Last week we experienced a kind of non-standing wave phenomenon in which small cloudless swells were flowing over the landscape (and possibly over each other…), generating variable periods of smooth, wide lift and sink. Six- to- ten-knot lift was reported straight above the airport as low as 3000 AGL – precisely while takeoffs were suspended briefly due to excessive sink over our field’s upwind end. And then the reverse might occur a few minutes later!
Though textbooks say you need at least 20-kt winds across a ridge top to generate wave, our wind aloft that day seemed much less, and was from the south, as opposed to westerly breezes on the ground.
All this demonstrates that, while surface conditions may indicate something about atmospheric flow aloft, they often do not. And among relationships that do exist, those that are actually useful for us are often more deductive than direct… (Editor’s note: if you know what this last sentence means, please explain it to the author.)