Wave - some of the conditions when it might occur in Britain

logo Wave 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.
But these negatives are really so obvious that they need not be emphasized here.

So listed here are some of the factors that favour wave.  Use the NOAA website and charts to get the information


Inexperienced 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.


1 There is a range of hills upwind, the orientation being within about 30 degrees of perpendicular to the 10,000 foot wind
2 Wind direction should be more or less constant to at least 18,000 feet (500 millibars)
3 Wind speed 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)
4 A reasonably stable layer within some 2 - 3,000 feet above hill tops, ie a lid on convection
But it is certainly not unknown for wave to occur in  remarkably unstable air masses
5 A stable layer at altitude, eg above about 15,000 feet, ie produces a lid to inhibit vertical development and thus showers.
But again, this is not a hard and fast rule.  It is sometimes possible to climb above nearby cu-nims.
A 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 34C
The net weather and NOAA websites (and others) will enable you to find out 
6
Factors 4 & 5 can often be inferred by the curvature of the isobars
While wave certainly does occur with isobars with gentle cyclonic curvature, slight anticylonic curvature is undoubtedly a plus. 
Pronounced cyclonic curvature usually (but not always) rules out wave.  Examples of both curvatures shown below.

A nearby front, eg quasi stationary or just after a cold front, often gives the right stability

synoptics

Cloud 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)

Widespread wave 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.

Rarely 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.


The 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!