Terms Used to Describe Pacific Northwest Weather

Lenticular Clouds Over Mt Rainier at Sunset
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The weather in the Pacific Northwest is influenced by both the large bodies of water and the complex topography of the region. The Pacific Ocean, the Olympic Mountains, Puget Sound, and the Cascade Mountain range all impact local weather conditions. These contributing factors lead to weather conditions that vary significantly from one location to the next; for example, it may be storming in Everett while it is clear and sunny in Tacoma.

Because these influences are unique in the continental United States, newcomers are often confused by the weather terms common to the Pacific Northwest.

Pacific Northwest Weather Terms

Air mass

A large expanse of air having similar temperature and humidity at any given height.

Beaufort scale

A scale of wind strength based on visual assessment of the effects of wind on seas and vegetation.

Chinook

A warm, dry wind on the eastern side of mountains, often resulting in a quick winter thaw.

Cloud base
The lowest portion of a cloud.

Cloud deck
The top of a cloud layer, usually viewed from an aircraft.

Condensation nuclei
Small particles in the atmosphere that serve as the core of tiny condensing cloud droplets. These may be dust, salt, or other material.

Convergence Zone
An atmospheric condition that exists when the winds cause a horizontal net inflow of air into a specified region. In the case of Western Washington, winds in the upper atmosphere are split by the Olympic Mountains, then re-converge over the Puget Sound region. The resulting updrafts can create convection currents, leading to rain showers or stormy conditions.

Cutoff high
Anticyclonic circulation system that separates from the prevailing westerly airflow and therefore remains stationary.

Cutoff low
Cyclonic circulation system that separates from the prevailing westerly airflow and therefore remains stationary.

Deposition nuclei
Tiny particles in the atmosphere that serve as the core of tiny ice crystals as water vapor changes to the solid form. These are also called ice nuclei.

Diffraction
The bending of light around objects, such as cloud and fog droplets, producing fringes of light and dark or colored bands.

Drizzle
Small drops between 0.2 and 0.5 mm in diameter that fall slowly and reduce visibility more than light rain.

Eddy
A small volume of air (or any fluid) that behaves differently from the larger flow in which it exists.

Halos
Rings or arcs that encircle the sun or moon when seen through an ice crystal cloud or a sky filled with falling ice crystals. Halos are produced by refraction of light.

Indian summer
An unseasonably warm spell with clear skies near the middle of autumn. Usually follows a substantial period of cool weather.

Inversion
An increase in air temperature with height.

Land breeze
A coastal breeze that blows from land to sea, usually at night.

Lenticular cloud
A cloud in the shape of a lens. This type of cloud can often be seen forming a cap over Mount Rainier.

Marine climate
A climate dominated by the ocean, because of the moderating effect of water, sites having this climate are considered relatively mild.

Maritime air mass
An air mass that originates over the ocean. These air masses are relatively humid.

Maritime polar air
Cool, humid air mass that forms over the cold ocean waters of the North Pacific and North Atlantic.

Offshore flow (or wind or breeze)
A breeze that blows from the land out over the water. Opposite of an onshore breeze. This condition results in warm, dry weather conditions for Western Washington.

Onshore flow (or wind or breeze)
A breeze that blows from the water onto the land. Opposite of an offshore breeze. Sometimes referred to as a "marine push."

Prevailing wind
The wind direction most frequently observed during a given period.

Radar
An instrument useful for remote sensing of meteorological phenomena. It operates by sending radio waves and monitoring those returned by such reflecting objects as raindrops within clouds.

Rain Shadow
The region on the leeside of a mountain where the precipitation is noticeably less than on the windward side. The occurs on the eastern sides of both the Olympic and the Cascade Mountain Ranges.

Sea breeze
A coastal local wind that blows from the ocean onto the land. The leading edge of the breeze is termed a sea breeze front.

Storm surge
An abnormal rise of the sea along a shore. Primarily due to the winds of a storm over the ocean.

Temperature inversion
An extremely stable air layer in which temperature increases with altitude, the inverse of the usual temperature profile in the troposphere.

Thermal
A small, rising parcel of warm air produced when the earth's surface is heated unevenly.

Upslope fog
Fog formed as moist, stable air flows upward over a topographic barrier.

Visibility
The greatest distance an observer can see and identify prominent objects.

Wind-chill factor
The cooling effect of any combination of temperature and wind, expressed as the loss of body heat. Also called wind-chill index.