Jump to content
  • Sign in to follow this  

    Meteorology For Glider Pilots


    Gary B

    Sitting snowbound in the house waiting for winter to end my mind is wandering to the flying field on a nice sunny day! Part of the daydream is plotting how to read the weather for soaring.

    I used to teach trainee glider pilots basic meteorology to get them through the ‘Bronze C’ examination, as part of my Private Pilots Licence training I also had to take an exam on meteorology. The difference between the two exams is that the glider pilot one is aimed at understanding soaring conditions and the PPL one is aimed at keeping the pilot out of trouble.

    Chatting with other F3J pilots at competitions last year showed that there are different levels of understanding of the weather, perhaps it is a boring subject but knowing a few basics might help.

    Right then! Meteorology is a big subject but what we need to know as model glider flyers is manageable. Being able to read a weather chart is a very useful skill and probably more obviously, reading the actual conditions by looking at the sky at the flying site is the other.

    Unavoidably there are some technical terms but I will try to explain them as simply as possible as we go along! Apologies if this is ‘egg sucking’ for some, it’s aimed at readers with limited knowledge of the weather.

    Air mass

    This is the term to describe a very big lump of air! We say ‘the air mass is coming off the sea’ or ‘the air mass is stable’. The British Isles has a maritime climate (greatly affected by the surrounding sea), air can come from four general directions, Polar Maritime (Iceland, cold and wet), Polar Continental (Norway, cold and dry),  Tropical Maritime (Bay of Biscay, warm and wet) and Tropical Continental (France/Germany, warm and dry). There is a fifth direction now, Arctic, you can guess what kind of air comes from there!

    Pressure systems   

    We have two general types of pressure systems in the UK, low pressure (also known as a depression or ‘low’) and high pressure. Air always moves from the high pressure area to the low and the systems rotate due to Coriolis force (caused by the Earth’s rotation, the same force that makes your bathwater swirl down the plug hole!)

    Atmospheric pressure is measured in Millibars (mb), the worldwide average sea level pressure is accepted as 1013 mb.

    Fronts and troughs

    A front is the leading edge of an air mass, several hundred miles long and up to 100   deep, there are three types, warm front, cold front and occluded front.

    A warm front generally means dull, damp conditions with stable air (overcast and raining!) where a cold front will be drier with unstable conditions (very likely to be thermic).

    An occluded front is a mixture of both warm and cold fronts. The areas behind fronts are called ‘sectors’ so we say ‘cold sector’ or ‘warm sector’. German glider pilots call the cold sector ruckseiten wetter, literally ‘back side weather’. What they mean is the back side of the cold front where thermic conditions are best.

    A trough is an isolated line of low pressure, often associated with stormy weather.

    Lapse rate

    Sounds technical but it is just the change of temperature with altitude! If the air mass is wet (saturated lapse rate) the temperature will decrease approximately 1.5 degrees C per 1,000 ft of altitude and if the air mass is dry the temperature will decrease 3 degrees C per 1,000 ft. The way to remember this (if you really want to!) is that drei (dry) is German for the number three. These are very general numbers, the lapse rate varies depending on the actual humidity of the air mass.

    Under high pressure conditions there is often an ‘inversion’, this is where the temperature increases with altitude (the reverse of normality) and prevents thermal activity or limits its height. You can often see this as a purple haze where pollution is trapped in a layer.

    Dew point

    The dew point is the temperature that water vapour condenses into visible liquid water (i.e. a cloud!). This temperature relates to an altitude so we can work out what height the cloudbase will be (using lapse rates).

    Technical again but very simply imagine that the ground level temperature is 10 degrees C, the air mass is saturated (humid) so it has a lapse rate of 1.5 degrees. If the dew point has been predicted as 4 degrees then the cloudbase will be 4,000 ft (10 minus 4 equals 6, 6 divided by 1.5 equals 4).

    The difference between the ground level temperature and the dew point is known as the dew point depression, the bigger the number the higher the cloudbase. A high cloudbase indicates large, strong thermals, in the UK a summer cloudbase of 5,000 ft is fairly normal, Germany can be up to 9,000 ft (hotter land mass) and Australia can go to 12,000 ft plus.

    A related rule of thumb that works well for soaring prediction is the difference between the lowest overnight temperature and the maximum expected the following day, a cold night followed by a hot day usually means strong thermals.

    Orographic cloud

    Related to the dew point is orographic cloud, this is the mist that suddenly forms on slopes (usually after we’ve walked a mile to get there!). As the air rises up the slope it cools and if the dew point at your level decreases then cloud will form suddenly from nowhere, this can be quite dangerous for full-size glider pilots, the brief is to know what direction is needed to fly away from the hill and use the compass to do that (swiftly!).

    Insolation

    Not to be confused with insulation this is the amount of sunshine (Sol) that the ground is receiving, a south facing slope gets the sunbeams directly on it and heats up, often making the air rise up the slope without any wind, this is known as Anabatic wind. It’s enough to keep full-size gliders airborne in the Alps and can be used for light weight models on our smaller slopes.

