I think Ally Pally may have missed the point! How often do we get still nights on the north sea?! They tend to happen more often in the summer and funnily enough it stays lighter for longer then!
p.s. Even with the technology available for the Wessex 3, we din't have too many problems establishing a doppler hover in light airs, the downwash actually came in handy to give the doppler something to look at!