This report describes the typical weather at the Philadelphia International Airport (Philadelphia, Pennsylvania, 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.
Philadelphia, Pennsylvania has a warm humid temperate climate with hot summers and no dry season. The area within 25 mi of this station is covered by croplands (32%), built-up areas (30%), forests (29%), grasslands (6%), and lakes and rivers (3%).
Over the course of a year, the temperature typically varies from 25°F to 87°F and is rarely below 13°F or above 94°F.
The warm season lasts from May 29 to September 16 with an average daily high temperature above 77°F. The hottest day of the year is July 25, with an average high of 87°F and low of 71°F.
The cold season lasts from December 3 to March 5 with an average daily high temperature below 48°F. The coldest day of the year is January 19, with an average low of 25°F and high of 39°F.
The length of the day varies significantly over the course of the year. The shortest day is December 21 with 9:20 hours of daylight; the longest day is June 20 with 15:00 hours of daylight.
The earliest sunrise is at 5:32am on June 15 and the latest sunset is at 8:34pm on June 26. The latest sunrise is at 7:32am on November 3 and the earliest sunset is at 4:35pm on December 7.
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 60% (partly cloudy) to 80% (mostly cloudy). The sky is cloudiest on December 27 and clearest on October 1. The clearer part of the year begins around June 17. The cloudier part of the year begins around November 13.
On October 1, the clearest day of the year, the sky is clear, mostly clear, or partly cloudy 58% of the time, and overcast or mostly cloudy 42% of the time.
On December 27, the cloudiest day of the year, the sky is overcast, mostly cloudy, or partly cloudy 68% of the time, and clear or mostly clear 31% of the time.
The probability that precipitation will be observed at this location varies throughout the year. Precipitation is most likely around April 28, occurring in 48% of days. Precipitation is least likely around September 13, occurring in 37% of days.
Over the entire year, the most common forms of precipitation are light rain, moderate rain, and thunderstorms.
Light rain is the most severe precipitation observed during 34% of those days with precipitation. It is most likely around April 26, when it is observed during 20% of all days.
Moderate rain is the most severe precipitation observed during 26% of those days with precipitation. It is most likely around April 20, when it is observed during 14% of all days.
Thunderstorms are the most severe precipitation observed during 16% of those days with precipitation. They are most likely around July 6, when it is observed during 18% of all days.
During the warm season, which lasts from May 29 to September 16, there is a 41% 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 thunderstorms (35% of days with precipitation have at worst thunderstorms), light rain (30%), moderate rain (26%), and heavy rain (6%).
During the cold season, which lasts from December 3 to March 5, there is a 46% 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 (27% of days with precipitation have at worst light rain), light snow (26%), moderate rain (21%), and heavy snow (8%).
The likelihood of snow falling is highest around February 2, occurring in 23% of days. The season in which it is relatively likely for snow to fall spans from November 26 to March 31.
Either snow rarely accumulates at this location or snow depth measurements are unavailable or unreliable.
The relative humidity typically ranges from 40% (comfortable) to 90% (very humid) over the course of the year, rarely dropping below 22% (dry) and reaching as high as 100% (very humid).
The air is driest around April 20, at which time the relative humidity drops below 49% (comfortable) three days out of four; it is most humid around September 24, exceeding 86% (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 13°F (dry) to 70°F (muggy) and is rarely below -3°F (dry) or above 76°F (very muggy).
There are two periods in the year that are most comfortable: The first is between April 29 and June 18 and the second is between September 7 and October 23. The air feels neither too dry nor too muggy during these periods.
Over the course of the year typical wind speeds vary from 3 mph to 18 mph (light air to fresh breeze), rarely exceeding 26 mph (strong breeze).
The highest average wind speed of 14 mph (moderate breeze) occurs around April 12, 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 August 9, at which time the average daily maximum wind speed is 13 mph (moderate breeze).