On December 3, 1984, approximately 40 metric tons of toxic methyl isocyanate (MIC) gas leaked from the Union Carbide plant in Bhopal, India. The catastrophic severity of the disaster was heavily dictated by the local microclimate and chemical properties of the gas. Methyl isocyanate has a vapor density roughly twice that of ambient air, meaning it naturally tends to hug the ground rather than dissipate into the upper atmosphere.
This hazardous property was exacerbated by a strong surface temperature inversion occurring that cold winter night. Under normal atmospheric conditions, air temperature decreases with altitude, allowing warm surface air and any suspended pollutants to rise and disperse via convective mixing. However, during a radiation inversion, radiation from the earth chills the lowest layer of air while a warmer layer remains above it.
This meteorological ceiling locked the cold, dense air in place over the city. As the heavy MIC cloud escaped, it behaved as a dense gravity current, flowing along natural topographies and sinking into low-lying residential areas adjacent to the plant. Coupled with extremely low ambient wind speeds of less than five kilometers per hour, the inversion layer prevented vertical dispersion, creating a concentrated, deadly blanket of toxic vapor that persisted for hours across densely populated neighborhoods.