Portland, Oregon, has a long-standing reputation for its gray skies, a perception that often overshadows its many other qualities. This image of a perpetually overcast city leads many to wonder if the frequent cloud cover is simply local legend or a verifiable climatic reality. To understand the truth behind Portland’s cloudiness, it is necessary to examine specific meteorological data. This analysis provides the objective context for the city’s annual cloud patterns, their seasonal distribution, and how Portland compares to other major U.S. metropolitan areas.
Defining Cloud Cover for Meteorological Data
The classification of a day as “cloudy” is not based on a general feeling but on a specific measurement system used by meteorological organizations like the National Weather Service. Scientists use a unit called the okta, which divides the sky into eight equal parts to quantify the amount of cloud cover. One okta represents one-eighth of the sky covered by clouds, with the scale ranging from zero oktas for a completely clear sky to eight oktas for a fully overcast sky.
A day is officially classified as a “heavy cloud” day when cloud cover is consistently measured at six, seven, or eight oktas, meaning at least 75% of the sky is obscured. For annual climate statistics, a day of heavy cloud is typically defined as one where clouds cover 80 percent or more of the sky during daylight hours. This precise definition ensures that reported annual averages are based on a uniform, objective standard.
The Annual Average Number of Cloudy Days
Based on long-term climate averages, Portland, Oregon, experiences approximately 222 days each year that are classified as having “heavy cloud” cover. This figure means that on roughly 61 percent of the days, more than three-quarters of the sky is covered by clouds. This count is derived from decades of climate data collection, establishing a clear pattern of frequent overcast conditions.
This annual count confirms the widespread perception of Portland as a city with predominantly gray skies. The 222 days represents the number of days when the sky is effectively blanketed by clouds, minimizing direct sunlight.
Seasonal Distribution of Cloud Cover in Portland
Portland’s cloudiness is not evenly distributed throughout the year but is instead heavily concentrated in the cooler months, defining a distinct seasonal pattern. The period from November through February is the cloudiest time, with the winter months averaging a high number of heavy cloud days. Specifically, December and January often vie for the title of cloudiest month, with sky cover being overcast or mostly cloudy roughly 75% of the time.
During the three winter months of December, January, and February, Portland averages about 71 days of heavy cloud cover. This dominance is due to the city’s location in the Pacific Northwest, where prevailing westerly winds bring mild, moist air from the Pacific Ocean inland. As this air encounters the cooler landmass and the Coast Range, it consistently generates low-lying clouds and frequent light precipitation.
The pattern shifts dramatically in the summer, which is characterized by a high-pressure system forming over the Pacific. This system diverts stormy weather to the north, allowing for a much drier and clearer climate in Portland. August is the clearest month, with the sky classified as clear, mostly clear, or partly cloudy about 77% of the time.
How Portland Compares to Other Major U.S. Cities
Portland’s annual average of 222 heavy cloud days places it among the cloudiest major metropolitan areas in the United States. This position is comparable to that of its regional neighbor, Seattle, Washington, which records an average of 226 days of heavy cloud cover annually. The similar coastal geography and prevailing weather systems cause both cities to share a climate characterized by frequent overcast conditions.
The city’s cloudiness is slightly higher than other major U.S. cities often associated with frequent cloud cover, such as Buffalo, New York, which averages 208 heavy cloud days. In contrast, cities in the Midwest and East Coast generally experience fewer days of heavy cloud, with Chicago, Illinois, averaging around 176 days. The difference becomes profound when comparing Portland to famously sunny locations, such as Phoenix, Arizona, which averages only about 70 days of heavy cloud per year.
Portland consistently ranks near the top of U.S. cities for the highest number of heavy cloud days. While other cities may experience a higher number of days with some cloud cover—including partly cloudy days—Portland’s ranking is based on the frequency of skies that are almost entirely obscured.