Cross Country Magazine Tuesday Tip

This week's tip is from Understanding the Sky, by Dennis Pagen

Cross country flight planning for beginners

A potential thermal may sit on the ground for many minutes as it is building.

Although this is an unstable situation, the warm air must push up through the cooler air and a flow under the thermal must begin.

If the warm air is expanding it may remain on the ground until a gust breaks it away, or it becomes so large that the expansion slows and cooler air pushes in from the side.

The sudden release of a thermal is called triggering.

Lots of things can trigger thermals: a passing car, a pilot landing, sheep moving in a field.

Even shadows passing over sloping ground can trigger thermals. The ground can cool quickly over several minutes when the solar heating is stopped. A quick slug of cool air can then form, which slides downhill to trigger any potential thermals in its path.

Yes sheep can trigger thermals

Understanding the Sky Understanding the Sky is available to buy at