If you see stratus clouds, that's the time to get inside and stay there. Stratus clouds mean rain if it is warm and snow if it is cold. They look like a huge gray blanket that hangs low in the sky. Sometimes stratus clouds are on the ground or very near the ground, and then we call them fog.
Usually stratus clouds and fog form when it has been cold out and then warmer, wet air blows in. As the warm air flows over the cold ground or over the cold air near the ground, the water vapor in the warm air condenses into drops of water that make a cloud. How thick the cloud is depends on how wet the air is and how big the difference in temperature between the cold air and the warm air is.