The heat and airborne debris created by a nuclear explosion can cause rain; the debris is thought to do this by acting as cloud condensation nuclei. During the city firestorm which followed the Hiroshima explosion, drops of water were recorded to have been about the size of marbles. This was termed black rain and has served as the source of a book and film by the same name. Black rain is not unusual following large fires and is commonly produced by pyrocumulus clouds during large forest fires. The rain directly over Hiroshima on that day is said to have begun around 9 a.m. with it covering a wide area from the hypocenter to the northwest, raining heavily for one hour or more in some areas. The rain directly over the city may have carried neutron activated building material combustion products, but it did not carry any appreciable nuclear weapon debris or fallout, although this is generally to the contrary to what other less technical sources state. The "oily" black soot particles, are a characteristic of incomplete combustion in the city firestorm.