From Alan Spilsbury
Feedback
(25 July)
reported the case of a pilot who couldn’t take off because
there were turtles on the runway. But what if he couldn’t land because there
were turtles in the sky?
The Roman army had a saying that you should never pitch camp in the lee of
hills when there were turtles in the sky. These turtles were in fact lenticular
clouds formed by lee waves when strong winds blow across lines of hills under
unstable atmospheric conditions. In Britain they can be seen quite frequently
and are used by glider pilots to soar in the rising air along the leading
edges.
The Romans encountered the associated problems of extremely turbulent rotor
flow at ground level under a wave peak. If the camp were at the position where
the air is descending, compression can cause higher local winds than general, in
a venturi effect. Both can wreak havoc to a tented camp site or an aircraft.
On 5 March 1966 a BOAC Boeing 707 broke up in rotor flow turbulence while
flying in the lee of Mount Fuji, Japan, with the loss of 124 lives.
Advertisement
Severe damage was done by compression flow in Sheffield, in the lee of the
Pennines, during a gale on 16-17 February 1962. Three people were killed and 250
were injured, while 7000 properties were damaged.
So, as they did back in Roman times, watch out for the turtles.
Moseley, Birmingham
