -40°C and 0°C ...

it's possible for the cloud and precipitation to be all ice crystals, all liquid water, or a mixture of both but any liquid water can freeze instantly when hit by an aircraft altitudes between -40°C and 0°C are the danger zones for ice formation on aircraft danger only exists if flying through liquid water clouds or rain clear air or 100% ice crystal clouds = no hazard of icing

Obstacle-wake turbulence; also can form as mountain-wake turbulence

large obstacles (such as mountains) that have created regions of strong obstacle turbulence downwind of them when the wind is STRONG and when air near the ground is relatively warm compared to air aloft (statically neutral or unstable) can affect your flight any time you're near or below the altitude of the mountain tops

Clear ice

large raindrops take a second or more to freeze when they hit the aircraft allowing some of the water to flow a short distance before freezing droplets in regions of 0 to -5°C air temperature freeze slowly. "slow" freezing allows any trapped air bubbles to escape --> ice is CLEAR (or dark) and very hard & difficult to remove

mixed ice

layers or mixtures of clear and rime ice most frequent when aircraft is flying in clouds where air temperature is -5 to -15°C

squall line

line of thunderstorms NOT along a front

supercooled droplets

liquid water that remains unfrozen at temperatures well below freezing in the atmosphere can be liquid between -40 °C and 0°C BUT supercooled liquid droplets freeze almost instantly when they touch something solid and cold such as an airplane, a snowflake, tree branches, power lines, runways, and road surfaces

ice crystal icing (ICI)

normally ice forms when aircraft flys through LIQUID water below freezing, and usually when flying through SOLID water (snow) it just blows around aircraft BUT when snow enters hot engine intake --> melts & re-freezes when aircraft flys through ice-crystal cloud, if conditions are right then ice instantly melts when it enters heated nacelle at jet-engine inlet but then refreezes on turbine blades in first part of the engine ice can build up on turbine blades --> degrades performance of the engine, and can damage the engine if chunks of ice break off and shatter the turbine blades further back in the engine. NOT a hazard most general aviation pilots would encounter

K Index

older measure for rain intensity in thunderstorms looks at temperature and dew-point (humidity) of environment at just a few key altitudes and is useful for indicating rain intensity in thunderstorms <20 = thunderstorm unlikely 20-30 = chance of scattered thunderstorms 30-40 many thunderstorms likely, some with heavy rain >40 many thunderstorms with very heavy rain

anti-icing

prevents ice from forming

Rotor clouds

ragged clouds below the lenticular clouds indicated extremely violent turbulence with up and downward motions that can exceed the climb speed of the aircraft

Froude Number(Fr)

ratio of natural wavelength to mountain width (W), when the most intense mountain waves occur with the most violent up- and down-drafts Fr<<1 mild top wiggle winds are light and stability is strong natural wavelength shorter than mountain width Fr= VIOLENT natural wave matches mountain width Fr>>1 super long slow wavelength Fr= infinity is CRAZY wake turbulence but not really waves

Temperature soundings (Upper air sounding)

record of outside air temperature at a bunch of different altitudes helps gather weather conditions to determine stability modern aircrafts have automated systems Rawinsondes are weather balloons launched by national weather services, they have small sensor packages and radio transmitter hanging from the bottom

jet stream

region of fast winds 6-12km altitude

de-icing

removes ice that has formed on aircraft

Light turbulence

slight erratic changes, slight rythmic changed (shop) slight strain against seat belts little or no difficulty walking

mountain-wave turbulence

smooth periodic motion that is different from the quasi-random nature of turbulence can can cause aircraft to be pushed up and down in a regular cycle (Chop) can create strong up or down drafts that move your aircraft from desired altitude can be so strong that its impossible to maintain correct altitude, and can even cause structural damage to the aircraft

Actual vs. standard atmosphere

standard atmosphere --> average conditions actual temperature structure is never average (varies by location, time, weather conditions, etc)

Mesosphere

temp decreases with increasing altitude -top of mesosphere = mesopause

Thermosphere and exosphere

temp increases with height above thermosphere is exosphere --> air so thin that molecules don't behave like a gas -bottom of exophere is exobase (500-1,000 km top is 197,000km .. halfway to the moon) exosphere: air molecules are so far apart ... at 400km altitude, air molecules are 16km apart from eachother

adiabatic lapse rate

temperature change for vertically moving blobs of air (air parcels) when air rises in the atmosphere, it moves into regions of lower pressure, which allows the air to expand and get cooler. Similarly, when air sinks in the atmosphere, it is compressed in the higher pressure and becomes warmer. 9.8 deg C/km

mountain waves

terrain-induced vertical oscillations of the air occurs when statically stable air blows towards a mountain range, and some lower altitude air may be blocked while the higher air can flow over the top. but top air is deflected upward by the mountain --> as it flow up and over the mountain it sinks back to its original altitude, and can even overshoot downward resulting airflow is wave downward of the mountain range type of motion depends on the relative strengths of the static stability and mean wind speed

Natural wavelength

the air has a natural wavelength that depends on stability and wind speed. faster wind speeds (M) create longer wavelengths greater stability (S) creates shorter wavelengths can use M and S in the Froude-number graphical tool to estimate natural wavelength

Turbulence hazards to aviation

weak --> no hazard, just inconvenient moderate --> sensitive passengers become airsick Severe --> exhausting for pilot to fly in because its hard work manipulating the controls to keep aircraft level, on course, and on altitude extreme --> things break (wings), never fly in extreme turbulence

Boundary-Layer Turbulence

when clear-buoyant updraft/downdrafts and wind shear create turbulence int he bottom 300m - 4km of the atmosphere this layer of the atmosphere is called the atmospheric boundary layer (ABL) or planetary boundary layer (PBL) and the turbulence in this layer is called boundary layer turbulence weak to moderate and not usually a hazard to aircrafts nonexistant/ weak & sporadic at night when winds are light But sunny day over land, convective boundary layer starts as a very thin layer near the ground, and gets thicker and stronger as the day progresses as thermals of warm air rise from the ground thermals carry air pollutants upward --> cause updrafts --> bumpy flight

Rime ice

when smaller cloud droplets freeze instantly upon hitting the leading edge of the wind and fuselage (body) of the aircraft droplets in regions of -15 to -20°C air temperature freeze faster. This traps air pockets between the frozen drops, which scatter the sunlight making the ice look white or milky. Also, this rime is relatively brittle and does not have much strength, so it can break off easier.

Kelvin-Helmholtz Waves (K-H waves)

when warmer air on top is moving faster than cooler air below --> wind shear across a statically stable region --> interface between the two layers forms waves in the air

Clear-air turbulence (CAT)

when wind-shear turbulence happens outside of (and not near) a thunderstorm cloud can happen at any altitude, but often most intense where winds are strongest such as near jet stream shaped like pancakes can often exit CAT by climbing or descending to a different altitude

stable static stability

whether the air will become or stay non- turbulent , or "laminar" (smooth flow)

unstable static stability

whether the air will become or stay turbulent

Atmospheric Stability

whether the air will become or stay turbulent (unstable static stability) or will become or stay non-turbulent (stable static stability)