Geography and Topography

Chester County is located in the Northeast U.S. in the Northern Mid-Atlantic Region. The County topography features rolling hills and valleys, characteristic of the Piedmont Plateau Region, with a high point of about 1,071 feet at Welsh Mountain and a low point along the Schuylkill River at approximately 70 feet. The land is shaped by major watersheds like the Brandywine and Octoraro creeks, contributing to a landscape that historically supported fertile agricultural land and is now a blend of suburban development and rural open spaces, including significant areas of woodlands and streams.

Much of Western Chester County PA lies at elevations over 600 feet above sea level. The National Weather Service split out this zone (PAZ101) from the relatively lower valley elevations seen across much of Eastern Chester County (PAZ102) several years ago to better distinguish the climate differences attributed to relative elevation and a more inland location from the Atlantic Ocean. We have taken care to ensure an appropriate mix of these higher and lower elevation locations to support the most accurate climate data and analysis for Chester County.

That elevation difference is measurable in the daily records, and it does not work the way most people assume. Comparing East Nantmeal against Avondale 2N across 7,190 days on which both stations reported, with 268 feet of elevation between them, the higher station runs 1.38 degrees cooler on daily highs and 1.40 degrees warmer on daily lows. That works out to roughly 5 degrees per 1,000 feet in each direction, and the same comparison run years earlier against Phoenixville 1E landed within a few tenths of it.

The sign flip is the point. The higher ground is cooler by day and warmer at night, because cold air drains off the ridges after dark and pools in the valleys below. Our four hilltop stations, East Nantmeal, Atglen DEOS, West Grove DEOS and London Grove, all between 600 and 685 feet, have the lowest average daily highs in the network, about 61 to 62 degrees from 2012 through 2025, and the narrowest day to night ranges, 14 to 16 degrees against 21 degrees at Phoenixville 1E in the Schuylkill valley. They are among the stations with the fewest freezing nights, 78 to 91 a year against 109 at Phoenixville, and at the same time among the most days that never rise above freezing, 18 to 20 a year against 12 at Phoenixville. Milder nights, colder days. This is why a single county figure for freeze nights or for 90 degree days is only ever half the story here, and why we hold on to stations at both ends of the elevation range. The full analysis is on our Elevation & Terrain page.

Climate Classification

Typical of the Piedmont Plateau, the climate is classified as Modified Humid Continental, moderated by its proximity to the Atlantic Ocean about 85 miles east. However, due to its inland position and relative high elevation, many winter storms that mix with or turn to rain in Philadelphia will remain as snow or another form of wintry precipitation in the higher elevations of the County. Snowfall in the Chester County Area is quite variable year to year, with the snowiest winter recorded in 1957-58 at Devault 1W when 106.7 inches of snow accumulated over the season. At the other end, genuinely near-snowless winters do occur, most recently in 2019-20 and 2022-23.

The Chester County Area Compared to Philadelphia

The Philadelphia International Airport sits 20 to 28 nautical miles southeast of Chester County, at 59 feet above sea level on the Atlantic Coastal Plain. Our stations sit on the Piedmont Plateau, most of them between 200 and 700 feet above sea level, and that difference turns up in the data every single year.

Across the period of record the airport runs about 3 degrees warmer than the Chester County Area station network. The gap is widest in summer. PHL reaches the 90 degree mark more than 30 times in an average season, while the median across our county network is about 12 days a year. At East Nantmeal, our highest reporting station at 677 feet, the average is under 2 days a year.

The two records are also moving apart. Over the airport’s own period of record, 1941 through 2025, PHL warms at +0.44 degrees per decade while the Chester County Area station composite over those same years warms at +0.12. Starting both series in 1950, so that neither one gets a longer runway than the other, gives +0.52 degrees per decade for the airport against +0.26 for the county. The difference is statistically significant either way.

Snowfall separates the two places even more sharply than temperature does. East Nantmeal averages 35.8 inches of snow a season across its 22 seasons of record, the highest seasonal average of any station in our network. The published seasonal normal for Philadelphia International is 23.1 inches. That is a difference of nearly 13 inches, and it is the whole reason a storm that gives the airport a cold rain can give the western end of this county a foot of snow. The March 31 to April 1 storm of 1997 is the cleanest example on record. It produced 13.7 inches of snow in the Chester County Area and 4 inches at the airport.

Two further findings are worth stating, because they are the ones that hold up under testing.

