# Are winds that are not from the prevailing wind direction more often gusty, and why?

It seems to me that, looking at the ground weather forecasts (as opposed to, say, low level forecasts), it's far more common for the wind to be predicted to be significantly gusty when it is blowing across the runway direction of my local airport, than along with it.

Now, obviously, the wind doesn't particularly care about the direction of the slab of asphalt on the ground, but the slab of asphalt is normally aligned with the prevailing wind in the area.

Is a wind that blows at a significant angle to the prevailing wind direction more likely to be gusty, than a wind blowing in the prevailing wind direction in the area? If so, what's the mechanism behind that? Or is this just a case of me being more frustrated about gusty crosswind conditions, thereby noticing it more?

My area is relatively flat.

To clarify, since there seems to be some confusion about this; I'm referring to area ground-level weather forecasts intended for the general public (as those offer a longer-term outlook, typically with reduced resolution and accuracy up to ten days ahead), not METARs or TAFs (however presented) which are about weather conditions specifically at the airport now and in the next several hours.

• Are you talking about a mixing winds condition where two wind patterns converge? – DLH Oct 1 at 21:11

## 4 Answers

A little more info about where your airfield is would be helpful.

The generic answer would be that a significant weather event would be required to over come the prevailing winds. Also, as winds changed from prevailing to cross, gusts and shears would be more likely. Local low level factors such as terrain, buildings, and trees may also taken into account. The airspace along the runway is generally more clear of these effects as compared with across it for a single runway. Larger airports with multiple runways provide more options.

• Well, the ground forecast isn't just for the airfield, it's for the area, so I doubt the factors you mention would have a significant impact on it. They probably do locally, as in, say, within the immediate vicinity of the airport, but there are both trees and buildings to any direction of an arbitrary point, so for the area, it seems like those effects should effectively cancel out. – a CVn Dec 27 '18 at 18:32
• My training field was 280 to 100. We almost always took off from 280 into prevailing northwest winds, which generally meant clear skies. Southeast winds, or especially northeast, generally meant storms or unstable air. I can only remember taking off on 100 a few times. – Robert DiGiovanni Dec 27 '18 at 19:20

You need to know what causes air movements to better answer this question.

Generally, there are two sources: Earth rotation and sunshine.

Earth rotation leads to large-scale air movements from Coriolis forces. This is the source of prevailing and less gusty winds. Local obstacles can still cause local changes in direction and even regular gusts from a vortex street, but that is rather unusual.

Vortex street behind the Juan Fernandez islands (picture source)

Sunshine will heat the ground which in turn heats the air above it, reducing its density. Periodically, this heated air will bubble up as a thermal and cause surrounding air to flow towards the thermal, being felt as a gust by an observer. Typical sources of thermals can be the edge of a forest or a local hill which will give the heated air a jump start.

Is a wind that blows at a significant angle to the prevailing wind direction more likely to be gusty?

If you observe the gusty conditions in fair weather with lots of sunshine, small cumulus clouds and from late morning to late afternoon? Then the answer is yes. The gusts are caused by thermals - look for typical sources of thermals to understand where they blow to.

Here is O'Hare's wind rose for 1961-1990.

So Chicago's prevailing winds are prominently southwesterly.

NOAA's historical data for Chicago for the past year, 2018 Sep 28 to 2019 Sep 28, includes:

• WDF2, direction of fastest 2 minute wind (degrees)
• WSFG, peak gust speed (mph)
• AWND, average wind speed (mph)

The first approximates daily wind direction. Dividing the last two approximates gustiness. Here's the resulting scatter plot of direction vs. gustiness.

Other than extreme gustiness when the wind is due north, this shows no strong correlation. Winds from the ESE (120 degrees) are rare (fewer X's), as confirmed by the wind rose. Winds from 180 to 270 degrees are more frequent (denser X's), as confirmed by the wind rose, but they're not gustier. I don't see any other conclusions to draw.

Plotting gustiness redefined as the mph difference between peak gust speed and mean wind speed, instead of the ratio, leads to the same conclusions.

Is wind at a significant angle to the prevailing direction more likely to be gusty?

No, at least not for Chicago in 2019. (The data for all of Illinois was too big to download. Smaller Illinois towns lacked some of these fields.)

Or is this just a case of me being more frustrated about gusty crosswind conditions, thereby noticing it more?

I think that is the case. Are there trees along the length as well, making for interesting wind rotor effects?