    Cloud cover has a marked effect on insolation, if the sun can’t heat the ground then the thermals will die or decrease in strength. Late afternoon ‘spread out’ or ‘over development’ (becoming overcast) is common in the summer and is the curse of cross-country glider pilots. Out of interest cloud cover is measured in Oktas (eighths) so a half covered sky is reported as four Oktas.

    Diurnal variation

    Diurnal variation is a posh term used to describe the difference between day and night weather conditions that are affected by the sun. Obvious effects are the drop in temperature at dusk and a drop in the wind speed which free flight model flyers wait for when they are trimming.

    Hot air balloonists fly in the mornings and evenings, taking advantage of the effect.

    Perhaps bundled in with this is ‘maximum heating’, the time when the ground has got as hot as it is going to get, usually taken as 1 or 2 o’clock, slightly after the sun has passed its highest point in the sky (Zenith!).

    Weather charts

    The nice pictures provided by the BBC and Met Office are good enough for us. This one is from June 2012.

    Weather chart from the BBC

    bbc-meteorology.thumb.png.484057609a299b

    Chart explanation and notes:

    Isobars: Isobars are lines connecting areas of equal atmospheric pressure, the closer they are together the windier it will be. On the chart they are the thin black lines with the millibar value written on them.

    Fronts: Warm fronts are marked in red with semi-circles facing the direction of movement, cold fronts are blue with triangles (icicles!) also facing the direction of movement.

    An occluded front is shown in purple with a mixture of semi-circles and triangles.

    Trough: A trough is shown as a thick black line (over France).

    From this chart I would expect good soaring conditions in Brittany and Cornwall.

    At the flying site

    Armed with all this gen we have looked at the charts and decided that it will be good for a trip to the field (lots of thermals) or slope (lots of wind and thermals!).

    There are still some weather related decisions to be made at the flat field flying site, ‘reading the air’ is often mentioned in model soaring books. What this means is taking a few moments before launching to work out where the best lift might be and formulate a plan to get there without losing too much height.

    On a good day there is a thermal under every cumulus cloud, a problem for us is that the thermals rise from the ground at an angle due to the wind so their source is probably quite a distance upwind, directly underneath a cumulus cloud is likely to be sinking air. It might be worth having a look above or downwind to see if a cloud is sitting on a thermal column that is much closer. Quite a guessing game but it’s the part of the sport (F3J) that I enjoy the most and still have a lot to learn.

    The strength of the wind can vary throughout the day, probably the biggest effect I notice is in the lower end of the landing circuit, too much spare height with no headwind makes hitting the tape tricky! I have watched many full-size gliders land downwind, quite remarkable at an airfield with two very dayglo orange windsocks!!

    I hope this article is useful, there is obviously a lot more to it, meteorology is covered in the older soaring books by Dave Hughes and George Stringwell etc, not always easy to get hold of nowadays. A very good book is ‘Meteorology and Flight’ by Tom Bradbury, available from the British Gliding Association online shop or Amazon etc.

    If all else fails follow the birds, there’s a friendly Red Kite at my home field!

    Cheers

    Gary Binnie

    • Thanks 1
    Sign in to follow this  


    User Feedback

    Recommended Comments

    There are no comments to display.


  •  

  •  

  • Our picks

    • by Sydney Lenssen, July 2018

      Brian Austin has been co-opted by the BARCS Executive Committee as the new President of BARCS. His three year term of office will be confirmed by the membership at the AGM 2019. I am confident that this announcement will be welcomed by all BARCS members. Many, if not all, members know Brian from his long record of achievements and activities in the silent flight field. He is especially known for his friendly cheerful manner, always at hand to help fellow modellers.


      Four years ago, Brian was awarded BARCS’ Eppler Trophy, in my opinion, the association’s the most prestigious award with a long list of distinguished aeromodellers such as Eppler himself.

      Graham James, BARCS President at that time, wrote the following citation: In the early years of BARCS, awarding was often a relatively straightforward decision as new construction methods, materials, wing sections, control methods and launch and landing requirements demanded continuous model development. Today, many of us have moved onto moulded ready builds and the skills of the true modeller are largely being lost.


      One person, Brian Austin, continues to lead the field in home design and build models. Responsible over the years for many familiar Open and 100s designs, his name is now better known in electric circles not only for his planes but also as a driving force behind competition rule progression. Names like Trilogy, Alacrity and more recently the Watts series of electric gliders, of which Watts New is the latest incarnation, will be familiar to us all. For many years, he has also been the responsible for running a very successful series of competitions in Essex.

      Although tempted by shiny plastic models too, he continues to fashion exquisitely beautiful soarers, built to standards that most of us can only
      aspire. They take the latest look and feel of moulded machines, but are built in more traditional ways. Brian pilots competition winning models.
        • Thanks
        • Like
      • 0 replies
    • Can F3J survive the treatment meant to save it
      What are the new rules?

