Climate and Weather

Climate describes the meteorological conditions, including temperature, rainfall, and wind, that prevail in a particular region. Weather, which is often confused with climate, describes the meteorological conditions at a specific point in time.1

The climate in the northern Puget Sound is maritime, which is characterized by a relatively moderate seasonal range of temperatures, high humidity, and ample rain and snow. The sound and the Pacific Ocean minimize large extremes in the air temperature, resulting in mild winters and cool summers. Over the year, the mean Bellingham temperatures only differ by about 30 degrees Fahrenheit, from the 70s in summer to the 30s in winter (National Oceanic and Atmospheric Administration, Climate Diagnostic Center 1998). Daily weather, however, can range from sunny and clear to hailing and rainy within the same afternoon. Most of the rain falls during late summer and fall. The majority of snow falls from November to March, and it frequently rains on warm winter days. Despite its maritime climate, the watershed does not receive as much precipitation as the nearby Olympic Peninsula. Both the high peaks of Vancouver Island and the Olympic Peninsula form the outer west coast. Steep fjords trap moisture-laden clouds from the Gulf of Alaska, preventing much rain and snow from reaching northern Puget Sound.1

The Big Picture: Regional Climate

Both continental and maritime climate systems influence the region. The Canadian Cascade Range to the north of the Strait of Georgia protects northern Puget Sound from the severe, continental, arctic cold fronts that come from the interior of British Columbia. However, cold continental air masses do come south through the Fraser River valley, and bring occasional bitter cold weather to the sound during winter months.1

The Aleutian Low, a semi-permanent low-pressure system over the Pacific Ocean, has a strong effect on the climate in the sound. As this low-pressure area moves and changes in intensity, it brings storms with wind, rain, and snow.1

The climate of northern Puget Sound varies considerably from that of nearby Vancouver Island or the Olympic Peninsula. Vancouver Island and the Olympic Peninsula form the outer coast and have a pronounced maritime climate, so they receive more rain and snow. The strong rain-shadow effect on the San Juan Islands is evident when comparison is made between the 130 inches of annual average rainfall in Quillaute on the Olympic Peninsula and the 29 inches received in Olga on Orcas Island in the San Juans.2

Northern Puget Sound Climatology and Data

The Bellingham airport in Whatcom County is centrally located in northern Puget Sound and has long-term climate data. Weather conditions can vary considerably in just the local Whatcom County area. Based on records from 1949 to 2000, the mean-daytime temperature is in the low 60s in the summer and in the low 30s in the winter. Annually, Bellingham receives an average of 35 inches of rain, and almost 14 inches of snow. Average maximum and minimum temperatures show the limited differences between the extremes experienced within a month and illustrate the small variability in northern Puget Sound temperatures.3 The annual average wind speed at the Bellingham Airport is 3.3 meters per second.4

Northern Puget Sound Climate

Graph representing 30 year (1971-2000) daily/monthly station climate normals for Bellingham, Washington [Data obtained from the National Oceanic and Atmospheric Administration's National Climatic Data Center]

Microclimates in Northern Puget Sound

Within northern Puget Sound watersheds, elevation, prevailing winds, glacier proximity, sheltering topography, and aspect (slope orientation) determine the microclimate of a given area. Vegetation growth and plant community composition can reveal helpful information about the microclimate of a given site, especially in the spring when growth stages are most apparent. As in most temperate regions, the higher elevations are colder than those at sea level. Northern locations do not receive as much direct sun, so they are colder and retain snow longer than those with southern exposure. As the winter sun moves low in the sky, some northern aspect sites may not receive much direct sunlight. The resulting shade and cooler temperatures contribute to the deep, persistent snowpack found on the higher elevations of the North Cascades National Park.1

Marine Weather1

Just as the ocean plays a major role in controlling the climate of northern Puget Sound, marine weather dictates whether conditions are safe for boating, fishing, or conducting marine research. Those planning to do research or to enjoy recreation should be aware of the swiftly changing conditions, including the day-breeze and the large swift-tides that can amplify waves. With winter surface water temperatures as low as 28 degrees Fahrenheit, and average maximum summer temperatures at a chilly 55 degrees Fahrenheit, capsizing can be fatal.

Extreme Conditions, Winds, and Waves: Strong southerly and Gulf of Alaska winds work with Strait of Georgia and Strait of Juan de Fuca tides to shape the marine weather in northern Puget Sound. With a range of nearly 14 feet, the tides combine with the winds to make life on the water difficult at times. Freezing spray, high seas, and rough chop from winds and tides commonly prompt small craft advisories. Ten-foot seas are not uncommon in the straits during winter and these treacherous conditions often force boats to seek refuge in the more sheltered waters of the inner San Juan Islands.

Why do the storms blow with such force in the straits? The North Pacific is one of the most meteorologically active areas on earth and feeds storms to the Washington coast through the Straits of Georgia and Juan de Fuca.

Northerly winds also come from continental areas. Consequently, they are colder in winter and warmer in summer than the winds blowing from marine zones.

Weather Phenomena: Fog and the Day Breeze: Two types of fog occur in the bay. Radiational fog forms at night and burns off at midday; dense advection fog moves into the bay from elsewhere and obliterates visibility. Of the two, advection fog is the more dangerous. It can move quickly, engulfing all boats in its path in a palpable shroud. Visibility can be so reduced that it is unsafe to travel by boats that are not equipped with radar.

A common but dangerous phenomenon is the "sea" or "day" breeze. This breeze typically occurs midday when air is blown across the water to the land in all directions. This onshore wind results from the unequal warming of air over land and water. Air over land warms more quickly than that over water. As air over the land warms, it moves upward. As it ascends, the cooler air over the water moves shoreward to fill the "empty" place left by the rising warm air. The greater the difference in the air temperature over land and sea, the faster the day breeze moves toward shore. The wind begins to diminish rapidly at sunset. Because of the sea breeze, the calmest time of the day on the bay is generally early in the morning and late in the evening.

References Cited

1. Adapted from Kachemak Bay Ecological Characterization. Kachemak Bay Research Reserve. Copyright ©2000 Alaska Department of Fish and Game. <http://www.csc.noaa.gov/lcr/kachemak/html/ecosys/physical/climate.htm>

2. Western Regional Climate Center. Western U.S. Historical Summaries by State.
<http://www.wrcc.dri.edu/COMPARATIVE.html>

3. National Oceanic and Atmospheric Administration. Climate Diagnostic Center. <http://www.cdc.noaa.gov>

4. National Renewable Energy Laboratory. Wind Energy Resource Atlas of the United States (Appendix C Table C-1). <http://rredc.nrel.gov/wind/pubs/atlas/tables/tablec1/wa.html>

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