Transcript 5AER200-MET3clouds, s+

Meteorology 3
FRONTS
Pg 6-7
Warm
Colder
Cold
Warm
Colder
Cold
Warm
Colder
Cold
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 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
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
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Convection
Orographic lift
Frontal Lift
Convergence
Mechanical turbulence
UNSTABLE vs. STABLE
Steep lapse rate
Shallow lapse rate
Cumulus type cloud
Stratus type cloud
Precipitation in showers
Precipitation is steady and lighter
Thunderstorms
Fog
Good visibility
Poor visibility
Bumpy conditions
Smooth conditions
UNSTABLE
STABLE
Precipitation
.
Pg 8-17
- Any precipitation, in order to form, requires
condensation nuclei.
- Temperature to dew point spread must be small
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:
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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:
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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 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 midlevels 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:
• Rain:
rapid barometric changes, altimeter may be unreliable.
reduced visibility, refraction on windscreen, hydroplane
Pg 15-18
Do’s and Don’ts of Thunderstorms
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:
Radiation
Upslope
Frontal
Advection
Steam
Ice
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
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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
ICE
part 9
• 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)
Heaviest Icing – Rate of Catch
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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
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Stratus
•
Cumulus
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icing distributed horizontally
if turbulent, top of cloud will
have the heaviest.
snow means less icing
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icing distributed vertically
top of mature stage
In 5000 ft above 0° level
large droplets to -25°
•
**Warm clouds have more droplets with altitude.
**Colder clouds have fewer droplets with altitude.
Pg 9-5
WEATHER TERMINOLOGY
• SKY CONDITION
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Clear:
Few:
Scattered:
Broken:
Overcast:
0/8
1/8 to 2/8
3/8 to 4/8
5/8 to 7/8
8/8
• IFR:
less than 1000 ft
• MVFR: 1000-3000 ft
• VFR: greater than 3000 ft
3 miles
3-5 miles
better 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
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850 mb
700
500
400
300
250
200
150
100
5000 feet asl
10000
18000
24000
30000
34000
40000
45000
53000
STEEP PRESSURE GRADIENT GIVES HIGHER WIND SPEEDS
JET
STREAM
Pg 12-11
Pg 12-13
Low Level Nocturnal Jet
40 KNOTS
Pg 11-13