Gliding and the Weather

Nothing makes as much difference aspicking the right days to fly on.

So:

●What is “good gliding weather”?●How can we forecast it?

Air masses and Fronts

●Air in different places has different temperture, humidity etc.

●Call a large area of similar air an air mass

●Fronts occur where air masess meet

●A cold front occurs where cold air is undercutting warmer air

●A warm front is where warm air is overriding cold air

●Both lead to cloud and precipitation

●We get lots of them!

Warm Front

Cold Front

Synoptic Charts

Mid-latitude Depressions

●Usually associated with fronts and precipitation

●Often also high winds near centre

●Move from west to east

●This is why we get lots of fronts!

Gliding Weather

●For a day to be any good we need to be able to stay up

●Can get lift from:

●Hills

●Thermals

●Wave

●What sort of weather do we need for each one?

Hill Lift

Hill Lift

●Wind must be blowing towards ridge—within about 45°

●Must be strong enough—about 10kts minimum

●Not too strong—depends on direction but about 25kts max

●Not raining!

A Nice Ridge Day

Thermals

Thermals

●Sun heats ground

●Ground heats air

●Air rises in bubbles

●Gliders and soaring birds circle in rising air

●Thermals may be topped by cumulus clouds or “blue”

What do we need?

●Sunshine (spring or summer)

●Air must be unstable enough to allow convection but not too unstable or it gets showery

●Not too windy

●Not easterly (at Portmoak)

A Good Thermal Day

Wave

How does it work?

●Wind blows over mountains

●Standing waves set up downwind

●Glider sits in rising part of wave and goes up

What do we need?

●Right wind direction—roughly NW

●Stable layer

●Moderate wind strength, typically 15–20kts at surface

●Increasing wind strength with height

●Roughly constant wind direction at height

●Straight isobars or slight anticyclonic curve

●Luck!

A Good Wave Day

Wave bars

What can go wrong?

●Depressions and Fronts

●Showers and Storms

●Sea Breezes

●Fog

●Flooding

●Easterlies

Depressions and Fronts

●Forecasting depressions is not too hard

●But forecasting the timing of fronts is (e.g. last Sunday)

●Look for gaps, even short ones

●Keep checking—long range forecasts are often wrong

Showers and Storms

●Can occur when there is deep instability (e.g. last Sunday)

●Common in northerly airstreams in winter

●Winter showers generally worst near coast so more of a problem in north-easterly winds than north-westerlies

●Summer showers and thunderstorms generally require some heating by the sun to get them started so most likely in afternoon

Sea Breezes

●Happen when heating inland produces inflow of air

●Thermals on the front often enhanced

●Sea air behind front is cold and damp, therefore dead

●Can cause a change of wind direction

●Can be held back by westerly wind

●Only solution to get away before it arrives

Easterlies

●No reliable ridges

●Poor or no thermals due to sea air

●Cold and damp in winter

●Can bring fog

What too much fog can make you do

Stability

●Rising air exapands and therefore cools (PV = nRT etc.)

●This is the Dry Adiabatic Lapse Rate (DALR)

●Approximately 3°C per 1000ft

●Saturated air (i.e. clouds) release latent heat

●Therefore cool more slowly

●At the Saturated Adiabatic Lapse Rate SALR

Stability

●Actual decrease of temperature with height is the Environmental Lapse Rate (ELR)

●Lapse rate > DALR => unstable

●Lapse rate < SALR => stable

●SALR > Lapse rate > DALR => conditionally unstable

Tephigram

Soundings

Doing something with it

Weather Forecasts

●Check 24, 48, 72 hour forecast synoptics for position of depressions, fronts, wind strength & direction

●Keep checking back—they can change a lot

●Compare with, say, met office 3-day forecast

●If there’s a chance of thermals or wave check the forecast sounding

Weather Observations

●Met office●Visible and IR satpics (hourly)●Rainfall RADAR●Observations for Edinburgh & Leuchars

●Portmoak●Webcam●Observations

●The Window