Aerogel particles mostly semi-transparent and extremely light in weight. It is also known as "solid smoke" or "frozen smoke". The density of common silica aerogel is approximately 0.2 grams per cubic centimeter, which is only one fifth of that of water. It is one of the lightest solids in the world and currently has the lowest thermal conductivity among all solid and porous materials. Its heat transfer coefficient is much lower than that of still air, usually only about half of that of still air.