Average Weather in Philadelphia Pennsylvania, United States
In Philadelphia, the temperature typically varies from 26°F to 87°F over the course of the year, and is rarely below 13°F or above 94°F.
The hot season lasts for 3.6 months, from May 29 to September 17, with an average daily high temperature above 77°F. The hottest day of the year is July 21, with an average high of 87°F and low of 70°F.
The cold season lasts for 3.2 months, from December 2 to March 8, with an average daily high temperature below 50°F. The coldest day of the year is January 30, with an average low of 26°F and high of 40°F.
Average High and Low Temperature
The figure below shows you a compact characterization of the entire year of hourly average temperatures. The horizontal axis is the day of the year, the vertical axis is the hour of the day, and the color is the average temperature for that hour and day.
Average Hourly Temperature
The length of the day in Philadelphia varies significantly over the course of the year. In 2017, the shortest day is December 21, with 9 hours, 20 minutes of daylight; the longest day is June 21, with 15 hours, 1 minute of daylight.
Hours of Daylight and Twilight
The earliest sunrise is at 5:31 AM on June 14, and the latest sunrise is 2 hours, 2 minutes later at 7:33 AM on November 4. The earliest sunset is at 4:35 PM on December 6, and the latest sunset is 3 hours, 58 minutes later at 8:33 PM on June 27.
Daylight saving time (DST) is observed in Philadelphia during 2017, starting in the spring on March 12, lasting 7.8 months, and ending in the fall on November 5.
Sunrise & Sunset with Twilight and Daylight Saving Time
In Philadelphia, the average percentage of the sky covered by clouds experiences mild seasonal variation over the course of the year.
The clearer part of the year in Philadelphia begins around June 24 and lasts for 4.7 months, ending around November 14. On September 23, the clearest day of the year, the sky is clear, mostly clear, or partly cloudy 64% of the time, and overcast or mostly cloudy 36% of the time.
The cloudier part of the year begins around November 14 and lasts for 7.3 months, ending around June 24. On January 3, the cloudiest day of the year, the sky is overcast or mostly cloudy 54% of the time, and clear, mostly clear, or partly cloudy 46% of the time.
A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. The chance of wet days in Philadelphia varies throughout the year.
The wetter season lasts 4.9 months, from March 31 to August 27, with a greater than 29% chance of a given day being a wet day. The chance of a wet day peaks at 37% on July 29.
The drier season lasts 7.1 months, from August 27 to March 31. The smallest chance of a wet day is 22% on January 28.
Among wet days, we distinguish between those that experience rain alone, snow alone, or a mixture of the two. Based on this categorization, the most common form of precipitation throughout the year is rain alone, with a peak probability of 37% on July 29.
Daily Chance of Precipitation
To show variation within the months and not just the monthly totals, we show the rainfall accumulated over a sliding 31-day period centered around each day in the year. Philadelphia experiences some seasonal variation in monthly rainfall.
Rain falls throughout the year in Philadelphia. The most rain falls during the 31 days centered around July 27, with an average total accumulation of 3.8 inches.
The least rain falls around February 7, with and average total accumulation of 2.1 inches.
Average Monthly Rainfall
We report snowfall in liquid-equivalent terms. The actual depth of new snowfall is typically between 5 and 10 times the liquid-equivalent amount, assuming the ground is frozen. Colder, drier snow tends to be on the higher end of that range and warmer, wetter snow on the lower end.
As with rainfall, we consider the snowfall accumulated over a sliding 31-day period centered around each day in the year. Philadelphia experiences some seasonal variation in monthly liquid-equivalent snowfall.
The snowy period of the year lasts for 4.2 months, from November 23 to March 28, with a sliding 31-day liquid-equivalent snowfall of at least 0.1 inches. The most snow falls during the 31 days centered around January 25, with an average total liquid-equivalent accumulation of 0.7 inches.
The snowless period of the year lasts for 7.8 months, from March 28 to November 23. The least snow falls around August 1, with an average total liquid-equivalent accumulation of 0.0 inches.
Average Liquid-Equivalent Monthly Snowfall
We base the humidity comfort level on the dew point, as it determines whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid. Unlike temperature, which typically varies significantly between night and day, dew point tends to change more slowly, so while the temperature may drop at night, a muggy day is typically followed by a muggy night.
Philadelphia experiences very significant seasonal variation in the perceived humidity.
The muggier period of the year lasts for 3.9 months, from May 29 to September 26, during which time the comfort level is muggy, oppressive, or miserable at least 15% of the time. The muggiest day of the year is July 29, with muggy conditions 58% of the time.
The least muggy day of the year is February 19, when muggy conditions are essentially unheard of.
