Meteorology 3Meteorology 3

FRONTSFRONTS

Pg 6-7

Colder

Colder

Colder

Cold

Cold

Cold

Warm

Warm

Warm

PrecipitationPrecipitation

• Precipitation occurs when water droplets grow sufficiently in size and weight and then fall due to gravity. – Showery precipitation: Cumulus– Steady precipitation: Stratus

• Condensation Nuclei– Smoke, sea salt, etc.

MOISTURE CONTENTMOISTURE CONTENT

@40OC one cubic metre of air can hold 50 grams of water vapour. One M3 of air weighs about 1.35 kg. This represents about 3.5% by weight.

+

Pg 1-3

– CLOUDS– PRECIPITATION– THUNDERSTORMS– FOG– ICING– VFR MVFR IFR WEATHER– UPPER WINDS

CLOUDSCLOUDS

• CLOUDS ARE FORMED IN TWO WAYS:– air is cooled to its saturation point and

condensation occurs OR– Air absorbs additional water vapour until

saturation is reached.

• LIFTING AGENTS • Convection• Orographic lift• Frontal Lift• Convergence• Mechanical turbulence

UNSTABLE vs. STABLEUNSTABLE vs. STABLE

UNSTABLEUNSTABLE STABLESTABLE

Steep lapse rate

Cumulus type cloud

Precipitation in showers

Thunderstorms

Good visibility

Bumpy conditions

Shallow lapse rate

Stratus type cloud

Precipitation is steady and lighter

Fog

Poor visibility

Smooth conditions

PrecipitationPrecipitation

.

- Any precipitation, in order to form, requires Any precipitation, in order to form, requires condensation nuclei. condensation nuclei.

- Temperature to dew point spread must be small- Temperature to dew point spread must be small

Pg 8-17

PrecipitationPrecipitation

Pg 9-2

THUNDERSTORMSTHUNDERSTORMS

• Requirements:– Unstable air to high levels– High relative humidity– Lifting agent

• The three stages:– Cumulus– Mature– Dissipation

THUNDERSTORMSTHUNDERSTORMS

Pg 15-2

• Cumulus Stage:– Cloud driven upward by the latent heat as water vapour condenses– Temperatures in cell are higher than outside temperature at the same level.– No precipitation as water and ice are suspended.– Strong updrafts prevail throughout.

• Mature Stage:– Heights may reach 60,000 feet– Updrafts up to 6,000 ft/min or over 100 km/hr.– As the water droplets grow large enough to fall, they drag air down with them.– Downdraft starts in the middle region of the cloud.– Lightning, microburst, hail, wind shear.– Arrival of precipitation on the ground in mature stage.

• Dissipation stage:– Downdraft spreads throughout the cell (except top)– Gradual cessation of rainfall and the passing rain may cool the lower regions– Anvil shape

• Total cycle:– Can form in 15-20 minutes although whole process usually 1 to 3 hours

Thunderstorm TypesThunderstorm Types

• Air Mass:– Form singly or in clusters on hot days.– Usually VFR weather around them– Convection and/or orographic lifting

• Frontal:– Usually advancing cold front– Squall line – very turbulent and slows front

• NOTE:– Either type can penetrate the tropopause which can

be based from 28,000 feet at poles to 54,000 feet at equator

Convective ThunderstormsConvective Thunderstorms

Pg 15-11

Orographic LiftOrographic Lift

Pg 8-15

Pgs 15-9,10,11

Thunderstorm WeatherThunderstorm Weather

• Turbulence: has potential to overstress a/c; max turb. near mid-levels 12,000 to 20,000 feet

• Lightning: may blow out electrical fuses; blindness; structural damage; ignite fuel cells; greatest potential between -5°/+5°

• Hail: encountered between 10,000 and 30,000 ft; may be encountered in clear air downwind of storm; airframe damage

• Icing: heaviest icing occurs just above freezing level where droplets are super cooled. Severe in the mature stage near cloud top.

• Pressure: rapid barometric changes, altimeter may be unreliable.