The divergence is a daytime one. On average daily high temperature, all three of the nearest first order airport stations, Philadelphia, Allentown and Reading, warm significantly faster than the Chester County Area cooperative network. On overnight low temperatures not one of the three separates from the county at all. That is not the textbook urban heat island signature, which is mostly a nighttime effect, and it is a result we would not have found without a county network this deep.

The method was checked against a control. Phoenixville 1E is a station inside the county composite, and tested against that same composite it returns +0.011 degrees per decade with a p value of 0.83. A station that is part of an average should show no differential trend against it, and it does not.

One caveat. These tests establish that the two records diverge. They do not establish why. Airport paving and siting, urban growth around the field, station relocations, instrument changes and changes in observation hour all sit inside that difference and none of them is separated out here.

No airport data is included in any Chester County Area figure anywhere on this site. The reference stations are held in a separate file, drawn in a separate color, and labeled as outside the county wherever they appear. Every comparison above can be run and re-run under Climate Trends, on the Trends on Demand page.

Historic Snowfall and Major Storms

The Chester County Area is home to two of the largest snowstorms ever recorded in the state of Pennsylvania. The largest is the 53 inch snowstorm at Coatesville 1SW that ended on February 14, 1899, and the second greatest is the 50 inches of snow that fell at Morgantown on the Berks and Chester County border between March 18 and 21, 1958.

Statistical analysis shows that the Chester County Area should expect a single storm to exceed twenty inches on average once every 10 years. Since 2003 the County has already had six such snowfalls: January 22-24, 2016, with 31 inches at New Holland 2 SE and 27.4 inches at Coatesville 1E; February 9-11, 2010, with 26.8 inches at East Nantmeal Township; February 5-7, 2010, with 26 inches at Octoraro Lake; February 14-17, 2003, with 25.2 inches at Octoraro Lake and 24 inches at West Chester; February 12-15, 2014, with 24.4 inches at Morgantown and Elverson; and February 11-13, 2006, with 21.1 inches at Coatesville 2W. Snowstorms will exceed 10 inches every other year, and snowstorms of 6 inches or more should be expected at least twice each winter.

The higher elevations of the Chester County Area average more than a foot of snow a season above what falls on the Atlantic Coastal Plain at Philadelphia, New York City, Long Island, New Jersey and Delaware.

Weather Systems and Airflow

Weather systems that affect the Chester County Area either develop in the Midwest and are steered eastward by the prevailing westerly flow aloft, or they form in the southeastern United States and track northward along the Atlantic Coast as powerful Nor’easters. The long overland trajectory of any arctic high pressure system, in conjunction with a downslope component off of the Appalachian Mountains to the west of Chester County, considerably modifies the accompanying cold and dry air masses, so that upon reaching Chester County they have warmed and gained moisture from either the Gulf or the Atlantic Ocean.

Summer Climate

Summers are warm and humid. Usually, highs reach into the mid-80s with nighttime lows near 60 degrees. However, summer nights in the 50s are to be expected several times each month of the summer season. The occasional maritime influence tends to produce a uniform weather pattern, resulting in occasionally prolonged hot, hazy and humid periods.

How often the Chester County Area reaches 90 degrees depends enormously on where in the county you stand. Measured across our network from 2012 through 2025, Phoenixville 1E, low in the Schuylkill valley, averages 32 days a year at or above 90 degrees. East Nantmeal, on high ground at 677 feet, averages 1.7. The county median sits near 12 days. This is in sharp contrast to the more than 30 such days reported at the Philadelphia International Airport, which lies close to the urban center of Philadelphia at only 59 feet above sea level on the Atlantic Coastal Plain.

That nineteenfold spread across a single county is not our stations disagreeing with each other. The annual average high temperature across those same stations spans only about 4 degrees, from 61.3 at East Nantmeal and London Grove to 65.5 at Phoenixville, and that 4 degree gradient explains 78 percent of the variation in hot day counts. A count taken at a fixed threshold magnifies a small difference in level enormously, which is worth remembering before reading too much into any map of 90 degree days.

Here in Chester County on the Piedmont Plateau, summer low temperatures will rarely go lower than 45 degrees. The all-time record high for the Chester County Area, though some dispute the reading, was 111 degrees at Phoenixville on July 10, 1936. The next hottest day was 107 degrees, set on August 7, 1918 at both Sadsburyville and Coatesville. Readings of 100 degrees or better are rare in the County and are essentially confined to the lower valley locations. East Nantmeal has not recorded a single 100 degree day in more than two decades of observations.

Cloud cover is at a minimum during the summer season, as better than 60 percent of available sunshine is received while nights are generally clear. Prevailing winds flow from the south southwest averaging 8 mph. Summer rainfall is adequate, falling as the result of thundershowers.

