The term 'killer cloud' has been assigned to various cloudlike atmospheric formations which have resulted in great and often rapid loss of life. While occasionally the term has colloquially been used for tornadic cumulonimbus clouds, especially those which have resulted in particularly deadly tornadoes, it is most commonly found in conjunction with volcanic or other gaseous emissions harmful to human life. While most such emissions escape quickly into the upper atmosphere and are dissipated more or less harmlessly, occasionally the released gas will be trapped near the surface by geography, by an atmospheric inversion, or even by the gas's own density relative to the surrounding air.
Probably the best known example of killer clouds is smog. Whenever warm, often humid air is trapped under a cooler layer of air, especially in areas which are already in a geographic depression surrounded by higher land (such as Los Angeles), surface pollutants such as sulfur dioxide and ozone are also trapped and begin to accumulate. Minor smogs create a slight haziness of the air, and may cause respiratory distress and even death. Major smogs, such as the 1952 pea-souper which trapped incendiary byproducts of coal over London, England, for four days, may kill by the thousands.
While modern anti-pollution laws have vastly decreased the intensity of smog, the frequency of air alert days seems to be increasing, especially in northern climes; while a three kilometre thick 'Asian Brown Hole', including heavy concentrations of black carbon, has been identified over virtually the entire Indian subcontinent. While detailed analysis is still lacking, respiratory illnesses are known to be increasing in the area. Air pollution is also expected to be a major performance factor during the 2008 Summer Olympics in Beijing, China.
Volcanic eruptions frequently release killer clouds. The 1783 Skaftareldar (eruption of the Laki fissure vent volcano in Iceland) was neither explosive nor a simple release of magma, although massive amounts of magma were also emitted. Rather, it continuously released millions of tons of fluorine and sulphuric acid aerosol over a period of almost eight months, in a lingering dry fog ('Laki haze') which prevailing winds spread over much of northern Europe and as far away as Syria. In these regions, the haze contributed to unusually severe thunderstorms through that entire year. Monsoons were disrupted in India and Egypt. In time its atmospheric effects were carried all the way around the world, where Benjamin Franklin observed that a dry fog, which the sun could not disperse, had spread as far as North America. The average temperature had fallen by almost 5 degrees C, ice was found in the Gulf of Mexico, and the Mississippi River froze at New Orleans. Around the world the sun was visibly dimmed; and weather patterns were disrupted for years.
In Iceland itself as well as through much of nearby Europe, most crops and grasses were contaminated by the fluorine, which in turn killed livestock and also killed a quarter of Iceland's population through starvation alone. Death rates in the entire region rose by a consistent 5%, mostly among rural workers. The cloud also killed directly in a manner similar to the London killer fog: not surprising, given that in addition to the fluorine, roughly three times modern Europe's entire annual production of sulphur dioxide had been released.
Two killer cloud incidents within modern history are the Lake Nyros tragedy and the Bhopal industrial disaster.
In 1986, a large cloud of carbon dioxide was released from Lake Nyros, Cameroon. Being heavier than air, it quickly displaced the lighter nitrogen-oxygen mix, instantly killing two thousand people who lived in the area. While the lake will continue to emit carbon dioxide in future, methods of controlling the rate of that emission are still under development. For now, a pump has been installed, but this is not a long-term solution.
Two years before the Lake Nyros gas release, somewhere between 3000 and 15,000 people died in Bhopal, India. Those deaths were traced to the accidental release of 40 tonnes of methyl isocyanate (MIC) gas by a Union Carbide pesticide plant. Being heavier than air, the MIC stayed next to the ground. Over half a million people were exposed to the gas; and it is believed that the health consequences continue to this day. The Bhopal killer cloud is considered one of the worst industrial accidents in history.