I've noticed on a couple of launches now, that shortly before sunset, the balloon starts to sink, and after 1.5 hrs, the ballon starts to rise again. I suspect this is due to the Diurnal Tide / Cooling of the Troposphere, but have been unable to find anything in the literature that references vertical wind speed so low down in the atmosphere and it's magnitude due to the diurnal tide.
For example, the latest flight (30g Pilot Ballon, GPS):
1, Avg Ascent Rate: approx 1.5m/s
2. 9:10pm @ 28,000ft, ballon begins to descend (sunset 9:25, Latitude 52N) at average 1.5m/s
3. 10:20pm @ 8,000ft, ballon begins to ascend at approx 1.5m/s to 36,500ft till power ran out
I.E. Ballon was in sinking air of speed 1.5m/s (ballon speed down) + 1.5m/s (ascent buoyancy) = 3m/s for 1.5 hours after sunset.
Any experts out there who are familiar with this effect on vertical wind speed and it's magnitude in the Troposphere and if this is the likely cause?