Average Weather For Philadelphia, Pennsylvania, USA

Location

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%).

Temperature

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.

Daily High and Low Temperature

The daily average low (blue) and high (red) temperature with percentile bands (inner band from 25th to 75th percentile, outer band from 10th to 90th percentile).

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.

Fraction of Time Spent in Various Temperature Bands

The average fraction of time spent in various temperature bands: frigid (below 15°F), freezing (15°F to 32°F), cold (32°F to 50°F), cool (50°F to 65°F), comfortable (65°F to 75°F), warm (75°F to 85°F), hot (85°F to 100°F) and sweltering (above 100°F).

Sun

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.

Daily Hours of Daylight and Twilight

The number of hours during which the Sun is visible (black line), with various degrees of daylight, twilight, and night, indicated by the color bands. From bottom (most yellow) to top (most gray): full daylight, solar twilight (Sun is visible but less than 6° from the horizon), civil twilight (Sun is not visible but is less than 6° below the horizon), nautical twilight (Sun is between 6° and 12° below the horizon), astronomical twilight (Sun is between 12° and 18° below the horizon), and full night.

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.

Daily Sunrise & Sunset with Twilight and Daylight Savings Time

The solar day over the course of the year 2012 . From bottom to top, the black lines are the previous solar midnight, sunrise, solar noon, sunset, and the next solar midnight. The day, twilights (solar, civil, nautical, and astronomical), and night are indicated by the color bands from yellow to gray. The transitions to and from daylight savings time are indicated by the "DST" labels.

Clouds

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.

Median Cloud Cover

The median daily cloud cover (black line) with percentile bands (inner band from 40th to 60th percentile, outer band from 25th to 75th percentile).

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.

Cloud Cover Types

The fraction of time spent in each of the five sky cover categories. From top (most blue) to bottom (most gray), the categories are clear, mostly clear, partly cloudy, mostly cloudy, and overcast. Pink indicates missing data. Outside of the United States clear skies are often reported ambiguously, leading them to be lumped in with the missing data.

Precipitation

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.

Probability of Precipitation at Some Point in the Day

The fraction of days in which various types of precipitation are observed. If more than one type of precipitation is reported in a given day, the more severe precipitation is counted. For example, if light rain is observed in the same day as a thunderstorm, that day counts towards the thunderstorm totals. The order of severity is from the top down in this graph, with the most severe at the bottom.

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.

Types of Precipitation Throughout the Year

Relative frequency of various types of precipitation over the course of a typical year.

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%).

Warm Season Precipitation

Cold Season Precipitation

Relative frequency of various types of precipitation during the warm and cold seasons respectively.

Snow

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.

Probability of Snow Fall Being Reported in a Given Day

Probability that snow will be reported at least once in a given day. The season is defined as the period during which the probability is greater than one third the maximum probability.

Either snow rarely accumulates at this location or snow depth measurements are unavailable or unreliable.

Humidity

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.

Relative Humidity

The average daily high (blue) and low (brown) relative humidity with percentile bands (inner bands from 25th to 75th percentile, outer bands from 10th to 90th percentile).

Dew Point

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.

Dew Point

The daily average low (blue) and high (red) dew point with percentile bands (inner band from 25th to 75th percentile, outer band from 10th to 90th percentile).

Wind

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).

Wind Speed

The average daily minimum (red), maximum (green), and average (black) wind speed with percentile bands (inner band from 25th to 75th percentile, outer band from 10th to 90th percentile).
The wind is most often out of the west (18% of the time), south west (17% of the time), north west (15% of the time), north (12% of the time), and south (11% of the time).

Wind Directions Over the Entire Year

The fraction of time spent with the wind blowing from the various directions over the entire year. Values do not sum to 100% because the wind direction is undefined when the wind speed is zero.

Fraction of Time Spent with Various Wind Directions

The fraction of time spent with the wind blowing from the various directions on a daily basis. Stacked values do not always sum to 100% because the wind direction is undefined when the wind speed is zero.