1. Gliding and the Weather

2. Nothing makes as much difference as picking the right days to fly on. So: ● What is “good gliding weather”? ● How can we forecast it?

3. What we're going to talk about: ● Background — general metoeorology ● Gliding weather ● What can (and will) go wrong ● Forecasts & Observations

4. Atmospheric Structure ● Tropopause between 7 and 17km ● All significant weather takes place in the troposphere ● Tempertaure decreases with height

5. Pressure ● Surface pressure varies between around 900mb and 1050mb ● Average sea level pressure is 1013mb ● Decreases by about 30mb per 1000ft ● Means that on a high wave flight you are above about 50% of the atmosphere! ● Variation (pressure gradient) leads to wind ● Due to Coriolis force wind doesn't go from high to low pressure but follows isobars

6. 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!

7. Warm Front

8. Cold Front

9. Synoptic Charts

10. ● Plot surface pressure and fronts ● Isobars join points of equal pressure ● Isobars typically spaced at 4mb intervals ● Wind blows approximately parallel to isobars ● Buys-Ballot's law: if you stand with your back to the wind the lower pressure is on your left (or right in the southern hemisphere) ● In other words, the wind goes clockwise around areas of high pressure and anti-clockwise around areas of low pressure

11. 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!

13. 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?

14. Hill Lift

15. ● 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!

16. Portmoak Ridges ● Bishop south face & upper south face: SSE–SW ● Bishop south-west face: SW–WNW ● Bowl: SW–NW ● Bishop north-west face: W–NNW ● Benarty: NW–N ● West Lomond: WNW–N ● Lomond hills: NW–NE ● Cemetery ridge: SE

17. A Nice Ridge Day

18. Thermals

19. ● 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”

20. 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)

22. A Good Thermal Day

23. Wave

24. How does it work? ● Wind blows over mountains ● Standing waves set up downwind ● Glider sits in rising part of wave and goes up

25. 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!

26. A Good Wave Day

27. Wave bars

28. What can go wrong? ● Depressions and Fronts ● Showers and Storms ● Sea Breezes ● Fog ● Flooding ● Easterlies

29. 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

30. 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

31. If you get cut off by a shower...

32. You end up in a field

33. 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

34. Easterlies ● No reliable ridges ● Poor or no thermals due to sea air ● Cold and damp in winter ● Can bring fog

35. What too much fog can make you do

36. 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

37. Stability ● Actual decrease of temperature with height is the Environmental Lapse Rate (ELR) ● Lapse rate > DALR => unstable ● Lapse rate stable ● SALR > Lapse rate > DALR => conditionally unstable

38. Tephigram

39. Soundings

40. Doing something with it

41. 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

42. Weather Observations ● Met office ● Visible and IR satpics (hourly) ● Rainfall RADAR ● Observations for Edinburgh & Leuchars ● Portmoak ● Webcam ● Observations ● The Window

43. Further Reading ● Ahrens, Meteorology Today ● Website (http://gliding.tardis.ed.ac.uk/) ● Take Met 1