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A gust front is the leading edge of gusty, cooler surface winds from thunderstorm downdrafts, convergences, cold fronts or sea breeze fronts.
We don't see many thunderstorms in the Southern Alps (except on the West Coast, but we don't fly there in thunderstorm conditions), so we're more likely to see a gust front associated with the sea breeze. As the sea breeze moves inland the cool dense air pushes a wedge of air ahead of the main airmass. As the wedge of air is thin at the leading edge the surface wind at the edge can be much stronger than overall speed of the front. If the front is moving inland at ten knots, it might be twenty knots or more at the leading edge because more air is required to thicken the wedge as it moves forward.
Once the leading edge has passed and the main airmass is overhead, the surface winds will drop to the speed of the front. This can be dramatic. Winds can go from a ten knot westerly before the front, switch quickly to a gusty twenty knots plus easterly, then after a few minutes drop to a ten knot easterly as the main front arrives.
A less common type of gust front that we see around the alpine areas on wave days are lines of dust being torn up off the ground travelling with the wind in the unstable low level airmass. I have seen these travelling down the Dobson and across Lake Ohau. Cool dense air comes over the main divide at the top of the Dobson and is squeezed into the narrow Dobson valley. This dense air pushes down the valley. The less dense air in the valley is lifted up and as it's unstable forms a line of thermal/convergence that travels down the valley ahead of the cool dense air. These can be strong enough to be visible as they pull dust and gravel up from the valley floor. They can travel quickly. I have been overtaken by these gust fronts when flying down the Dobson, so they were moving at more than sixty knots.
These gust fronts under the wave can also be observed travelling across the Southern Mackenzie, pulling up a wall of dust from the fields on strong wave days. Out in the open they tend to be slower moving, moving at twenty or thirty knots. They tend to be a few kilometers across (although that might just be the visible area, just the width of a field that it picked the dust up from).
Briefing prepared by Phil Plane.
This work is licensed under a Creative Commons Attribution 4.0 International License