Seattle's temperate, rainy climate is usually described as temperate Oceanic or Marine west coast, with mild, damp winters and relatively dry and mild summers. Like much of the Pacific Northwest, according to the Köppen climate classification it falls within a cool, dry-summer subtropical zone (Csb), with cool-summer Mediterranean characteristics such as its usually dry summers. Other climate classification systems, such as Trewartha, place it firmly in the Oceanic zone (Do).

Temperature extremes are moderated by adjacent Puget Sound, the greater Pacific Ocean, and Lake Washington. The region is partially protected from Pacific storms by the Olympic Mountains and from Arctic air by the Cascade Range. Despite being on the margin of the rain shadow of the Olympic Mountains, the city has a reputation for frequent rain. This reputation derives from this frequency of precipitation (150 days of precipitation > 0.01 in/0.3 mm) as well as the fact that it is cloudy an average of 201 days and 93 partly cloudy days per year. At 37.1 inches (942 mm), the city receives less precipitation than New York, Atlanta, Houston, and most cities of the Eastern Seaboard of the United States. Seattle was also not listed in a study that revealed the 10 rainiest cities in the continental United States. Most of the precipitation falls as drizzle or light rain. Thunderstorms occur only occasionally. Seattle reports thunder on just seven days per year (according to 'Cities Ranked and Rated' - Bert Sperling and Peter Sander.2007). For comparison Fort Myers, Florida reports thunder on 93 days per year. Kansas City reports 52 'thunder days' and New York City reports 25. There are occasional downpours. One of these downpours occurred in December 2007 when widespread rainfall hit the greater Puget Sound area. It became the second wettest event in Seattle history when a little over 5 inches of rain fell on Seattle in a 24 hour period. The rain also caused five deaths and widespread flooding and damage.[82] Spring, late fall, and winter are filled with days when it does not rain but looks as if it might because of cloudy, overcast skies. Winters are cool and wet with average lows around 35–40 °F (1.7–4.4 °C) on winter nights. Colder weather can occur, but seldom lasts more than a few days. Summers are dry and warm, with average daytime highs around 73–80 °F (22.8–26.7 °C). Hotter weather usually occurs only during a few summer days. Seattle's hottest official recorded temperature was 103 °F (39.4 °C) on July 29, 2009; the coldest recorded temperature was 0 °F (–18 °C) on January 31, 1950.

Eighty miles (130 km) to the west, the Hoh Rain Forest in Olympic National Park on the western flank of the Olympic Mountains receives an annual average rainfall of 142 inches (3,610 mm), and the state capital, Olympia—south of the rain shadow—receives an annual average rainfall of 52 inches (1,320 mm). Snowfall is very infrequent, especially at lower elevations and near the coast, and is usually light and fleeting, lasting only a few days. Heavier snowfall happens infrequently; a recent example happened from December 12–25, 2008, when over one foot of snow fell and stuck on much of the city's roads, causing widespread difficulties in a city so unaccustomed to heavy snow. Average annual snowfall, as measured at Sea-Tac Airport, is 13 inches (33 cm). Seattle's daily record snowfall was 20 inches (51 cm) on January 13, 1950. A sunnier and drier climate typically dominates from mid-July to mid-September. An average of 0.8 inches (20 mm) of rain falls in July and 1.0 inch (25 mm) in August. Although the summer climate is considerably drier and less humid than in areas with humid continental climates, a slight dampness can be occasionally felt, usually when temperatures reach above 80 °F (26.7 °C). This dampness is typically more noticeable during the evening when the temperatures have dropped. Because of this, Seattle experiences occasional summer thunderstorms.

The Puget Sound Convergence Zone is an important feature of Seattle's weather. In the convergence zone, air arriving from the north meets air flowing in from the south. Both streams of air originate over the Pacific Ocean; airflow is split by the Olympic Mountains to Seattle's west, then reunited by the Cascade Mountains to the east. When the air currents meet, they are forced upward, resulting in convection. Thunderstorms caused by this activity can occur north and south of town, but Seattle itself rarely receives worse weather than occasional thunder and ice-pellet showers. The Hanukkah Eve Wind Storm in December 2006 is an exception that brought heavy rain and winds gusting up to 69 mph (111 km/h).

Another exception to Seattle's dampness may occur in El Niño years, when the marine weather systems track as far south as California and little precipitation falls in the Puget Sound area. Since the region's water comes from mountain snowpacks during the drier summer months, El Niño winters can not only produce substandard skiing but can result in water rationing and a shortage of hydroelectric power the following summer

Luciano Mende

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