Transcript Bronze & Beyond

Meteorology
Tim Freegarde
www.uskgc.co.uk
Meteorology
• what you need for the Bronze exam
• useful knowledge for flying
• terminology
• principles
• how it all works
Q. Questions from Bronze & Beyond
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Terminology
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air mass
anabatic
anticyclone
cloud (common types)
convection
convergence
Coriolis force
cyclone, high, low
depression
dew point
diurnal
drizzle
fog
Föhn effect
front (warm, cold, occluded)
geostrophic
hail
humidity
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inversion
katabatic
lapse rate
lee waves
mist
orographic
pressure
rain
ridge
saturation
sea-breeze/-front
snow
synoptic chart
temperature
thermal
trough
visibility
wind/-gradient/-shear
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Principles of meteorology
1.
2.
3.
4.
5.
hot air rises
the Earth rotates
solar power drives everything
warm air can hold more moisture
Newton was right
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The Earth’s atmosphere
• layer of gas surrounding earth
• weather occurs in troposphere
• at sea-level pressure, ~8.5 km thick
PRESSURE /hPa
0.1
1
10
250
1013
Randy Russell, UCAR
PRESSURE
• 1 mbar = 1 hPa
• 1 mbar per 27 ft
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Convection & thermals
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sunlight heats the ground
ground heats the air
hot (less dense) air rises
atmosphere is transparent
THERMAL
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WIND
1013
Randy Russell, UCAR
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Global circulation
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more dense
sunlight heats the ground
ground heats the air
hot (less dense) air rises
atmosphere is transparent
warm air rises at the equator
cool air descends at poles
but…
less dense
Remote Sensing Tutorial, NASA
THERMAL
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WIND
1013
Randy Russell, UCAR
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Coriolis effect
H
Coriolis
300 m s-1
L
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pressure
gradient
L
460 m s-1
tendency to turn right in N hemisphere
to observer moving with Earth, appears as virtual force
pressure gradient balances Coriolis & centrifugal forces
at altitude, flow follows isobars around high/low pressure
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Buys-Ballot’s law
H
Coriolis
300 m s-1
L
pressure
gradient
L
• with the wind on your back,
in the N hemisphere,
the low pressure is on the left
460 m s-1
Low
High
1. What does Buys-Ballots law state for the northern hemisphere?
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Global circulation
H
L
Coriolis
pressure
gradient
L
• Coriolis effect breaks up the
circulation into cells and cyclones
• prevailing/trade winds
• weather & climate
• real situation complex & dynamic
HIGH
LOW
COLD
HIGH
NOAA
WARM
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Global circulation
HIGH
LOW
LOW
• sea, land, ice affect heating
• UK rarely far from low pressures
HIGH
LOW
COLD
HIGH
NOAA
WARM
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Global circulation
OCCLUDED
FRONT
HIGH
PRESSURE
WARM
FRONT
LOW
PRESSURE
COLD
FRONT
• sea, land, ice affect heating
• UK rarely far from low pressures
HIGH
LOW
COLD
HIGH
NOAA
WARM
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Global circulation
OCCLUDED
FRONT
HIGH
PRESSURE
WARM
FRONT
LOW
PRESSURE
COLD
FRONT
• sea, land, ice affect heating
• UK rarely far from low pressures
HIGH
LOW
COLD
HIGH
NOAA
WARM
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Global circulation
SUMMARY
• convection
• sunlight warms earth
• hot air rises at tropics
(LOW)
• cool air descends at poles (HIGH)
• spinning Earth
• Coriolis effect breaks up circulation
into cells and cyclones
• weather where air masses converge
• fronts tend to distort and twist
Low
High
HIGH
LOW
COLD
HIGH
NOAA
WARM
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Wind
360
090
270
AT ALTITUDE
• geostrophic wind
• follows isobars
• Buys-Ballot’s law
• speed from isobar spacing
NEAR GROUND
• friction slows air movement
• reduced centrifugal/Coriolis force
• flow from High to Low pressure
• wind veers with height
STRENGTH & DIRECTION
• bearing from which wind comes
• speed in knots (1 kt = 1.15 mph)
180
24012KT
12 knots from SW
04515G25KT 25 knot gusts
3.
does
the wind
direction
vary
youbydescend
to which
grounddirection
level from
2. How
If a due
westerly
wind
is forecast
toas
veer
10°, from
will2000'?
it then come?