Winter Climate

Cloudiness persists to a much greater degree in the winter due to the increased frequency of cold fronts and coastal low pressure systems known as Nor’easters. Prevailing winds during the wintertime are from the west northwest averaging 10 to 12 mph. Daytime highs average in the upper 30s during January with nighttime lows around 20 degrees.

A normal winter season here in the Chester County Area will have around 100 to 115 days with sub-freezing low temperatures. That figure ranges across the County from about 116 such nights at West Bradford and 109 at Phoenixville down to 78 at London Grove, and as described above it is elevation and cold air drainage, not simply latitude, that drives the spread. Around 17 winter days will not make it to the freezing mark for a high temperature. Only two days in the record stayed below zero all day. On January 19, 1994 at Coatesville 2W the high for the day was one degree below zero, after a morning low of eleven below, and on January 9, 1856 the Pocopson observer’s three readings were all below zero, the warmest of them one below.

Temperature Extremes and Snow Cover

Summer-like readings have been experienced in the winter but are rare, the highest being 80.2 degrees on February 21, 2018 at the Warwick Township DEOS station. The all-time lowest reading was nineteen degrees below zero, which has happened four times: at Pocopson on January 24, 1857, at Coatesville 1SW on January 14, 1912, at Phoenixville on January 22, 1961, and at Octoraro Lake on January 22, 1984.

The first snowfall of any consequence may occur as early as November but is much more likely in December. On the National Weather Service definition, at least one inch of snow on the ground on Christmas morning, the Chester County Area has recorded a White Christmas in 30 of the 121 years with snow depth reports since 1890, or 25 percent. Widen the question to snow on the ground at any point in the ten days before Christmas and it rises to 50 percent. The last White Christmas here was 2012, and the deepest was 25 inches in 1966.

The record monthly snowfall for this area is the 69.8 inches that fell at Coatesville 1SW in February 1899, a month that carried the 53 inch storm of February 11-14 and a 16.3 inch storm the week before it. In January 1996 over 36 inches of snow was on the ground following the Blizzard of January 6-10. Historically, however, snow cover does not persist for any great length of time here, and the longest period of an inch or more of cover was 63 days from December 9, 1995 to February 9, 1996 at Coatesville 2W. The threat of snow diminishes rapidly after mid-March. However, large snowfalls do continue to occur, as evidenced by the 13.7 inch snowfall on March 31 and April 1, 1997, a system that produced only 4 inches of snow at the Philadelphia International Airport.

Transitional Seasons

Spring and fall are the transition seasons. Prevailing winds are from the west northwest at the start of spring but swing back from the southwest by late spring. As fall progresses the prevailing flow gradually changes back to a west northwest flow once again. Wind speed averages 8 to 10 mph during both spring and fall. By the latter part of April daytime highs in the 70s are noted more and more frequently, and after the warm summer these same seventy degree readings may be observed into October. These mild dry fall days with plenty of sunshine, along with the autumnal change in colors amidst the rolling hills of Chester County, make the fall season one of the most pleasant times of the year.

Growing Season and Freeze Dates

The growing, or freeze free, season averages 184 days in length over the last thirty years, and 180 days across the full period of record since 1890. County-wide it has run from 135 days in 1956 to 211 days in 2017, and individual stations have gone further in both directions, from 118 days at Coatesville 1SW in 1956 to 249 days at Octoraro Lake in 1978. April 21st is the average date of the last spring freeze over the last thirty years, with the latest observed on May 29, 1936 at Phoenixville. The average date of the first fall freeze is October 24th, with the earliest on September 17, 1986 at Phoenixville and the latest on December 2, 1985 at Devault 1W.

Precipitation and Thunderstorms

A little over one half the annual precipitation usually falls in the spring and summer. The average annual precipitation, melted snow and rain, across the County is 49.07 inches over the last thirty years and 45.95 inches across the full period of record since 1890. The greatest monthly total was 18.15 inches at Morgantown and Elverson in August 1955, and the heaviest single day on record was 9.23 inches at Phoenixville 1E on October 1, 2010, at the end of the September 30 deluge. Hurricane Jeanne put 8.91 inches into the West Chester 2NW gauge on September 28, 2004, and Hurricane Floyd brought 8.28 inches to Thorndale on September 16, 1999. While primarily a warm season phenomenon, thunderstorms have been observed in every month of the year. As evidenced by Floyd and Jeanne, hurricanes and tropical storms occasionally track inland enough to provide heavy rain and wind.