Weather and Climate Inventory, Klamath Network, National Park Service, 2007
2.0. Climate Background
2.1. Climate and the KLMN Environment
Topography and coastal influences are instrumental in defining the climate of the KLMN (Odion et al. 2005). The Klamath-Siskiyou subregion contains REDW, ORCA, and WHIS. This subregion is characterized by extremely rugged topography. Topography works along with the proximity of the subregion to the Pacific Ocean to create exceptionally severe climatic gradients. The climate of the Klamath-Siskiyou subregion is typified by cool, wet winters and warm, dry summers. In winter, a strong low pressure area generally sets up in the Gulf of Alaska, with a relatively weak subtropical Pacific high-pressure system south of that. As a result, the prevailing westerlies that are common in temperate zones strengthen and move southward during the winter months, increasing the numbers of cyclonic storms (Bryson and Hare 1974; Miller 2002). These winter storms are responsible for the majority of the subregion’s annual precipitation. Topography also affects the distribution of precipitation during the winter months, with precipitation generally decreasing in the subregion from windward (west-facing) slopes and higher elevations to leeward (east facing) slopes and lower elevation areas (Miller 2002). Precipitation also tends to decrease from west to east across the Klamath-Siskiyou subregion. Despite deep, late-lying snowpacks, winters at high elevations in the Klamath-Siskiyou subregion are relatively mild and the ground rarely freezes.
In summer, the subtropical Pacific high-pressure system strengthens and the prevailing westerlies weaken and move northward. These shifts create dry conditions in the Klamath-Siskiyou subregion (Bryson and Hare 1974). When summer precipitation occurs, it usually comes in association with weak frontal disturbances and occasional thundershowers, especially at higher elevations. Lightning associated with thunderstorms commonly ignites fires in late summer and fall.
Although the Klamath-Siskiyou subregion is strongly moderated by the Pacific Ocean throughout the year, coastal influences are especially marked in summer. From June to September, warm, moist Pacific air is advected eastward by prevailing winds across the cold, upwelling coastal waters of the California current, creating a layer of moist and relatively cool air along the coast (Miller 2002). This moist, cool air is overlain by warmer, drier air, making this moist, marine layer relatively stable. The coastal mountains add to this stability by blocking the moist air from moving inland (Mitchell 1976), although occasionally a “marine push” can develop that will move cool, moist air from the Pacific Ocean over the Coast and Cascade mountain ranges into the interior (Mock 1996). The frequency and length of time a given site is under the influence of this maritime air plays a major role in the ecology of the Klamath-Siskiyou subregion. Maritime stratus and fogs decrease the amount of solar radiation that reaches the ground, lowering maximum temperatures and increasing the humidity during the otherwise dry summers. All these factors differentiate the maritime-influenced western slopes of the Klamath-Siskiyou subregion from the drier eastern slopes (Waring 1969). Coastal slopes and valleys that are favorably oriented to northwest summer winds are bathed in summer fogs and fog drip, vital sources of moisture for redwood trees (Burgess and Dawson 2004). The marine air masses effectively delimit the landward extent of the redwood biome.