There’s no smoke without a fire. Smoke is an unwanted by-product of fire. When a fire burns, the air around it becomes heated as some of the burning fuel is changed into invisible gases, and the heated air sweeps up water vapor and tiny specks of the fuel into a dark cloud of smoke.
The more incompletely something burns the more smoke it produces, because more particles are left to be swept up into the air.
Smoke is a collection of airborne solid and liquid particulates and gases emitted when a material undergoes combustion together with the quantity of air that is mixed into the mass. It is commonly an unwanted by-product of fire.
Wood smoke, for example, consists mainly of carbon dioxide and water vapor. These hot gases rise and become part of the air. As the water vapor in the rising gases cools, it forms tiny drops of water that may make the smoke appear white. But often, bits of ash and tiny particles of burned carbon, called soot, are also carried away in the rising cloud of gases. They color the smoke gray to black.
Smoke gradually spreads out and drifts away, with gravity pulling the heaviest bits back to the ground. When a fire first starts to burn, there is usually a lot of smoke, which decreases as more of the fuel is burned completely. If a fire is extremely hot, it may burn without much smoke at all.
Content for this question contributed by Wendy Ashwell, resident of Cincinnati, Ohio, USA