Oso, Ontario - Any country dweller could tell you that this dizzying "mackerel sky" is a harbinger of drizzle and downpour.

Unstable air is rising over colder air, producing clouds as the air cools to the dew point.

This wide-angle view shows a vortex of cirrocumulus clouds swirling just above the tree tops. The approaching warm front is setting up ascending air currents, to create cumulonimbus clouds and thunderstorms in its path.

Take an umbrella.