Transcript Bronze Weather - Oxford Gliding Club

The Atmosphere
 Atmosphere is 100km thick
 Troposphere is about 10km
 Contains 80% of atmosphere.
 Air above us has weight.
 Weight reduces with
increased height.
 Creates a pressure of about
15psi.
High pressure always moves towards low pressure.
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Pressure
 ICAO standard altimeter setting is 1013.2mb
 Regardless of local conditions
 Pressure reduces by 1mb per 30 feet
 Scale set to QNH represents altitude
 i.e. height above mean sea level
 Scale set to QFE represents height
 i.e. height above ground
Altimeter reads HIGH when flying towards a LOW.
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Coriolis Effect
 Named after Gustave Coriolis
 (who had absolutely nothing to do with it)
 Air would tend to move in straight line.
 Coriolis effect causes veering.
 To the right in northern hemisphere
 To the left in southern hemisphere
 Force increases towards the poles.
 Zero at the equator.
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Real Life Example
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Wind
 Wind is pressure differences trying
to reach equilibrium.
 Air movements deflected to the
right due to Coriolis effect.
 Anticlockwise rotation around a LOW.
 Clockwise rotation around a HIGH.
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Why is the prevailing wind in the
British Isles South Westerly?
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Atmospheric Circulation
Antitrades
Horse Latitudes
Trade Winds
Doldrums
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Surface Wind
 Surface wind is measured at 10m.
 Near the surface, the wind will:
 Slow due to surface friction
 Turn inward across isobars
 With height, the wind will:
 Increase
 Veer
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Solar Heating
 During the day heat from the sun
causes convection.
 During the night the Earth gives
off its heat.
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Wind & Hills
 Sun shines on slope during day
 Air warms and becomes less dense.
 Rises up the slope
 Known as Anabatic
 Slope cools at night
 Air cools and becomes more dense.
 Flows down the slope
 Known as Katabatic
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Föhn Wind
 Prevailing wind pushed up slope
 Pressure decreases
 Air expands and cools
 Water vapour falls out
 Wind descends on other side
 Pressure increases
 Wind speed increases
 Temperature increases
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Humidity
 Most air contains some water vapour.
 Percentage is called Relative Humidity.
 Compared with maximum amount that
could exist at a given temperature.
 Cooler air cannot hold as much water.
 Temperature at which air becomes 100%
saturated is the DEW POINT.
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Clouds
 Coverage reported in Oktas.
 Orographic cloud
 Moist air flowing over a hill
 Rotor cloud
 Over hill tops
 In the lee of hills
Thermals can still develop under an extensive layer
of strato-cumulus if there is sufficient instability in
the atmosphere.
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Visibility
 Less than 1000m is FOG
 Air being cooled to below dew point
 Radiation fog – moist air cooling to below
dew point overnight.
 Hill fog – moist air being forced uphill and
temperature cooled to below dew point.
 Advection fog – warm moist air being
cooled from below
 If cooled below freezing creates Hoar Frost
 1000m to 2000m is MIST
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What are Air Masses?
 Large volumes of air.
 Characteristics of:
 Temperature
 Atmospheric pressure
 Water content
 Cover many hundreds of square miles
 Most will be moving
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Origins
 Designated by their origin
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Tropical Maritime
Tropical Continental
Polar Continental
Arctic Maritime
Polar Maritime
Returning Polar Maritime
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Air Mass Characteristics
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Synoptic Charts
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Buys-Ballot’s Law
 Wind blows at right angles to the
atmospheric pressure gradient.
 Observed by Dutch meteorologist
Christopherus Buys-Ballot in 1857.
 In the northern hemisphere, if you
stand with your back to the wind,
the LOW will always be on your left.
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Fronts
 Boundary between two air masses.
 Principal cause of significant weather.
 Polar & Tropical air masses typically
clash in the middle latitudes.
 Produce changeable climates
 Such as those experienced in the UK
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Warm Fronts
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Identified by a line of semi-circles on chart.
Warm air advancing over cold.
Usually preceded by precipitation and fog.
Pressure falls then rises.
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Cross Section through a Warm Front
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Signs of a Warm Front
 In the summer:
 High layer cloud slowly approaching.
 Weakening soaring conditions.
 Often bring spells of prolonged and sometimes
heavy rainfall, with strong winds.
