What is a Pyrocumulus Cloud? (Photo Credit: World Meteorological Organization)

Weather patterns in the summer months, particularly throughout western portions of the United States, provide hot and dry conditions ideal for the growth of wildfires. These fires destroy large areas of vegetation, damage property and cause significant human health problems. In addition wildfires, along with volcanic eruptions, can contribute to the formation of what are known as pyrocumulus clouds. These clouds can cause a number of issues that help contribute to the negative social impact of fire weather events.

Most are familiar with the large, white, puffy cumulus clouds commonly associated with calm weather days in the summertime. One of the most basic cloud forms, general cumulus clouds form as warm air at the Earth’s surface rises, cools and eventually condenses on particles called cloud condensation nuclei (or CCN). Condensation is simply the transformation of a substance from a gas to liquid state but, in the case of clouds, water vapor gas condenses into liquid water droplets. In the atmosphere, this process creates the characteristic puffy, cotton ball like form of fair weather cumulus clouds. Pyrocumulus clouds thus form as a result of intense heating at the surface, most commonly associated with wildfires or volcanic eruptions. 

The term “pyrocumulus” is Latin and translates literally to “fire cloud”, an incredibly accurate description. Since hot air is less dense than cold air, relatively warm air will rise upward while colder air sinks to the surface. In addition to heat, fires also produce large amounts of water vapor and carbon dioxide as byproducts of a chemical reaction known as oxidation. Therefore the upward motion of hot air at the ground transports water vapor and, in the case of wildfires and volcanic eruptions, smoke and ash to higher levels in the atmosphere. Smoke, ash and other similar particles can serve as CCN onto which water vapor may condense, eventually creating a pyrocumulus cloud. While pyrocumulus clouds share the puffy, cotton ball like appearance of fair weather cumulus clouds, the incorporation of smoke and ash turn pyrocumulus a dark gray color. Often times, this makes it difficult to distinguish between a pyrocumulus cloud and general smoke and ash produced by a burning fire or recent volcanic eruption. 

While seemingly harmless, pyrocumulus clouds can and do cause a number of problems. In some cases, these fire clouds can develop into pyrocumulonimbus, essentially thunderstorms capable of producing copious amounts of lightning. When lightning occurs away from precipitation (known as dry lightning) and in an already fire prone area, even more wildfires are likely to be produced. Additionally, these storms can be associated with strong downdrafts of cold air that spread out as they reach the surface. This outward movement of cold air at the surface can cause the wildfire to spread and make it even more difficult to keep under control. Despite the calm nature of regular cumulus clouds, pyrocumulus can create potentially hazardous conditions in areas already under the dangerous influence of fire weather. 

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©2019 Weather Forecaster Dennis Weaver