Surface wind direction in gusts
 

Just picked up this intersting comment from the Freight Dogs forum.

The original post was discussing the eventful landing of a 747 at Manchester. Since I'm in the unititiated group can anyone tell me by how much the bearing is likely to change? (No marks for saying it depends on the strength of the gust) For example we had 20kt gusting 30 over the weeekend, would the bearing change be significant?
Tone

You can expect up to a 30 degree variation in a gust. And the bearing will increase - in other words the wind "veers".

Gust strength is a factor, but also the terrain, since that determines the difference, both in strength and direction, of the free wind vs. surface wind.

Gusts tend to veer (increase in bearing or more to the right) in the northern hemisphere. They go the other way in the southern hemisphere. The closer you get to the equator, the less noticeable the effect.

This is something we have looked at a lot in sailing and there are a huge number of factors that influence the gust velocity. Surface roughness, stability (vertical airflow) and vertical wind-sheer are the key ones.

Perhaps surprisingly, very little research has been done into wind flow at ultra-low altitudes (less than 5m) except in cities where a good body of information exists. The standard measuring height for wind (10m) did not come about by accident....

Also bear in mind that gusts are a three-dimensional phenomenon. It's easy to think of wind as always blowing parallel to the surface of the earth and only changing direction in terms of compass bearing. In reality, most gusts will also include an increased downwards component of air movement.

I always bear that in mind when listening to the tower wind reports. At LHR for example, a 250/25G35 you know is going to put the gusty bits more in line with the runway (270).

In general a gusting crosswind from the left is more favorable then a gusting crosswind from the right as the wind will veer clock wise in the Northern hemisphere.
Asked my Meteo instructor the same question, there is no formula to calculate the degrees of shift.
Mind you she had a Phd in Mathematics and a degree in Met, so I took it that there is none...:O

I'd be very surprised if she hadn't heard of Gustav Gaspard de Coriolis.

Increase wind velocity, Coriolis force increases, deflecting wind to the right in the N hemisphere and to the left in the S Hemisphere.

Various formulae apply, basic one is Coriolis Force= 2 x (Angular speed of earth) x (Sin Latidude) x (Wind Velocity). Having established Coriolis force, apply this to pressure gradient force (derived from another formula), et voila!

Sod the physics though, my personal rule of thumb is that gusts of 1/4 windspeed = 10 deg veer, 1/2 windspeed = 20 deg veer, 2/3 windspeed = 30 deg veer(In N Hem.). (The same as backing factors for surface vs 2000' wind)

EG 250/20G25 is likely to be 260 in gusts, and 250/20G30 is likely to be 270 in gusts, and 250/20G35 likely to be 280 in gusts