 After the front passes:
 Weather usually clears quickly.
 Wind shifts.
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Cold Fronts
 Cold air pushing underneath warmer air at the surface.
 Identified on weather charts as triangles.
 Bring short spells of heavy rainfall & squally winds.
 Lots of cloud in the warmer air ahead of the cold front
 Pressure rises throughout the approach and passage.
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Cross Section through a Cold Front
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Occluded Fronts
 Cold fronts move faster than
warm fronts.
 Can catch and overlap a warm
front causing an occluded front.
 Curve naturally poleward into the
point of occlusion.
 Similar characteristics to a cold
front – but less intense.
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Ridges
 Elongated areas of high pressure.
 Bring similar weather to that associated with anticyclones.
 Good soaring likely - reduces the instability behind the
cold front preventing over convection.
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Troughs
 Elongated areas of low pressure.
 Bring similar weather to that associated with depressions.
 Deep trough:
 Particularly violent weather at the passage of the trough, including
strong winds, heavy rain and Cb activity.
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Cols
 Area of slack pressure bounded by two high
pressure systems (anticyclones) and two low
pressure systems.
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Sea Breeze Front
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Land heats more quickly during the day
Air over land rises causing advection.
Wind blows in from the sea.
Meets warmer land air & creates shallow cold front.
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Environmental Lapse Rate
 As air rises, it expands due to the
reduced pressure.
 If no heat transfer occurs into or out of
the parcel, the process is adiabatic.
 A rising air mass cools at a given rate.
 Known as the Environmental Lapse Rate.
 The ELR varies from day to day.
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Dry Adiabatic Lapse Rate
Dry or unsaturated air cools at 3°C per 1000 ft.
Known as the Dry Adiabatic Lapse Rate (DALR).
This is a theoretical rate and can be calculated.
Most air masses contain a proportion of water
vapour.
 DALR only applies if RH is less than 100%.
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Saturated Adiabatic Lapse Rate
 Cooler air cannot hold as much water vapour
as warmer air
 Rising air eventually becomes 100% saturated.
 This point is called the dew point.
 Marks the start of cloud vertical development.
 Saturated air cools at a different rate.
 Known as the Saturated Adiabatic Lapse Rate.
 SALR is 1.5°C per 1000 ft.
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The temperature drop of
a rising parcel of air
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Stable Atmosphere
 If the ELR is less than the DALR the air mass is stable.
 Smoke, haze and dust may result in poor visibility.
 Cloud formation in stable air unlikely.
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Unstable Atmosphere
 If ELR greater than DALR, atmosphere is unstable.
 Often happens in the afternoon over land masses.
 Likelihood of cumulus & good soaring conditions.
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Rule of Thumb
 Cloudbase = (Surface Temperature - Dew point) x 400
If:
Dew point = 16º
and Surface Temperature = 26º
Then:
Cloudbase = (26 – 16) x 400 = 4000 feet
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Inversions
 Increase of temperature with height.
 Two main causes are:
 Descending air warming due to compression
and resting on the cooler air mass beneath.
 Cooling of the surface on a clear night,
where the air in contact becomes colder
than the air above.
 Once inversion is below the dew point,
cumulus will not form.
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Adverse Conditions
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Thunderstorms
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Three conditions necessary:
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Plenty of moisture.
A mass of warm unstable air.
A source of energy to lift the warm,
moist air mass rapidly upward.
Three main stages called in the life
cycle are:
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Cumulus stage
Mature stage
Dissipating stage
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Icing
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Causes weight to be increased.
May alter the C of G position.
Pitot tubes & statics may become blocked.
Radio communications degraded.
 Hail is water molecules freezing in the up
draughts of a Cb, growing with each cycle until
too heavy to be sustained by the rising air.
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Ideal Conditions
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Unstable air mass
Surface heating
Cloud amount
Cloud base
Wind strength
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What to Look For
 Air fairly close to centre of a high or the
axis of a ridge.
 Pressure 1017 to 1028mb.
 Isobars with strong anti-cyclonic curvature.
 Northerly component to the wind.
 Air coming from well to the north of UK.
 Geostrophic wind less than 16kts.
 Inland surface winds less than 12kts.
 Temperature spread between 12º – 15ºC.
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The Perfect Day!
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March 2008