• Rain: reduced visibility, refraction on windscreen, hydroplane

Do’s and Don’ts of ThunderstormsDo’s and Don’ts of Thunderstorms

Pg 15-18

Gust Front HazardsGust Front Hazards

Pg 15-18

Anvil, Multi-Cell Anvil, Multi-Cell & Lightning& Lightning

Positive build up in top and negative build up in cloud base is usual

Pgs 15-3,5,7

FOGFOG

FOGFOG

• Fog is a cloud (usually stratus) that is in contact with the ground.– forms in relatively stable air where the temperature

to dew point spread is small, wind may be present– requires condensation nuclei– Usually needs a cooling process

• Types of Fog:Radiation Advection

Upslope Steam

Frontal Ice

Radiation FogRadiation Fog: clear night, light : clear night, light wind, high humidity, often a high wind, high humidity, often a high

pressure area.pressure area.

Pg 10-7

Radiation Fog at CYKZRadiation Fog at CYKZ

Advection FogAdvection Fog: : warm moist air warm moist air

moving over colder moving over colder land, horizontal land, horizontal

movement, warm movement, warm fronts/oceans, fronts/oceans,

sometimes strong sometimes strong winds (+25kts)winds (+25kts)

Pg 10-8

Upslope FogUpslope Fog

Pg 10-9

Ice FogIce Fog: Byproduct of engine is : Byproduct of engine is water…added to cold crisp air. water…added to cold crisp air.

(sublimation: vapour to ice)(sublimation: vapour to ice)

Pg 10-10

WATER VAPOUR – WATER - ICEWATER VAPOUR – WATER - ICE

Pg 1-2

Frontal FogFrontal Fog: vapour addition raises : vapour addition raises the dew point under a warm frontthe dew point under a warm front

Pg 10-10

Steam fogSteam fog: cold air moving over : cold air moving over a warm surfacea warm surface

Pg 10-9

Steam Fog at CollingwoodSteam Fog at Collingwood

• Clear Ice: skin warms to 0° while droplets freeze

– large super cooled water droplets– bottom layers of cold clouds or tops of unstable clouds – freeze just below 0° to -15°– tend to hit wing

– high collection efficiency– large spreading droplets

• Rime Ice: skin temperature < 0°

– small super cooled droplets can exist down to -40°– stable clouds, usually rime only -25° to -40°– tend to flow around wing

– low collection efficiency– leading edge only (no spread)

ICEICEpart 9part 9

Heaviest Icing – Rate of CatchHeaviest Icing – Rate of Catch

– Skin temperature at or below freezing– High water content held up in updrafts– Large droplets– Collection efficiency is inversely proportional to the

surface geometry - thick vs. thin wings

Pg 9-10

Ice Formation - StabilityIce Formation - Stability

Pg 9-4

ICINGICING

• Cumulus• icing distributed vertically• top of mature stage• In 5000 ft above 0° level• large droplets to -25°

• Stratus• icing distributed horizontally• if turbulent, top of cloud will

have the heaviest.• snow means less icing

**Warm clouds have more droplets with altitude.**Colder clouds have fewer droplets with altitude.

Pg 9-5

WEATHER TERMINOLOGYWEATHER TERMINOLOGY

• SKY CONDITION– Clear: 0/8– Few: 1/8 to 2/8 – Scattered: 3/8 to 4/8– Broken: 5/8 to 7/8– Overcast: 8/8

• IFR: less than 1000 ft 3 miles

• MVFR: 1000-3000 ft 3-5 miles

• VFR: greater than 3000 ft better than 5

Clear: Clear: 0/80/8

Few: Few: 1/8 to 2/81/8 to 2/8

Scattered: Scattered: 3/8 to 4/83/8 to 4/8

Broken: Broken: 5/8 to 7/85/8 to 7/8

Overcast: Overcast: 8/88/8

UPPER AIR CHARTS UPPER AIR CHARTS standard pressure levelsstandard pressure levels

• 850 mb 5000 feet asl

• 700 10000

• 500 18000

• 400 24000

• 300 30000

• 250 34000

• 200 40000

• 150 45000

• 100 53000

STEEP PRESSURE GRADIENT GIVES HIGHER WIND SPEEDS

JET JET STREAMSTREAM

Pg 12-11

Pg 12-13

Low Level Nocturnal JetLow Level Nocturnal Jet

40 KNOTS Pg 11-13