At temperatures as low as 120 °C (248 °F), fuel-air mixtures can react chemically and produce very weak flames called cool flames. The phenomenon was discovered by Humphry Davy in 1817. The process depends on a fine balance of temperature and concentration of the reacting mixture, and if conditions are right it can initiate without any external ignition source. Cyclical variations in the balance of chemicals, particularly of intermediate products in the reaction, give oscillations in the flame, with a typical temperature variation of about 100 °C (212 °F), or between "cool" and full ignition. Sometimes the variation can lead to an explosion.
In the year 2000, experiments by NASA confirmed that gravity plays an indirect role in flame formation and composition. The Common distribution of a flame under normal gravity conditions depends on convection, as soot tends to rise to the top of a flame (such as in a candle in normal gravity conditions), making it yellow. In microgravity or zero gravity environment, such as in orbit, natural convection no longer occurs and the flame becomes spherical, with a tendency to become bluer and more efficient. There are several possible explanations for this difference, of which the most likely is the hypothesis that the temperature is sufficiently evenly distributed that soot is not formed and complete combustion occurs. Experiments by NASA reveal that diffusion flames in microgravity allow more soot to be completely oxidized after they are produced than do diffusion flames on Earth, because of a series of mechanisms that behave differently in microgravity when compared to normal gravity conditions. These discoveries have potential applications in applied science and industry, especially concerning fuel efficiency.
Flame temperatures of Common items include a candle at 829 °C (1,500 °F), a blow torch – at around 1,600 °C (2,900 °F) a propane torch at 1,995 °C (3,620 °F), or a much hotter oxyacetylene combustion at 3,000 °C (5,400 °F).
