This report describes the typical weather at the Portland International Airport (Portland, Oregon, United States) weather station over the course of an average October. 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 October is characterized by rapidly falling daily high temperatures, with daily highs decreasing from 70°F to 58°F over the course of the month, exceeding 81°F or dropping below 51°F only one day in ten.
Over the course of October, the length of the day is rapidly decreasing. From the start to the end of the month, the length of the day decreases by 1:33 hours, implying an average day-over-day decrease of 3.0 minutes.
The shortest day of the month is October 31 with 10:07 hours of daylight; the longest day is October 1 with 11:40 hours of daylight.
The earliest sunrise is at 7:10am on October 1; the latest sunset is at 6:50pm on October 1; the latest sunrise is at 7:50am on October 31; the earliest sunset is at 5:58pm on October 31.
Daylight savings time is observed in 2012 but it neither starts nor ends in October, so the entire month is in daylight time.
The median cloud cover ranges from 68% (partly cloudy) to 91% (mostly cloudy). The trend is for the sky to be becoming rapidly cloudier with the median cloud cover increasing by 22% from the start to the end of the month.
On October 1, the clearest day of the month, the sky is clear, mostly clear, or partly cloudy 51% of the time, and overcast or mostly cloudy 50% of the time.
On October 31, the cloudiest day of the month, the sky is overcast, mostly cloudy, or partly cloudy 78% of the time, and clear or mostly clear 21% of the time.
The probability that precipitation will be observed at this location varies throughout the month. Precipitation is most likely around October 31, occurring in 58% of days. Precipitation is least likely around October 1, occurring in 40% of days.
Throughout October, 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 October 31, when it is observed during 25% of all days.
Moderate rain is the most severe precipitation observed during 40% of those days with precipitation. It is most likely around October 31, when it is observed during 25% of all days.
Either snow is exceptionally unlikely to fall at this location in October or this station does not reliably report precipitation types.
The relative humidity typically ranges from 47% (comfortable) to 95% (very humid) over the course of a typical October, rarely dropping below 25% (dry) and reaching as high as 100% (very humid).
The air is driest around October 1, at which time the relative humidity drops below 58% (mildly humid) three days out of four; it is most humid around October 19, rising above 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 typical October, the dew point typically varies from 40°F (dry) to 54°F (very comfortable) and is rarely below 31°F (dry) or above 59°F (comfortable).
Over the course of October typical wind speeds vary from 0 mph to 13 mph (calm to moderate breeze), rarely exceeding 21 mph (fresh breeze).
The highest average wind speed of 7 mph (light breeze) occurs around October 31, at which time the average daily maximum wind speed is 13 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).