Microburst m/s or knots?

[Startup]

Got some Met feedback that says Microbursts are quoted in 'kts/100ft'. This sounds like a load of cobblers, because in my experience it's always in m/s....

Can't find microbursts listed in the ICAO annexes, so can anyone shed any light on this one?

[tom775257]

For what it's worth (without my notes) I seem to recall windshear being measured in kts/1000ft. kts/100ft sounds wrong!

The only question I remember from the exam re: specifically microbursts was: What is the size and duration of a typical microburst (or along those lines).
The question regarding measurement was specifically referenced to 'windshear.'

[Dick Whittingham]

Slightly tricky one. Vertical windshear is not wind blowing up or down as you might expect, but the change in horizontal wind you get when climbing or descending. Thus it is measured in speed change per height up or down, typically kts per 100ft. Horizontal windshear is the change in horizontal wind you get as you travel along horizontally, and is measured in speed change per distance travelled, the Met bible giving kts per 1000ft.

I doubt the units are cast in concrete. I have quoted the Handbook of Aviation Meteorology.

Microbursts can be variously categorised. One way is to quote the horizontal head-wind to tail-wind shear across the whole thing, across several km. This has been assessed as about 50kt on average with 100kt maximum.

The microburst will have originated in a downdraft from a big CB, and you could quote this speed, usually a 4000 to 6000ft per min downdraft. Of course these ft/min speeds correspond roughly to 40 to 60kt, so you could use those as well.

The pearl in the oyster here is to remember the definitions of horizontal and vertical windshear.

Dick W

[WX Man]

Good answer, Dick W, but may I add:

Any windspeed's SI units are metres per second.... depending on what the question says (exactly), eg "how are microburst speeds quoted in an ATIS" (kt/1000 ft it appears) vs "what is the measurement windspeed in a microburst" (m/s or kt/1000 ft depending which way you look at it), it may or may not be worth appealing the question.

Of course I like to pick holes in any question I can! In particular met ones (having done it at university).

[witchdoctor]

Form 215 would give you the info in ft/min for the general sector forecasts when such conditions are expected to exist.