In mountain valleys, winds follow a predictable pattern. Here are 4 different ways slope winds can affect your approach to a mountainous airport.
1) Airspeed fluctuations on short final.
Upslope winds, which are caused by warm air rising due to sun-radiated mountain slopes, can create wind speed fluctuations as you cross the threshold of the runway. Upslope winds tend to be strongest around 33-66 feet into the jet-speed profile, and since this is the height you'll be crossing the threshold, you should expect to see wind fluctuations here.
When mountain slopes absorb heat from the surrounding air after the sun sets, it causes the airmass to cool down and sink due to an increase in density. This causes downslope winds. Downslope winds tend to have very shallow jet-speed profiles, so you'll experience the strongest winds around 3-10 feet into the core, so as you begin your flare, you might begin to notice speed fluctuations.
The height of a mountain slope will determine how strong the downslope and upslope winds will be. A taller mountain means stronger winds. Since airports tend to be located closer to the base of mountains, you'll want to pay closer attention to the presence of strong upslope or downslope winds.
If you're flying into an airport that doesn't have AWOS or ASOS, you'll have to rely on the windsock. Based on what time of day it is, you can make an educated guess to determine whether you can expect upslope or downslope winds so that you aren't surprised when you take off or land.
Mountain weather isn't just a Rockies problem. Learn to evaluate terrain weather in planning and in flight, and fly confidently around the mountains anywhere you go.
Corey is an Airbus 320 First Officer for a U.S. Major Carrier. He graduated as an aviation major from the University of North Dakota, and he's been flying since he was 16. You can reach him at corey@boldmethod.com.
