Steven Yelle, age 11, of Ottawa, Ontario, Canada, for his question:

WHAT IS FOG EXACTLY?

Fog is a cloud formed so low that it rests either on the ground or very near it. It can also appear above a body of water. Fog may be thin or dense.

You'll find that fog forms much the same way that a cloud appears above your tea kettle. The moisture in the warm air coming from the kettle condenses and forms small water drops when it hits the cooler air. In much the same way, fog appears when wind blows warm and moist air over a cold surface. The sudden chill condenses the moisture.

Advection fogs often occur along seacoasts and along the shores of rivers and inland lakes. It happens when warm air blows over cold water.

Radiation fogs form when air near the ground loses warmth through outward radiation. This type of fog can be found in valleys during winter.