Fumes is a grain or small particles that arise as a result of burning. The size of the particle varies, it can be tens of nanometers (if the particles are 10 nanometers in size, then 1 meter will line up about 100 million particles, so the particles are very small), up to microns (1 micron equals one millionth of a meter). Such particles may consist of a variety of molecules, depending on the burning material. May contain carbon dioxide, nitrogen oxides, and other molecules. As a result of combustion, the fumes has a high temperature, consequently the distance between the particles is relatively far apart so that the density or density is small compared to the surrounding air. If this happens then the fumes will rise upwards. As in water, objects that are denser in mass are lighter than water will float.
But in fact the direction of fumes is not necessarily upward, depending on the temperature difference of fumea with the environmental temperature and the current wind conditions. In a house or factory with a chimney, if the air outside is very cold then the fumes coming out of the chimney can instead slide down instead of going up. This happens because the temperature is very cold, so very quickly the fumes cools. As a result the density increases, causing a mass density of fumes greater than the surrounding air so that the fumes "drown" alias down. So the rise or fall of fumes coming out of the chimney can be a signal of the temperature of the air around the chimney at that time....
Photo was taken by using my smartphone camera xiaomy mi4i.