This report describes the typical weather at the Portland International Airport (Portland, Oregon, United States) weather station over the course of an average year. It is based on the historical records from 1974 to 2012. Earlier records are either unavailable or unreliable.
Portland, Oregon has a mediterranean climate with dry warm summers and mild winters. The area within 25 mi of this station is covered by forests (69%), croplands (15%), built-up areas (11%), and lakes and rivers (4%).
Over the course of a year, the temperature typically varies from 35°F to 82°F and is rarely below 25°F or above 93°F.
The warm season lasts from June 24 to September 20 with an average daily high temperature above 74°F. The hottest day of the year is July 30, with an average high of 82°F and low of 59°F.
The cold season lasts from November 16 to February 23 with an average daily high temperature below 52°F. The coldest day of the year is December 29, with an average low of 35°F and high of 44°F.
The length of the day varies significantly over the course of the year. The shortest day is December 21 with 8:40 hours of daylight; the longest day is June 20 with 15:42 hours of daylight.
The earliest sunrise is at 5:20am on June 16 and the latest sunset is at 9:04pm on June 24. The latest sunrise is at 7:54am on November 3 and the earliest sunset is at 4:26pm on December 12.
Daylight savings time (DST) is observed in this location during 2012, starting in the spring on March 11 and ending in the fall on November 4.
The median cloud cover ranges from 45% (partly cloudy) to 99% (overcast). The sky is cloudiest on January 1 and clearest on August 14. The clearer part of the year begins around June 25. The cloudier part of the year begins around October 7.
On August 14, the clearest day of the year, the sky is clear, mostly clear, or partly cloudy 64% of the time, and overcast or mostly cloudy 36% of the time.
On January 1, the cloudiest day of the year, the sky is overcast, mostly cloudy, or partly cloudy 84% of the time, and clear or mostly clear 15% of the time.
The probability that precipitation will be observed at this location varies throughout the year. Precipitation is most likely around December 15, occurring in 71% of days. Precipitation is least likely around August 4, occurring in 21% of days.
Over the entire year, the most common forms of precipitation are light rain and moderate rain.
Light rain is the most severe precipitation observed during 46% of those days with precipitation. It is most likely around March 11, when it is observed during 32% of all days.
Moderate rain is the most severe precipitation observed during 38% of those days with precipitation. It is most likely around November 30, when it is observed during 31% of all days.
During the warm season, which lasts from June 24 to September 20, there is a 26% average chance that precipitation will be observed at some point during a given day. When precipitation does occur it is most often in the form of light rain (50% of days with precipitation have at worst light rain), moderate rain (28%), thunderstorms (10%), and drizzle (7%).
During the cold season, which lasts from November 16 to February 23, there is a 68% average chance that precipitation will be observed at some point during a given day. When precipitation does occur it is most often in the form of light rain (40% of days with precipitation have at worst light rain), moderate rain (40%), and light snow (7%).
The likelihood of snow falling is highest around January 16, occurring in 9% of days.
During peak snow season, accumulation at this location on a given day is very unlikely. The chances of there being snow on the ground are highest around December 31, occurring 7% of the time.
The snow is typically at its deepest on November 13, with a median depth of 2.2"; the depth exceeds 2.7" only one year out of ten.
The relative humidity typically ranges from 40% (comfortable) to 95% (very humid) over the course of the year, rarely dropping below 24% (dry) and reaching as high as 100% (very humid).
The air is driest around July 30, at which time the relative humidity drops below 46% (comfortable) three days out of four; it is most humid around October 19, exceeding 93% (very humid) three days out of four.
Dew point is often a better measure of how comfortable a person will find the weather than relative humidity because it more directly relates to whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid.
Over the course of a year, the dew point typically varies from 30°F (dry) to 58°F (comfortable) and is rarely below 18°F (dry) or above 63°F (mildy humid).
The time of the year between May 15 and October 21 is the most comfortable, with dew points that are neither too dry nor too muggy.
Over the course of the year typical wind speeds vary from 0 mph to 17 mph (calm to moderate breeze), rarely exceeding 26 mph (strong breeze).
The highest average wind speed of 10 mph (gentle breeze) occurs around December 30, at which time the average daily maximum wind speed is 17 mph (moderate breeze).
The lowest average wind speed of 6 mph (light breeze) occurs around October 7, at which time the average daily maximum wind speed is 12 mph (gentle breeze).