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Synoptic charts
OCCLUDED
FRONT
HIGH
PRESSURE
WARM
FRONT
GEOSTROPHIC WIND
• closer isobars
→ higher Coriolis force
→ stronger wind
LOW
PRESSURE
COLD
FRONT
• isobars: pressure contours (4 hPa)
• cold/warm fronts divide air masses
AIR MASSES
• cold/warm; dry/humid
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Synoptic charts
36
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THU 19 MAR 2015
H
1028
L
990
• note times of forecast & validity
• …but why the weather?
SUMMARY
• geostrophic wind
• follows isobars
• Buys-Ballot’s law
• strength from isobar
spacing
• low pressure
• cyclone
• wind, rain, cloud
• high pressure
• anticyclone
• calm, clear/fog
• fronts & air masses
• cold/warm
• dry/humid
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Weather fronts
36
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THU 19 MAR 2015
H
1028
L
990
WARM
COLD
convection stops
at warm air
WIND
COLD FRONT
WARM FRONT
WARM FRONT
• shallow slope
• slower than wind
• cloud layers
• descending cloudbase
• steady rain/drizzle
• poor visibility
COLD FRONT
• steep slope
• moves with wind
• towering cloud
• rising cloudbase
• squalls & showers
WIND
• strengthens & veers as
cold front passes
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Warm front
CIRRUS
CIRROSTRATUS
ALTOSTRATUS
WARM FRONT
• shallow slope
• slower than wind
• cloud layers
• descending cloudbase
• steady rain/drizzle
• poor visibility
NIMBOSTRATUS
12. In the summer, what is often the first sign of an
approaching warm front?
Cloud Appreciation Society
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Cold front
CUMULONIMBUS
CUMULONIMBUS
COLD FRONT
• steep slope
• moves with wind
• towering cloud
• rising cloudbase
• squalls & showers
• wind strengthens & veers
as cold front passes
CUMULUS
17. It is July and the weather feels fresh. A ridge of high
pressure is forecast. What weather would you expect?
Cloud Appreciation Society
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THERMAL
WIND
1000’ height
Convection
TOP OF
THERMAL
RISING
AIR
SURROUNDING
AIR
3°C
temperature
LAPSE RATES
/1000’
• fall in temperature with increasing altitude
• clear, rising air
dry adiabatic lapse rate
3°C
• actual temperature
environmental lapse rate
• condensing air
saturated adiabatic lapse rate 1.5°C
AIR RISES
• if it is warmer than the
surrounding air
RISING AIR
• expands as the air
pressure falls
EXPANDING AIR
• cools as it pushes the
air aside
COOLING AIR
• continues to rise if it is
warmer than the air
around
9. What are the environmental lapse rate, the dry adiabatic lapse rate and the
saturated lapse rate?
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height
Convection
THERMAL
WIND
INVERSION
TOP OF
THERMAL
RISING
AIR
UNSTABLE
SURROUNDING
INVERSION
AIR
temperature
LAPSE RATES
/1,000’
• fall in temperature with increasing altitude
• clear, rising air
dry adiabatic lapse rate
3°C
• actual temperature
environmental lapse rate
• condensing air
saturated adiabatic lapse rate 1.5°C
STABILITY
• inversion if environmental lapse rate < dry adiabatic
i.e. rising air cools more quickly than air around
• unstable air mass if environmental LR > adiabatic LR
THE AIR AROUND
• is heated by mixing
with the warm air
IF CONDITIONS STEADY
• convection continues
until environmental =
dry adiabatic lapse rate
HEATING CAN CHANGE
• cloud cover
• time of day
• geography
• radiative cooling
AIR MASS CAN CHANGE
• fronts
• sea breezes
• downdrafts
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height
Water
TOP OF
THERMAL
RISING
AIR
THERMAL
WIND
SURROUNDING
AIR
temperature
CONDENSATION NUCLEI
• condensation generally requires a surface or
seed particle (salt crystal, pollution)
