A Pacific maritime climate characterized by rainy, mild winters and cool, dry summers controls northwest weather pattens. Winter storms, generated out of the Gulf of Alaska, strike the coast from the southwest and bring warm moist air into the Pacific Northwest. In summer, the North Pacific High dominates the weather pattern, bringing winds over cooler water and striking the region from the northwest. The water vapor in this cool air condenses to form fog in the areas in the summer. The driest weather in the region occurs when high pressure systems develop east of the Cascade Mountains, causing warm, dry air from the interior to blow westward toward the coast.
Mountains make their own weather by forcing incoming air masses upward as they strike the mountains.
As the air rises it cools at an average rate of 3.5 degrees Fahrenheit per 1000 feet, forming clouds and increasing precipitation as the air rises over the mountains. On the leeward side of the mountains the air descends down slope, warms, and increases the amount of water vapor it can hold. Mount Rainier lies west of the Cascade Mountains, only 100 miles east of the Pacific Ocean, and rises to an elevation of 14,410 feet. Mount Rainier makes its own weather. Clouds often collect around the mountain forming a cap on the summit, indicating a change in weather.