What is frost?

As temperatures begin to cool, the first frost of the season can be an important sign that summer is coming to an end.

Frost forms when surfaces near the ground cool to or below the freezing point of water — 0 C — allowing water vapour in the air to freeze into tiny ice crystals.

Unlike snow, frost does not typically fall from the sky. Instead, it forms directly on surfaces such as grass, car windshields, rooftops and outdoor plants.

Clear and calm nights are often the best conditions for frost. After the sun goes down, the ground loses heat through radiation. With little wind or cloud cover to slow that cooling, surfaces can become colder than the air temperature measured a few metres above the ground.

That's why frost can sometimes appear even when the official air temperature remains slightly above freezing.

There are several types of frost. Radiation frost, sometimes called hoarfrost, is common on clear, calm nights. Advection frost can occur when very cold air moves into an area and chills surfaces.

For gardeners, frost can be particularly important because temperatures near the ground can damage or kill plants that are sensitive to freezing.

The first frost of the season can also vary considerably from one location to another. Higher elevations, inland areas and places sheltered from the moderating influence of the ocean can often experience frost earlier than nearby coastal communities.

And while a frost means temperatures at the surface reached freezing, it doesn't necessarily mean the entire region experienced an air temperature of 0 C.