- some of the conditions when it might occur
is most incredibly fickle and can fail to occur when confidently expected.
In some ways, it would be easier to give the factors that most definitely
preclude wave, eg in East Anglian when plagued by low stratus with the
wind off the sea.
these negatives are really so obvious that they need not be emphasized
listed here are some of the factors that favour wave. Use the NOAA
website and charts
to get the information
pilots sometimes encounter wave without realising it and talk about
"prolonged sink". Certainly, wave is not always the cause of such
problems, but reports (and experience) suggests that wave effects can
often occur well away from mountains. More about this later.
Meanwhile, the basic conditions for real wave, that is wave that enables climbs above cloud are given here.
is a range of hills upwind,
the orientation being within about 30 degrees of perpendicular to the 10,000
should be more or less constant to at least 18,000 feet (500 millibars)
not decreasing with height, (increasing
speed is often the case in good wave conditions) but the wind does not
have to be particularly strong at hill top height (10 kts might be enough)
reasonably stable layer within some 2 - 3,000 feet above hill
tops, ie a lid on convection
it is certainly not unknown for wave to occur in remarkably unstable
layer at altitude, eg above about 15,000
feet, ie produces a lid to inhibit vertical development and thus showers.
again, this is not a hard and fast rule. It is sometimes possible
to climb above nearby cu-nims.
useful clue to the stability or otherwise is found by checking the temperature
at 18,000 feet (500 mbs). The difference between the surface level temperature
and the 18,000 feet should normally not be more than about 34°C
net weather and NOAA websites (and others) will enable you to find out
4 & 5 can often be inferred by the curvature
of the isobars.
wave certainly does occur with isobars with gentle cyclonic curvature,
slight anticylonic curvature is undoubtedly a plus.
cyclonic curvature usually (but not always) rules out wave. Examples
of both curvatures shown below.
front, eg quasi stationary or just after a cold front, often
gives the right stability
Streets The requirements for cloud streets to form are remarkably similar to those that favour wave.
Streets can suddenly collapse and wave forms instead - and vice versa. Sometimes wave occurs above streets!
Alignment of wave
is approximately parallel to the generating hills and not necessarily
precisely at right angles to the actual wind (see item 1 above)
over flatlands is more widespread than is often appreciated. The
general topography of Britain, ie the alignment of mountain ranges, is
that wave is often aligned approximately 210° / 030°.
Upper winds with directions between 290° and 320° are thus ideal.
Now it so happens that winds from this direction are often ideal
for thermals as well. A steep lapse rate in the lower levels
capped by an inversion (good thermal potential,) is also ideal for wave.
of course is it possible to climb above cloud in wave over the Fens,
but the influence of the wave on the thermals can be pronounced.
I have been told very reliably of exceptional thermal rates of
climb over Ely, for example, when the thermal coincided (or was
trigged) by the rising part of the wave. Equally of course, dead
areas will be associated with the "down" part of the wave or where the
air has descended and been adibatically warmed - warm air at say, 4,000
feet, acts as a strong damper to thermal development.
Thermals can in these wave-induced circumstances seem to align cross wind -
worth bearing in mind.
wave in these west-north-westerly winds can produce long lasting medium
cloud that is a real nuisance as it cuts off the sun. I
learnt the hard way about this at once competition where I was
forecasting and had expected the big area of cloud to drift away in the
wind. It didn't. Two weeks later at another competition, a
virtually identical scenario occurred. I got it right that time!