Mountain Wave
I fly in northern Scotland and often plan my routes to avoid the local mountain wave we experience. Thinking back over my last 10 trips, i've experienced some mountain wave effect on at least half of them. Any reasonable (say >20KTS at or below 2000) westerly or south westerly and its a 99.9% certainty you will be sinking if you head off in the wrong direction (high terrain is around 10NM's away). On this basis, its hard to avoid it being taught when you learn to fly here! We do get some very nice lenticular clouds, particularly in April & May!