• in clear air, supersaturation
AIR CONTAINS WATER
• above a certain
amount, water vapour
will condense
• saturated air contains
maximum water
• warm air can hold
more water than cold
RISING AIR
• cools until saturation
• vapour condenses
• dew point
8. What is the dew point?
14. What is the cause of radiation fog?
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height
Water
TOP OF
THERMAL
RISING
AIR
THERMAL
WIND
SURROUNDING
AIR
dew point
temperature
CUMULUS CLOUD
• flat base, where rising air reaches dew point
• latent heat gives extra buoyancy
CONDENSING RISING AIR
• releases latent heat
• cools less quickly
• saturated lapse rate
• condensing air will rise
higher than dry air
BRADBURY RULE
• height of cloudbase =
(max-min temp) x 400’
• if inversion lower,
thermals will be blue
10. The maximum temperature is forecast to be 22 °C, and the dew point is 7 °C.
Roughly how high will the cumulus cloudbase be?
15. What would be the effect on thermals of an inversion layer in an anticyclone in the
summer?
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Convective cloud
THERMAL
WIND
GROUND LEVEL WINDS
• flow along ground towards thermal
→ local changes in wind strength and direction
• convection mixes lower and upper air
→ wind picks up by day, slackens over night
5. What causes the windspeed to increase in the
morning and decrease in the evening?
WHAT COULD POSSIBLY
GO WRONG?
• powerful convection
• strong downdrafts
• wind shifts, turbulence
• lightning
• heavy rain, hail
• visibility
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Convective cloud
THERMAL
WIND
13. You are landing your glider near a thunderstorm.
What is your greatest risk?
16. You notice that the cumulus clouds are rising to
great heights above their bases and flattening at
their tops. What could happen next?
19. You are on a gliding holiday in the Pyrenees and
because thunderstorms are forecast, you decide
not to fly that day. Should you de-rig the glider?
WHAT COULD POSSIBLY
GO WRONG?
• powerful convection
• strong downdrafts
• wind shifts, turbulence
• lightning
• heavy rain, hail
• visibility
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Topography
AIR CANNOT PASS THROUGH ROCK
• wind blows along valleys
• wind rises over hills (lift), falls into valleys (sink)
• air rising over hills can form orographic cloud
SUNSHINE HEATS ROCK
• air flows up sun-facing slopes (anabatic wind)
• air flows down unlit slopes (katabatic wind)
• warm terrain creates thermals
• mountain ranges draw in air
WEATHER can be very local
11. You are ridge soaring and find that cloud is forming
over the hill. What is the type of cloud called?
21. In which direction would the wind tend to blow in the
mountains in high summer during the afternoon?
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Topography
AIR CANNOT PASS THROUGH ROCK
• wind blows along valleys
• wind rises over hills (lift), falls into valleys (sink)
• air rising over hills can form orographic cloud
THE WHOLE ATMOSPHERE CAN RIPPLE
• strong wind over mountains can cause wave
• lenticular cloud can mark wave crests
• strong, smooth lift and sink aloft
• violent rotor below
Gordon Boettker
4. There is a strong cross-wind. You want to land
alongside and downwind of a row of trees and large
hangar. What might happen?
20. What might you experience near a rotor cloud?
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Sea breezes
SEA HEATS & COOLS VERY SLOWLY
• warmer land by day forms sea
breezes
• cool air from sea suppresses
convection
• front forms inland where cool
and warm air meet
• night air leaves cooler land as
land breeze
6. What effect will a sea-breeze front have on your plans if you have to fly across
one?
7. What effect will the passage of a sea-breeze front have had if you make a field
landing?
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Satellite photographs
Analysis, midday
24 hour visible satellite view
18. There has been a shower while your glider has been at the front of the winchlaunch queue but flying is about to restart. What should you do?
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Books and websites
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www.metoffice.gov.uk/learning/learn-about-the-weatherhow it all works
www.metoffice.gov.uk/learning/library/publications/factsheets
www.weatherjack.co.uk
www.meteoblue.com
www.xcweather.co.uk
[email protected]
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