This report describes the typical weather at the Portland International Airport (Portland, Oregon, United States) weather station over the course of an average May. 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%).
The month of May is characterized by rising daily high temperatures, with daily highs increasing from 64°F to 70°F over the course of the month, exceeding 82°F or dropping below 55°F only one day in ten.
Over the course of May, the length of the day is increasing. From the start to the end of the month, the length of the day increases by 1:06 hours, implying an average day-over-day increase of 2.1 minutes.
The shortest day of the month is May 1 with 14:21 hours of daylight; the longest day is May 31 with 15:27 hours of daylight.
The earliest sunrise is at 5:25am on May 30; the latest sunset is at 8:52pm on May 31; the latest sunrise is at 5:57am on May 1; the earliest sunset is at 8:18pm on May 1.
Daylight savings time is observed in 2012 but it neither starts nor ends in May, so the entire month is in daylight time.
The median cloud cover ranges from 85% (mostly cloudy) to 90% (mostly cloudy). The trend is for the sky to be clearing with the median cloud cover decreasing by 6% from the start to the end of the month.
On May 16, a representative day, the sky is clear or mostly clear 21% of the time, partly cloudy 17% of the time, and mostly cloudy or overcast 61% of the time.
The probability that precipitation will be observed at this location varies throughout the month. Precipitation is most likely around May 1, occurring in 57% of days. Precipitation is least likely around May 31, occurring in 47% of days.
Throughout May, the most common forms of precipitation are light rain and moderate rain.
Light rain is the most severe precipitation observed during 50% of those days with precipitation. It is most likely around May 1, when it is observed during 28% of all days.
Moderate rain is the most severe precipitation observed during 36% of those days with precipitation. It is most likely around May 1, when it is observed during 22% of all days.
Either snow is exceptionally unlikely to fall at this location in May or this station does not reliably report precipitation types.
The relative humidity typically ranges from 46% (comfortable) to 90% (very humid) over the course of a typical May, rarely dropping below 29% (dry) and reaching as high as 98% (very humid).
The air is driest around May 15, at which time the relative humidity drops below 56% (mildly humid) three days out of four; it is most humid around May 1, rising above 87% (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 typical May, the dew point typically varies from 39°F (dry) to 53°F (very comfortable) and is rarely below 33°F (dry) or above 58°F (comfortable).
Over the course of May typical wind speeds vary from 0 mph to 14 mph (calm to moderate breeze), rarely exceeding 20 mph (fresh breeze).
The highest average wind speed of 7 mph (light breeze) occurs around May 1, at which time the average daily maximum wind speed is 14 mph (moderate breeze).
The lowest average wind speed of 7 mph (light breeze) occurs around May 15, at which time the average daily maximum wind speed is 13 mph (moderate breeze).