Although many fires were caused by human activities, most ignitions were undoubtedly natural. The term 'natural ignition' is nearly synonymous with a lightning strike. Afternoon thunderstorms occur frequently in the Northern Rockies but release little precipitation, a condition known as dry lightning. In a typical season there are thousands of lightning strikes in Yellowstone. However most lightning strikes never result in a wildfire because fuels are not in a combustible state.
Fuels are typically wood, foliage or grass.
Fuels may be fine, such as twigs and needles, or heavy, such as logs, branches and whole trees that have blown down in a storm. Fuels may also be living such as an understory layer of tree regeneration or a layer of shrubs. In order for an ignition to take place, the fuels must be dry. On average in Yellowstone, fuels dry out enough to ignite the first wildland fire of the year about the middle of June. Young forests generally do not have the fuel loads that mature and old-growth forests do.