The general or macroclimate of the study region is characterized by rapid daily and day-to-day temperature fluctuations, erratic rainfall patterns, generally low humidity, plentiful sunshine, nearly continuous air movement, and a relatively short frost-free period. Plant growth and productivity also depend on microclimate factors that are affected by landform, such as exposure, slope angle, length, and aspect, as well as soil color, texture, and surface and vegetation. The most critical plant-growth factor is available water, which is determined by water input (precipitation) and water loss (evapotranspiration). Spring and fall precipitation is generally gentle and soil-soaking, but summer rains frequently come in the form of intensive, soil-washing, localized convective thunderstorms of short duration. The water loss due to runoff and consequent soil erosion are major problems resulting from such "cloudbursts." Generally clear skies over the region facilitate receipt of abundant sunshine, which, coupled with long days and nearly continuous wind causes evapotranspiration to exceed precipitation by an average of 6 inches between June and September. The water deficit is most severe on southern exposures, which receive the most direct solar radiation.