Meteorology 3

FRONTS Pg 6-7

Colder Cold Warm

Precipitation 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 O C one cubic metre of air can hold 50 grams of water vapour. One M 3 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

CLOUDS 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. STABLE

UNSTABLE STABLE 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

Precipitation. - Any precipitation, in order to form, requires condensation nuclei. - Temperature to dew point spread must be small Pg 8-17

Precipitation Pg 9-2

THUNDERSTORMS Requirements: –Unstable air to high levels –High relative humidity –Lifting agent The three stages: –Cumulus –Mature –Dissipation

THUNDERSTORMS 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 minutes although whole process usually 1 to 3 hours

Thunderstorm 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 Thunderstorms Pg 15-11

Orographic Lift Pg 8-15

Pgs 15-9,10,11

Thunderstorm 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 Thunderstorms Pg 15-18

Gust Front Hazards Pg 15-18

Anvil, Multi-Cell & Lightning Positive build up in top and negative build up in cloud base is usual Pgs 15-3,5,7

FOG

FOG 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: RadiationAdvection UpslopeSteam FrontalIce

Radiation Fog: clear night, light wind, high humidity, often a high pressure area. Pg 10-7

Radiation Fog at CYKZ

Advection Fog: warm moist air moving over colder land, horizontal movement, warm fronts/oceans, sometimes strong winds (+25kts) Pg 10-8

Upslope Fog Pg 10-9

Ice Fog: Byproduct of engine is water…added to cold crisp air. (sublimation: vapour to ice) Pg 10-10

WATER VAPOUR – WATER - ICE Pg 1-2

Frontal Fog: vapour addition raises the dew point under a warm front Pg 10-10

Steam fog: cold air moving over a warm surface Pg 10-9

Steam 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) ICE part 9

Heaviest 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 - Stability Pg 9-4

ICING 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 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 ft3 miles MVFR: ft3-5 miles VFR: greater than 3000 ftbetter than 5

Clear: 0/8

Few: 1/8 to 2/8

Scattered: 3/8 to 4/8

Broken: 5/8 to 7/8

Overcast: 8/8

UPPER AIR CHARTS standard pressure levels 850 mb5000 feet asl

STEEP PRESSURE GRADIENT GIVES HIGHER WIND SPEEDS

JET STREAM Pg Pg 12-13

Low Level Nocturnal Jet 40 KNOTS Pg 11-13