How tall is the cumulus?

 

Harter Fell in the distance is 650m above sea level so 4 fingers are probably worth 500m. The tall cumulus subtends twice as many fingers so would be worth 1000m if it were the same distance away as Harter Fell. I think it was further away so would be taller. I think that they can be up to 2000m high so that's not a bad estimate.