The climate of an area is determined by numerous factors such as temperature, precipitation, fog, and wind; "the sum total of atmospheric conditions that influence plant growth and development." It is also influenced by the area's latitude and longitude, topography, and position relative to large bodies of water. Climates have different levels of influence; there are regional climates, local climates, and microclimates. Regional climates are determined primarily by precipitation, temperature, and other large-scale influential factors, e.g., at the level of San Luis Obispo County.

Local climates are more specifically distinguished by their individual topographies and the resulting differences in slope, exposure, temperature inversions, cold air drainage, groundwater levels, soil development, stream flow, etc. For example, we could compare the local climates of adjacent chaparral and oak woodland communities in Poly Canyon. Another example can be found within a single community where there is variation among sites whose slope angle and slope aspect differ. Slope angle is the inclination of a slope relative to an imaginary vertical line. Slope aspect is the disposition of the slope relative to directional coordinates (north, south, etc.). These two factors combined, along with seasonal variations in the relative positon of the sun, determine the angle of incidence of solar radiation at any given site. In turn, this influences the temperature of the soil as well as the amounts of evaporation by the soil and transpiration by the plants. For example, a slope that faces north receives less incoming sunlight per unit area (insolation) than one facing south. Therefore, the north-facing slope tends to be cooler and moister than the south-facing slope. In terms of the vegetation that occurs across such a boundary, depending on other many factors (soils, prevailing winds, etc.), one may notice a dramatic change in community. For example, an oak woodland may dominate gently inclined north-facing slopes, whereas a grassland prevails on steeper south-facing slopes. On the other hand, the changes in slope aspect and angle may be observed through subtler differences in the expression of a single community. These may involve the overall size and fullness of the plants present and/or the species which predominate. In a coastal scrub community, one may find more black sage (Salvia mellifera) and coyote bush (Baccharis pilularis) on a north-facing slope and more California buckwheat (Eriogonum fasciculatum) on a south-facing slope.

Microclimates are, of course, dependent on regional and local climate regimes, but are determined most specifically by properties of the vegetation and soil, on the time it has taken the ecosystem to develop, and on fire. Within an oak woodland, the microclimate under the umbrella of an oak is decidedly different from that on an exposed rock four feet from the oak's canopy.

San Luis Obispo has a Mediterranean regional climate. The summers are hot and dry. The average high temperature is 77F, although highs occasionally reach the low 100F, with a record of 118F in October 1961. Winters are cool and wet. The average low temperature is 47F. Winter temperatures rarely reach freezing. However, 63% of Januaries between 1933 and 1988 had freezing temperatures. The average frost-free season is 328 days ; occasional frosts occur as early as October 22 and as late as April 21. Extreme fluctuations in temperature are mediated by the proximity of the Pacific Ocean and the occurrence of fogs. Annual precipitation (measured from July 1 to June 30) totals about 22 inches, most of which falls between late October and April. Years with extreme amounts of rainfall include 1884 (49.99 inches) 1969 (48.76 inches) 1983 (49.89 inches) and 1995 (50.37 inches).

Rainfall is important to an area, as is the amount of moisture retained by the soils and the length of time it is available to the plants growing there. Water may not be able to penetrate the soils, flowing as run-off into streams and creeks or first puddling at the surface, then evaporating. Water that does penetrate the soil surface may percolate too deeply to be accessible to plants or it may evaporate. Water is also moved from the soil to the air by the transpiration of plants. Because evaporation and transpiration are closely associated processes, the term "evapotranspiration" is used to refer to them collectively. "'Potential evapotranspiration' is the amount of water that could be lost from a site if moisture were continually available." When trying to determine whether an area has a wet or dry climate, one calculates the area's index of moisture (Im) by comparing its potential evapotranspiration (PE) with its precipitation (P) using the formula Im= (P/PE - 1). Arid places tend to have lower Imvalues (from-100 to -33; PE exceeds P), whereas humid areas have much higher Imvalues (from 20 to over 100; P exceeds PE).

How does this type of climate affect the vegetation? If we examine the dominant plants of several communities we can see characteristics and habits which enable them to tolerate the stresses presented by the Mediterranean climatic regime. Some of these adaptive mechanisms include evergreen leaves that are small, hardened, and grayish; drought-deciduous leaves that fall off during the dry season; photosynthetic rates that are higher in winter or spring when water is most available and temperatures are mild and do not stress plants to excessive evapotanspiration; shallow roots that collect the earliest rains; and deeper tap-roots that take in water at different levels in the soil. These are elaborated in discussions of the individual plant communities.