mild winters, and warm, dry summer seasons. The area typically does not experience frost, with more than 200 frost-free days annually. Temperature can vary widely, reaching a historic maximum of , though the usual July maximum is below , and the average minimum for January is above . Yearly, precipitation averages between in most river valleys, with a mean of from 1871 through 1952. It shows variability, however, with a historic low of at Portland in 1929 and a maximum of in 1882. More than 75 percent of this precipitation occurs between October and March; July and August mark the driest months with means below , while November, December, and January represent the wettest with averages greater than . Prevailing winds originate from the south during winter and from the northwest during the summer season, with the exception of prevailing winds at the mouth of the Columbia River Gorge, where winds predominantly move to the east. The southern winds have the highest velocities of the three, only rarely occurring with potentially destructive force. Geology There are a number of volcanic centers within a radius of Troutdale and more than 32 vents within a radius of Kelly Butte. Mostly small cinder cone vents, these volcanoes also include some larger lava domes from shield volcanoes at Mount Sylvania, Highland Butte, and Larch Mountain. Known as the Boring Lava Field, this cluster includes more than 80 known small vents and associated lava flows, with more volcanic deposits likely present. The Global Volcanism Program reports that the field includes somewhere between 32 and 50 shield volcanoes and cinder cones, with many vents concentrated northwest of the town of Boring. Considered an outlier of the Cascade Range, the Boring Lava Field lies about to the west of the major Cascade crest. It marks one of five volcanic fields along the Quaternary Cascade arc, along with Indian Heaven, Tumalo in Oregon, the Mount Bachelor chain, and Caribou in California. Like the Cascade Range, the Boring field was also generated by the subduction of the oceanic Juan de Fuca tectonic plate under the North American tectonic plate, but it has a different tectonic position, with its eruptive activity more likely related to tectonic rifting throughout the region. The Boring Lava Field has erupted material derived from hot mantle magma, and the subducting Juan de Fuca plate may be as shallow as in depth at their location. Many