Some morning soon you'll step outside to a white film on the lawn, the car roof and the top rail of the fence, then it will be gone within an hour of being in the sun. Frost forms on clear, still nights, when the ground loses its heat straight up into an open sky and the surface cools below freezing. The air a metre and a half above it, which is where thermometers live, is still a degree or two warmer. That's why the overnight low can read 2C and your windshield is iced anyway. Cold air also drains downhill like water, so the low ground gets frosted first. River flats, ditches and the bottom of the park whiten while the slope above them stays green, and you can stand in one place and see the line. This is the night that stops the crickets and blackens the annuals, and after it the leaves start falling in earnest.