This report describes the typical weather at the Hillsboro Airport (Portland, Oregon, United States) weather station over the course of an average year. It is based on the historical records from 1982 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 (62%), croplands (23%), built-up areas (11%), and lakes and rivers (4%).
Over the course of a year, the temperature typically varies from 33Â°F to 82Â°F and is rarely below 24Â°F or above 93Â°F.
The warm season lasts from June 23 to September 20 with an average daily high temperature above 74Â°F. The hottest day of the year is July 29, with an average high of 82Â°F and low of 55Â°F.
The cold season lasts from November 14 to February 23 with an average daily high temperature below 51Â°F. The coldest day of the year is December 27, with an average low of 33Â°F and high of 44Â°F.
The length of the day varies significantly over the course of the year. The shortest day is December 21 with 8:43 hours of daylight; the longest day is June 20 with 15:42 hours of daylight.
The earliest sunrise is at 5:22am on June 17 and the latest sunset is at 9:05pm on June 23. The latest sunrise is at 7:55am on November 3 and the earliest sunset is at 4:28pm on December 9.
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 19% (mostly clear) to 99% (overcast). The sky is cloudiest on December 28 and clearest on August 14. The clearer part of the year begins around June 24. The cloudier part of the year begins around October 4.
On August 14, the clearest day of the year, the sky is clear, mostly clear, or partly cloudy 65% of the time, and overcast or mostly cloudy 32% of the time.
On December 28, the cloudiest day of the year, the sky is overcast, mostly cloudy, or partly cloudy 79% of the time, and clear or mostly clear 19% of the time.
The probability that precipitation will be observed at this location varies throughout the year. Precipitation is most likely around December 11, occurring in 68% of days. Precipitation is least likely around August 4, occurring in 18% 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 67% of those days with precipitation. It is most likely around March 29, when it is observed during 44% of all days.
Moderate rain is the most severe precipitation observed during 20% of those days with precipitation. It is most likely around December 10, when it is observed during 17% of all days.
During the warm season, which lasts from June 23 to September 20, there is a 22% 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 (72% of days with precipitation have at worst light rain), moderate rain (13%), and thunderstorms (8%).
During the cold season, which lasts from November 14 to February 23, there is a 65% 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 (59% of days with precipitation have at worst light rain), moderate rain (23%), and light snow (9%).
The likelihood of snow falling is highest around January 13, occurring in 10% of days.
Either snow rarely accumulates at this location or snow depth measurements are unavailable or unreliable.
The relative humidity typically ranges from 40% (comfortable) to 98% (very humid) over the course of the year, rarely dropping below 25% (dry) and reaching as high as 100% (very humid).
The air is driest around July 25, at which time the relative humidity drops below 49% (comfortable) three days out of four; it is most humid around November 21, exceeding 98% (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 30Â°F (dry) to 58Â°F (comfortable) and is rarely below 20Â°F (dry) or above 63Â°F (mildy humid).
The time of the year between May 13 and October 17 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 13 mph (calm to moderate breeze), rarely exceeding 20 mph (fresh breeze).
The highest average wind speed of 6 mph (light breeze) occurs around March 19, at which time the average daily maximum wind speed is 13 mph (moderate breeze).
The lowest average wind speed of 4 mph (light breeze) occurs around October 15, at which time the average daily maximum wind speed is 10 mph (gentle breeze).