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Gliding and the Weather
Nothing makes as much difference as
picking the right days to fly on.
So:
What is “good gliding weather”?
●How can we forecast it?
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Air masses and Fronts
Air in different places has different temperture, humidity etc.
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Call a large area of similar air an air mass
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Fronts occur where air masess meet
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A cold front occurs where cold air is undercutting warmer air
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A warm front is where warm air is overriding cold air
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Both lead to cloud and precipitation
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We get lots of them!
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Warm Front
Cold Front
Synoptic Charts
Mid-latitude Depressions
Usually associated with fronts and precipitation
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Often also high winds near centre
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Move from west to east
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This is why we get lots of fronts!
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Gliding Weather
For a day to be any good we need to be able to stay up
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Can get lift from:
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Hills
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Thermals
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Wave
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What sort of weather do we need for each one?
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Hill Lift
Hill Lift
Wind must be blowing towards ridge—within about 45°
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Must be strong enough—about 10kts minimum
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Not too strong—depends on direction but about 25kts max
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Not raining!
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A Nice Ridge Day
Thermals
Thermals
Sun heats ground
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Ground heats air
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Air rises in bubbles
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Gliders and soaring birds circle in rising air
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Thermals may be topped by cumulus clouds or “blue”
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What do we need?
Sunshine (spring or summer)
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Air must be unstable enough to allow convection but not too
unstable or it gets showery
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Not too windy
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Not easterly (at Portmoak)
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A Good Thermal Day
Wave
How does it work?
Wind blows over mountains
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Standing waves set up downwind
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Glider sits in rising part of wave
and goes up
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What do we need?
Right wind direction—roughly NW
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Stable layer
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Moderate wind strength, typically 15–20kts at surface
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Increasing wind strength with height
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Roughly constant wind direction at height
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Straight isobars or slight anticyclonic curve
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Luck!
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A Good Wave Day
Wave bars
What can go wrong?
Depressions and Fronts
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Showers and Storms
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Sea Breezes
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Fog
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Flooding
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Easterlies
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Depressions and Fronts
Forecasting depressions is not too hard
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But forecasting the timing of fronts is (e.g. last Sunday)
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Look for gaps, even short ones
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Keep checking—long range forecasts are often wrong
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Showers and Storms
Can occur when there is deep instability (e.g. last Sunday)
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Common in northerly airstreams in winter
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Winter showers generally worst near coast so more of a problem
in north-easterly winds than north-westerlies
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Summer showers and thunderstorms generally require some
heating by the sun to get them started so most likely in afternoon
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Sea Breezes
Happen when heating inland produces inflow of air
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Thermals on the front often enhanced
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Sea air behind front is cold and damp, therefore dead
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Can cause a change of wind direction
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Can be held back by westerly wind
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Only solution to get away before it arrives
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Easterlies
No reliable ridges
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Poor or no thermals due to sea air
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Cold and damp in winter
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Can bring fog
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What too much fog can make you do
Stability
Rising air exapands and therefore cools (PV = nRT etc.)
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This is the Dry Adiabatic Lapse Rate (DALR)
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Approximately 3°C per 1000ft
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Saturated air (i.e. clouds) release latent heat
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Therefore cool more slowly
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At the Saturated Adiabatic Lapse Rate SALR
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Stability
Actual decrease of temperature with height is the Environmental
Lapse Rate (ELR)
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Lapse rate > DALR => unstable
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Lapse rate < SALR => stable
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SALR > Lapse rate > DALR => conditionally unstable
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Tephigram
Soundings
Doing something with it
Weather Forecasts
Check 24, 48, 72 hour forecast synoptics for position of
depressions, fronts, wind strength & direction
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Keep checking back—they can change a lot
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Compare with, say, met office 3-day forecast
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If there’s a chance of thermals or wave check the forecast
sounding
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Weather Observations
Met office
●Visible and IR satpics (hourly)
●Rainfall RADAR
●Observations for Edinburgh & Leuchars
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Portmoak
●Webcam
●Observations
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The Window
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