Relationship between Thermal Height, Strength and Cloudcover
It is not easy to get reliable figures for Strength of Thermals. Pilots who have achieved the best speeds usually report the strongest thermals. The last person back complains that lift was much weaker than forecast.
The French devised the above graph for predicting thermal strengths; this is an empirical method based on numerous reports by their pilots.
The system depends on the cloudbase. The dotted line shows and example of its use.
1. Move up the left-hand scale to the cloudbase in thousands of feet. Suppose it is 5,500ft.
2. Follow a horizontal line until it meets one of the diagonals labeled with cloud amount.
3. The cloud amount lines got 6/8. Nil, 1/8, 2 or 5/8, 3 or 4/8. The French found that the lift varied according to the amount of cu. Follow the dotted line along tot he 1/8 line.
4.The take the Line down to the Average Lift Scale (which reads about 4.25 knots in this example)
5. Continue down the same line into the lower half of the diagram where there is another diagonal line.
6. From this turn to the Max Lift scale and read off the value (nearly 7kts).
Thus if the cloud was predicted to be 1/8 cloudcover at 5,500ft the forecast is for lift to average 4.25 knots with peaks to about 7 knots.
Source: Soaring July 1999, Tom Bradbury
4 July 2000