Ecologists have long considered the prairie to be a unique grassland. The area had a humid temperate climate favorable for the growth of deciduous forests. Elsewhere in the world, areas with similar climates are forested, and grasslands normally develop in more arid regions.
An analysis of various factors led to the following conclusions about the key climatic conditions favoring the prairie. The northern boundary is established by high thunderstorm and frequent severe drought conditions to the south of the boundary and low thunderstorm and less frequent severe drought conditions to the north of the northern boundary. As a result, lightning-induced fires are favored to the south of the boundary. The southern boundary is established by the frequency of cold seasons (November-March) with less than 38 cm of precipitation with a high frequency to the north and low frequency to the south of the southern boundary. The western boundary is established by the frequency of warm seasons (May-October) with a precipitation to potential evapotranspiration ratio of greater than 0.75 with a high frequency to the east and a low frequency to the west of the western boundary.