24 http://www.yachtingmonthly.com November 2017Temperature differences are responsible
for another kind of downslope wind,
which is rarely described in print. It is
usually fairly weak but on calm days it
may be the only useful wind for sailing.
When warm air passes over a cold sea, a
layer of cooled air is inclined to develop
above the water, and this is known
as a temperature inversion. (Several
other factors can create or encourage
temperature inversions but I won’t
describe them here). Even if the warm air
is blowing as a wind at high altitude, the
cool layer is denser and will be reluctant
to move – hence the calm at sea level.
If a land ridge, such as an island or
peninsula, lies across the direction of the
warm wind, it is often possible to find
air blowing off that land and on to the
water, but then fading away again, so
that the only surface breeze is close under
the weather shore. This counter-intuitive
effect is apparently caused by the warm
air pouring down the ridge’s leeward
slope, before rising again as its lighter
density lifts it back above the temperature
inversion (Diagram 6).
We have frequently experienced these
weather-shore breezes on the west of
Scotland, where the mountains are mixed
up with sea lochs and are inclined to hold
pools of cool air over the water. Last year,
when setting out to sail south past the
long, narrow Lismore Island, we could
see that clouds were moving briskly from
the west but there was only faint wind
on the water. We decided to try the eastside of the island and soon after entering
its lee encountered patches of stiffer
breeze that were falling off the shore and
spreading out towards us (Photo 13). That
inshore breeze helped us for the next
five miles to our lunch stop, after which
the temperature inversion broke up and
the afternoon reverted to normal sailing
conditions.
The effect can also occur further
south. On one passage through Ramsey
Sound, in North Wales (Diagram 7), we
approached the Sound in feeble wind
and rather hazy visibility, which is typical
of a temperature inversion (Photo 14).
After we had gone through the Sound
and turned east for Solva, we came in the
lee of the peninsula, whereupon the air
cleared and the wind suddenly picked up
(Photo 15), eventually reaching a good
Force 4 as we enjoyed a splendid beam
reach and even contemplated reefing.
On a late-season eastward passage
around Portland Bill, I managed to get a
photograph that demonstrates the role
played by a high ridge. In Lyme Bay, west
of The Bill a predominantly NW wind had
been weak and indecisive, although the
forecast predicted a brief backing to SW
before the NW flow reasserted itself. This
suggested a slight meteorological trough
would pass over. Sure enough, shortly
after we rounded The Bill a new breeze
swept down off the high Isle of Portland
from the SW, while Portland Harbour,
immediately to the north, remained
resolutely calm (Diagram 8 and Photo 16).Inversion effects
Cold seaDraw-down of wind from layer of warm air above a temperature inversionWarm airBreezy CoolingWind lifting
off again
Cool air – surface calmDiagram 61316EXPERT ON BOARD How to unlock tHe secrets of winds on a weatHer sHore
In the lee of Lismore, a catspaw of breeze hooks down on to the waterIn the lee of the St David’s
peninsula, the air has cleared
and the wind is much stifferWind off the high Isle
of Portland has slid
down to sea level but
PHOTOS: KEN ENDEAN there is calm elsewhere