MET 214 in application when planning (choosing) a day to thermal XC.

If on your chosen day the information in the MET 214 is as follows

[10,000ft] 290 10kt -06*c
[5,000ft] 290 10kt -02*c
[2,000ft] 260 05kt 10*c
[1,000ft] 240 05kt 15*c

The surface temperature from the METAR of the local airfield information gives 18*c, while the TAF predicts a cloud base above the airfield (50ft above sea level) of 8,000ft

Ok what does this mean?

Between sea level and 1,000ft you would expect a temp drop due to DRAL (Dry Adiabatic Lapse Rate) of 3*c/1000ft, this is what is shown on the days 214, so any ‘parcel of air’ warmed by contact with a shopping center car park out in front of your take off hill, to say 23*c once it breaks free of the ground will by the time it has risen to the hill top, drifting in the wind, now have a temperature of around 20*c….(ignoring the effect of mixing with the surrounding air).

As our thermal at 1,000ft is still 5*c hotter that the rest of the air at that height, it will keep rising, taking our XC pilot with it. By 2,000ft the thermal should have dropped its temp to 17*c (20-3) but here the 214 prediction for the overall air mass is 10*c… as a result the difference in temp (thermal/surrounding air) has in fact increased to seven degrees from five degrees (at 1,000ft…) so the thermal is getting stronger, on the other hand it is also likely to be larger and so may be smoother to fly….

Getting even higher as we climb from 2,000ft up towards 5,000ft our thermal still cools, just like the surrounding air, which would also tend to cool a further 9*c (DRAL x 3,000ft – 3*c x 3 = 9*c), all we need do to asses the chance of a good XC is ‘work the numbers’ AND COMPARE WITH THE 214 PREDICTION, if the 'numbers are good' instead of planning to head back to the hill we will have arranged prior to launch for some one to take the car home/retrive etc. etc... you DID leave the keys with someone not intending to xc I hope! So our thermal was 17*c as we passed 2,00ft and applying DRAL indicates it will still be 8*c (17-9=8)  as we reach the flight level FL50, while the prediction for the overall air mass at that level was given as being -02*c.... result our thermal is really booming as it is around TEN degrees warmer than the overall air at that altitude… (again this is ignoring the effect of the thermal mixing with the air it is passing through)

Passing above the FL50 mark however things on this day start to slow down as the air here (perhaps the leading edge of a warm front arriving tomorrow morning…)  is predicted to only be -06*c at FL100 (10,000ft), so some where between FL50 & FL10 there is a significant inversion, probably some where around the 8,000ft alto stratus layer forecast in the 215 and TAF…

Time to leave the weakening thermal and commence the down wind glide, with the upper level winds we track approximately west on our compass/gps (the info ‘290’ is true rather than magnetic by the way, in a written weather report), the plan is to hit the next thermal at or ABOVE the level  of any slight tendency for an inversion, in this case the biggest temp variation, thermal/general air, is found above the 1-2 thousand foot mark, sticking to ‘express elevators’ cuts down on the time spent circling and maximizes the time where you have a tail wind.

Finding the next 'boomer' back up we go, as we left early in the day and the di-urnal temperature variation means the strongest thermals should be around 1-3pm the day should be good as long as we are below the wedge of warm air from the approaching warm front coming in from the west (behind us) so here is another hint, on light wind days and thermal flying flying away from warm fronts is good and flying towards cold front (provided the weather is OK) is also good, further points to consider with cold fronts is they can give rise to notable changes in wind direction etc.

Ok so instead of fixating on distance, on any day with a 'lively' lapse rate simply aim to get high and the distance will follow as a matter of course, if four good 3,000ft decent glides will give you 60km all you need is another couple of climbs from 2,000 back you to 5,000ft to approach the 100km.... in the UK it very much comes down to how far it is till you are 'Feet Wet' as the RAF phrase it!

I hope you find the MET stuff as intresting as I do, always remember good MET understanding helps keeps pilots alive!

Yours Murray Hay

NB. I hope there are no arithmetic errors above!

Note://cycles & thermal 'tallness':  To get a rough idea of how high (long in time) a thermal can lift you we need to judge not just how strong it is but perhaps more important.... how 'tall' is it? Taking off JUST as the thermal is about to reach hill top height our, lucky/well judged pilot, enters the top of a thermal which at this moment is still attached at its base to the car park...... however in this example almost all the hot air has been drawn from the car park and now the (relatively) cool air that has been drawn in there will 'wait' for the sun to heat the ground, to heat the air in contact, to give the next thermal cycle in say 5 more mins.... (i.e. you have felt increasingly strong thermal cycles passing the hill every five mins for the last half hour as the day just starts to 'work')

Ok so how long (time) can you climb in a thermal that is 1,000ft top to bottom? Simple in a PERFECT bit of flying you are looking at a decent rate (relative to the air in the thermal) of 200ft/min, a 1000/200 gives you around five mins circling time, regarding height; if the thermal is going up at say 700ft/min then you will gain altitude at 500ft/min for five mins, so end up a about 3,500ft (you started at 1,000ft).

In the example where the source only generates consistent 1,000ft tall 'bubbles', just like step towing you head back into wind to quickly meet up with the next cycle this taking about five mins (the cycle time) so entering the following thermal at around 3,000ft (500ft 'spent' getting there) you ride this one up while already in the height band DUE TO THE BETTER LAPSE RATE (214  prediction) where your five mins of lift (1,000ft tall thermal) gives an overall gain of say 700ft/min taking you to near FL65 OR LOWER IF THE INVERSION IS LOWER.....

The aim for the rest of the xc is of course to stay in the 'band' (altitude range) where thermals have the greatest variation in temp relative to the overall air mass.....