When your student asks.....
 

....so if I've understood what you said, that the wind is a movement of air from an area of high pressure to an area of low pressure, moving at a speed dictated by the pressure gradient, and in a direction that is goverened by a combination of Bernoulis theorem and the correolis effect........then where do wind gusts come from?

When God farts.


:D

When lots of butterflies flap their wings all at the same time?

Or maybe this is a better explanation?

"The proposed approach assumes that surface gusts result from the deflection of air parcels flowing higher in the boundary layer, which are brought down by turbulent eddies."

When I admitted to my instructor that I wasn't particularly wind aware, he suggested I tried Baked beans and/or cabbage :O

Gusts are caused by vortices due to terrain, ground obstructions, rising and falling currents caused by "thermals".

Nothing worse than a vortex in your thermals.... :ooh:

Wind
 

All the answers so far are true!
However most students find it so difficult to understand the air at first. Why - because we cannot use the knowledge of our five natural senses; sight, smell, taste, hearing and touch. The air is invisible to us and like electricity strange.
Water is much more tangible and therefore easier to understand. The Ocean currents are 'wind' just the same and caused by temperature changes and the rotation of the earth (Coriolis effect). Sudden local changes in direction and speed are caused by channels and protrusions on the sea bed and above it. Ships are crashed against rocks and people are sucked under to drown because of these sudden changes. If we wish to cross a river direct then we have to head upstream or into the flow or current to reach a point on the other side.
I find with my students that if we can understand the effects in water first then it is much easier to move on to the air and Coriolis, gradiant winds and the like.

Well said Homeguard:D

The concept that air is a liquid is one that simply doesnt get aired often enough with students. Combining that with a basic understanding of molecules usually clears up most questions. Too often in the PPL world they tend to start with effects not causes.

Regards

Xraf:ok:

Think again!
 

QUOTE Gusts are caused by vortices due to terrain, ground obstructions, rising and falling currents caused by "thermals".

Put your terrain and ground obstuructions in a wind tunnel and the pass a constant flow of air over them. Using your theory the air on the downwind side will GUST! Or will it?

Stunned! I'd lost the thread - didn't get back to it - but....

All the answers that one would expect from a survey of professional pilots and school children - but tell me, the day that I wrote the original message, our airfield forecast included "Wind 030/12G27".

OK, so as I'm at a coastal airfield and the ONLY thing 030° from here is the entire width of the Atlantic Ocean - tell me which buildings and trees the wind had to battle through to gain gusts of more than double the original strong(ish) wind.

Keygrip
I was fortunate in that I was a racing dinghy sailor for many years before I learnt to fly. Characteristics of winds at suface level whether at sea or with many varieties of surface obstruction, when on rivers etc., get learnt about the hard way, especially when you have worked so hard during a race to use the gusts, etc to come in first, and the other guy judges it better!

Gusts over open water can be very sudden, and from a significantly different direction than the average. They arise from the consequence of thermals - what goes up must come down. Downdrafts will carry the 2000ft+ wind right down to the surface which is sometimes over twice the average surface speed, and in the northern hemisphere will be veered by as much as 15deg from the surface average. They are generally very local to cumulus clouds.

I understand that the Met Office uses forecasts of the amount of vertical activity and wind speed at the various altitudes to forecast the gust speed.

Much more severe are the downdraughts in the vicinity of thunderstorms, microbursts, one of which caused the fatalities in the DC10 crash landing at Faro.

MikeJ

You will also get oragraphic movement of air simply by it hitting the coast and rising. The vertical movement mentioned above, which is usually thermally generated will also cause a circulation of the airstream which is faster at height than on the surface causing large fluctuations of airspeed even when there is no cloud in evidence.

Let's try and keep things simple.

If air is flowing in a uniform manner over a perfectly flat surface, something is bound eventually to cause a disruption in the status quo. It doesn't matter what, it's just the nature of things.

If a parcel of air is caused to rise, it tends to leave behind it a vacuum, that thing which nature abhors, by all accounts. So, air will try to move from elsewhere in order to fill this space. As the rising air ascends, it will compress the air it passes through, and so the air at altitude will try to relieve the extra pressure by moving to where the pressure is less.

So, probably in a roundabout way, air at altitude changes places with air on the surface, each parcel tending to take with it its original properties, i.e. speed, direction and temperature.

Hope this helps.

Just tell them that the air is a female entity.

Therfore it doesn't need a reason to be awkard.

If further pushed say its all to do with navier Stokes equation and chaos theory. There is only about 200 people in America that truely understand how and why. Most of them work for NASA or MIT. And I certainly ain't one of them. The female entity works for me.

Wind is governed by Coles' Law

Phil

Paco - Coles' Law? say more please.

Say it out loud.....


:uhoh:

Gusts are mostly caused very localised heating from the sun from say scattered cloud cover. Hot air rises in the sun exposed areas and air from cool shaded areas rushes in - hence a gust :ok:

Awkward
 

Could explain the female logic gate. The 2 states are "if" and "maybe!":)

Nice one Paco :D