Humidity Comfort Levels
This section discusses the wide-area hourly average wind vector (speed and direction) at 10 meters above the ground. The wind experienced at any given location is highly dependent on local topography and other factors, and instantaneous wind speed and direction vary more widely than hourly averages.
The average hourly wind speed in Philadelphia experiences mildly seasonal variation over the course of the year.
The windier part of the year lasts for 6.4 months, from October 27 to May 9, with average wind speeds of more than 4.0 miles per hour. The windiest day of the year is February 26, with an average hourly wind speed of 5.0 miles per hour.
The calmer time of year lasts for 5.6 months, from May 9 to October 27. The calmest day of the year is August 13, with an average hourly wind speed of 3.0 miles per hour.
Average Wind Speed
The predominant average hourly wind direction in Philadelphia varies throughout the year.
The wind is most often from the south for 2.3 months, from July 21 to September 29, with a peak percentage of 37% on August 2. The wind is most often from the west for 9.7 months, from September 29 to July 21, with a peak percentage of 45% on January 14.
Philadelphia is located near a large body of water (e.g., ocean, sea, or large lake). This section reports on the wide-area average surface temperature of that water.
The average water temperature experiences significant seasonal variation over the course of the year.
The time of year with warmer water lasts for 3.5 months, from June 19 to October 3, with an average temperature above 67°F. The day of the year with the warmest water is August 7, with an average temperature of 74°F.
The time of year with cooler water lasts for 3.6 months, from December 21 to April 9, with an average temperature below 46°F. The day of the year with the coolest water is February 19, with an average temperature of 39°F.
Average Water Temperature
This section discusses the total daily incident shortwave solar energy reaching the surface of the ground over a wide area, taking full account of seasonal variations in the length of the day, the elevation of the Sun above the horizon, and absorption by clouds and other atmospheric constituents. Shortwave radiation includes visible light and ultraviolet radiation.
The average daily incident shortwave solar energy experiences very significant seasonal variation over the course of the year.
The brighter period of the year lasts for 3.9 months, from April 27 to August 23, with an average daily incident shortwave energy per square meter above 5.8 kWh. The brightest day of the year is June 29, with an average of 6.9 kWh.
The darker period of the year lasts for 3.1 months, from November 6 to February 10, with an average daily incident shortwave energy per square meter below 2.9 kWh. The darkest day of the year is December 23, with an average of 1.8 kWh.
Average Daily Incident Shortwave Solar Energy
For the purposes of this report, the geographical coordinates of Philadelphia are 39.952 deg latitude, -75.164 deg longitude, and 30 ft elevation.
The topography within 2 miles of Philadelphia contains only modest variations in elevation, with a maximum elevation change of 105 feet, and an average elevation above sea level of 37 feet. Within 10 miles also contains only modest variations in elevation (443 feet). Within 50 miles contains very significant variations in elevation (1,250 feet).
The area within 2 miles of Philadelphia is covered by artificial surfaces (92%), within 10 miles by artificial surfaces (90%), and within 50 miles by trees (41%) and artificial surfaces (27%).
This report illustrates the typical weather in Philadelphia, based on a statistical analysis of historical hourly weather reports and model reconstructions from January 1, 1980 to December 31, 2016.
Temperature and Dew Point
There are 4 weather stations near enough to contribute to our estimation of the temperature and dew point in Philadelphia.
For each station, the records are corrected for the elevation difference between that station and Philadelphia according to the International Standard Atmosphere , and by the relative change present in the MERRA-2 satellite-era reanalysis between the two locations.
The estimated value at Philadelphia is computed as the weighted average of the individual contributions from each station, with weights proportional to the inverse of the distance between Philadelphia and a given station.
The stations contributing to this reconstruction are: Philadelphia International Airport (48%, 11 kilometers, southwest), Wings Field (19%, 19 kilometers, northwest), North Philadelphia Airport (20%, 19 kilometers, northeast), and South Jersey Regional Airport (12%, 27 kilometers, east).
All data relating to the Sun's position (e.g., sunrise and sunset) are computed using astronomical formulas from the book, Astronomical Tables of the Sun, Moon and Planets , by Jean Meeus.
All other weather data, including cloud cover, precipitation, wind speed and direction, and solar flux, come from NASA's MERRA-2 Modern-Era Retrospective Analysis . This reanalysis combines a variety of wide-area measurements in a state-of-the-art global meteorological model to reconstruct the hourly history of weather throughout the world on a 50-kilometer grid.
Land Use data comes from the Global Land Cover SHARE database , published by the Food and Agriculture Organization of the United Nations.
Elevation data comes from the Shuttle Radar Topography Mission (SRTM) , published by NASA's Jet Propulsion Laboratory.
Names, locations, and time zones of places and some airports come from the GeoNames Geographical Database .
Time zones for aiports and weather stations are provided by AskGeo.com .