Slowly moving air currents can be directly observed by using smokes. These may range from simple dust clouds, through various chemical smokes, to more refined techniques employing gas and radioactive tracers. Various chemicals have been used, including stannic chloride, titanium tetrachloride, and pyrosulfuric acid. These materials give off white fumes when their vapors come into contact with atmospheric moisture. The method in common use is to carry the chemical in sealed glass ampules, which can be broken when an observation is to be made.