How is coal dust explosion severity measured?

The severity of a coal dust explosion is primarily measured by the ability of the coal dust to be suspended in the air, forming a cloud that can then be ignited. When coal dust becomes airborne, it can mix with oxygen and create a combustible mixture. If this mixture is ignited, it can lead to an explosion, and the severity of that explosion is largely influenced by how well the dust is dispersed in the air. A cloud of fine coal dust will explode more violently than larger clumps that do not suspend effectively.

Factors such as temperature, particle size, and moisture content can influence the behavior of coal dust, but they are not direct measures of explosion severity. For instance, while smaller particle sizes can increase surface area and potentially enhance explosiveness, it's the ability to form and ignite a cloud that ultimately determines the severity of the explosion. Therefore, measuring explosion severity centers around the conditions that allow coal dust to become airborne and ignitable, which is encapsulated in the third option.