condition type that shows "...natural (historical) range of variability of vegetation characteristics; fuel composition; fire frequency, severity and pattern; and other associated disturbances." This condition type, in keeping with the wilderness designation applied to most of the park, equates to a natural "let it burn" policy overall, so long as people and historical property are not threatened and the fire was lightning-caused. As part of the management plan, the few historic structures in the FMU are prioritized for fire protection including backcountry shelters like Beaver Pass Shelter, and fire lookouts such as Sourdough, Desolation and Copper lookout, all of which are on the National Register of Historic Places. Climate The Cascade Range is tall enough to cause a significant rain shadow on the eastern slopes. Annually, the western portions of the park receive more precipitation than the eastern sections. The higher precipitation in the west also correlates to increased snowfall with more measured in the west than the east annually. On the western slopes, snowfall depths range from in lower altitudes annually, increasing to at elevations between above sea level. Snow depths peak in early March and range from depending on altitude. Snow covers the ground more than six months out of the year, even at lower elevations, and State Route 20, the only highway through the park, is generally closed from late November until late April. Numerous avalanche chutes, as many as traverse any state or federal highway, and including some over long, cross the highway and make snow removal during that period infeasible. Heavy snow and frequent avalanches are common, especially on the western slopes, from autumn to spring. The high elevation trails are generally open by mid-summer and the majority of tourism is between mid-June and late September. The east side of the park is generally warmer and drier, especially in the summer with highs reaching . Since the 1950s, there has been a five-degree Fahrenheit (2.77°C) mean winter minimum temperature increase at elevations above . This has led to a reduced winter snowpack as the mean winter freezing level is now higher. Overall the relative proximity of the Pacific Ocean moderates temperatures in the park, and it is warmer than other regions at a similar latitude farther inland. Air and water quality While North Cascades National Park is in a remote region for the most part, the prevailing westerly winds bring various pollutants into the park