I'm sure you all remember the international standard atmospheric lapse rate from your ground school (it's 1.98°C per 1000 ft in case you need a hint) but it's that time of year again when you may have noticed your outside air temperature gauge increasing with altitude.

This year in Whitecourt we've been getting a lot and many with some big temperature changes. Some days it's been 15 degrees centigrade warmer at circuit height than on the ground. So how are these inversions formed and what does it mean for your flying?

The most common way this time of year is on a clear night (although thunderstorms and frontal systems can also cause wind shear). With the suns rays no longer heating the surface the ground radiates it heat out and the now cold ground cools the air above it. Typically these inversions are fairly shallow. Usually between 1000 – 2000 ft above ground level although can go much higher.

You may have noticed the horizon looking a little like the photo on the right. A flat mountain top with valleys and ridges that appear and disappear as you climb or descend. This is due to the different air temperatures having different densities. The light traveling through is bent or refracted causing you to see things that aren't really there.

With this change in temperature and air density you can expect to encounter some wind shear. This change in wind speed and/or direction can be barely noticeable to quite dramatic. It may change the height you were planning to fly at. By climbing or descending you may be able to switch from a headwind to a tailwind. You may find it considerably smoother above the inversion than below it. The most important consideration with wind shear is when making your approach to land. Always keep an eye on where the wind is from. Just because you started your approach into wind doesn't mean you'll still be into wind closer to the ground. If you're aware of this you might not need to change your approach but the wind can bite you when it's not where you think it is.