This report describes the typical weather at the Portland International Jetport (Portland, Maine, 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, Maine has a humid continental climate with warm summers and no dry season. The area within 25 mi of this station is covered by forests (57%), oceans and seas (38%), and lakes and rivers (4%).
The month of May is characterized by rapidly rising daily high temperatures, with daily highs increasing from 58°F to 68°F over the course of the month, exceeding 79°F or dropping below 48°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 1:02 hours, implying an average day-over-day increase of 2.0 minutes.
The shortest day of the month is May 1 with 14:11 hours of daylight; the longest day is May 31 with 15:13 hours of daylight.
The earliest sunrise is at 5:03am on May 30; the latest sunset is at 8:16pm on May 31; the latest sunrise is at 5:34am on May 1; the earliest sunset is at 7:44pm 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 is 72% (partly cloudy) and does not vary substantially over the course of the month.
On May 16, a representative day, the sky is clear or mostly clear 37% of the time, partly cloudy 15% of the time, and mostly cloudy or overcast 48% of the time.
The average probability that some form of precipitation will be observed in a given day is 49%, with little variation over the course of the month.
Throughout May, the most common forms of precipitation are light rain, moderate rain, and thunderstorms.
Light rain is the most severe precipitation observed during 47% of those days with precipitation. It is most likely around May 21, when it is observed during 24% of all days.
Moderate rain is the most severe precipitation observed during 25% of those days with precipitation. It is most likely around May 8, when it is observed during 12% of all days.
Thunderstorms are the most severe precipitation observed during 12% of those days with precipitation. They are most likely around May 31, when it is observed during 8% of all days.
The likelihood of snow falling is highest around May 1, occurring in 5% of days.
Either snow rarely accumulates at this location in May or snow depth measurements are unavailable or unreliable.
The relative humidity typically ranges from 47% (comfortable) to 93% (very humid) over the course of a typical May, rarely dropping below 24% (dry) and reaching as high as 100% (very humid).
The air is driest around May 1, at which time the relative humidity drops below 62% (mildly humid) three days out of four; it is most humid around May 28, rising above 90% (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 31°F (dry) to 54°F (very comfortable) and is rarely below 20°F (dry) or above 63°F (mildy humid).
Over the course of May typical wind speeds vary from 0 mph to 17 mph (calm to moderate breeze), rarely exceeding 23 mph (fresh breeze).
The highest average wind speed of 9 mph (gentle breeze) occurs around May 3, at which time the average daily maximum wind speed is 17 mph (moderate breeze).
The lowest average wind speed of 8 mph (gentle breeze) occurs around May 30, at which time the average daily maximum wind speed is 15 mph (moderate breeze).