      Two weeks ago the RC Soaring Technical Meeting in Lausanne took the bull by the horns and introduced new rules aimed at saving F3J glider contests from sliding off the world and continental championship schedules - the death of what for many soaring pilots is the most popular of silent flight competitions.

      Joe Wurts, the first F3J World Champion in 1998 at Upton-upon Severn, UK. Twenty years later with the latest F3J WC about to take place in Romania, many soarers are fearing that this could be the last.


      THE NEW RULES

      From next year pilots can use electric winches - either/or hand held winches - for launching their models. The models must have a maximum surface area of 150 dm2 and a minimum loading of at least 20 gm/dm2. There will be no dropped round in fly-offs, and no reflights for mid-air collisions after 60 seconds into the slot.

      CIAM, the world ruling body for this class is hoping that its new rules will halt the massive fall in numbers of F3J pilots wishing to compete, sixty per cent over the past five years and still falling, and restore its popularity.

      But among many F3J pilots, the bull is still shaking its horns. There has been an extraordinary shock reaction: hundreds of pilots from all over the world have reacted on social media, protesting, angry and forecasting the end of this class. Many pilots are concerned, ranging from previous finalists and champions to your typical enthusiast who enjoys travelling across country and continental boundaries to participate in their friendly sport. Only a few can see the logic and reasoning and are prepared to wait and see how the changes work in practice. More than a few want CIAM to think again!



       
      • 12 replies
    • Rule changes to halt terminal decline
      Uncle Sydney’ Gossip column returns

      FAI’s Aeromodelling Commission meets next month, 27/28 April 2018 in Lausanne, Switzerland. For F3J pilots the main topic on the agenda is how to halt the decline in silent flight contests. What does CIAM want to change?  What chance for these changes to save terminal decline?

      Winches to be allowed.

      If this proposal goes through the “launch of the model aircraft will be by hand held towline or winch.” Ever since 1998 when the first F3J world championships were held at Upton on Severn, pressure has been on CIAM to bring in winch launching. At numerous team managers’ meetings held by Jury President Bartovsky during World and European championships, arguments for and against have raged. Many countries do not have enough people to give one or two man tows, so they run their qualifying comps to local rules using electric winches. I guess more than half of countries do this. When they turn up at FAI championships, their pulleys and hand winches are brought out. In the UK perhaps we had one or two practice sessions at home before leaving.

      Certainly there is a difference between a regulation F3B winch and a two man tows. The best pilots still gain the most height either way. The big difference is what you need to carry on your travels, especially by airline. Winches and batteries are bulky and heavy. So far all votes have been to stick with hand towing.

      In CIAM agendas, any rule amendment is followed by its reasoning. 

      The winch proposal stems from Slovakia and they say: “The majority of pilots are older persons who are no longer physically capable of towing models. ( Uncle’s note: I have not seen anyone on crutches yet!) .....


       
      • 27 replies
    • by Sydney Lenssen, BARCS President and Gary Binnie, BARCS Chairman

      We and the BARCS executive committee wish all members, and indeed everyone who enjoys model flying and thermal soaring, a very happy Christmas, and also a very special year ahead in 2018. May all your achievements, higher scores and hopes be realised.

      Year 2017 has been a mixed year, probably for everybody. The biggest triumph by far has been the successful opening of BMFA’s National Flying Centre at Buckminster. BARCS can be very proud that it was the first group of aeromodellers to utilise the facilities on offer by organising a successful Radioglide 2017 at the end of May. 

      There is still a long way to go until BMFA realises all its ambitious plans for the NFC. Very sensibly, they are taking a careful financial route. Many members will not have even seen the site so far. Don’t hesitate. Many other members are in the band of volunteers, regularly making the Centre bigger and better. Offer to help if you can!

      One of the prime movers to establish the National Flying Centre is Chris Moynihan as chairman of the BMFA and also a member of the BARCS executive committee. Many years ago, it was Chris who tackled the difficult job of persuading BARCS to grow closer to the BMFA. He then went on to become chairman of the BMFA with his dedicated drive and skill at bringing together proponents and opponents. Very sadly, due to health problems, Chris has stepped down from both the BMFA chairman role and the BARCS committee. We shall all miss his wise counselling. 

      All the very best - and plenty of thermals - for 2018!

      Sydney Lenssen, BARCS President
       
      • 0 replies
    • Interglide F5J 2017 Report and Results
      This year’s Interglide over the weekend 24-25 June run by BARCS saw a necessary change from F3J to the electric launch format of F5J which proved to be very popular.

      Cracking flying site. Forty-seven pilots booked in. Prizes acquired, particular thanks going to UK KST agents, Flightech and C & M Rapid (Model Glasses) Ltd. for their generosity. The previous week saw fantastic weather. So what could possibly go wrong at Interglide 2017. Well, being the UK in June it’s no surprise, the weather changed for the week. Full report in the link above
      • 0 replies
×

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.