Freeze-thaw daysPhiladelphia International AirportWinter 2025-26

December 2025 had 28 freeze-thaw days at Philadelphia's airport, the most of any month since 1940

Winter 2025-26 brought 70 freeze-thaw days to Philadelphia International Airport, the most since 2018-19, led by a record 28 in December.

December 2025 came in at nearly twice its 14.6-day average

Freeze-thaw days by month at Philadelphia International Airport, October through April. Blue bars: winter 2025-26. Gray line: the 1990-91 to 2019-20 average for each month.

Source: NOAA daily highs and lows for station USW00013739, from the Applied Climate Information System. Counted by Conshy Roofers.

Philadelphia International Airport recorded 28 freeze-thaw days in December 2025, more than in any month since the station's record began in July 1940. That is this site's count from NOAA's daily temperature records, and a freeze-thaw day here is one when the low reached 32°F or colder and the high rose above 32°F.

The winter as a whole, July 2025 through June 2026, had 70 freeze-thaw days. That is the most since 2018-19 and above the 64.4 average of the 30 winters from 1990-91 to 2019-20, though well short of the 80.1 average from 1941-42 to 1970-71.

How the count works

A freeze-thaw day here has a low of 32°F or colder and a high of 33°F or warmer

Low: 32°F or colder
High: 33°F or warmer
Day, at the airport: Midnight to midnight, local standard time
Winter: July 1 through June 30

The airport station, NOAA's USW00013739, has a daily high and low for every day from July 1, 1940 through September 30, 2026, with no days missing. For each day, this count checks two things: the low was 32°F or colder, and the high was 33°F or warmer. The record lists whole degrees, so a high above 32°F means 33°F or more. Each winter runs from July 1 through June 30.

The day is the station's own. The National Weather Service's Chicago office, describing how airport stations report, says the observation period for a calendar day runs from midnight to midnight local standard time, which is 1 a.m. to 1 a.m. during daylight saving time.

Researchers define freeze-thaw cycles in several ways. The Great Lakes Integrated Sciences and Assessments program (GLISA) at the University of Michigan lists a common one as a day when the high is above 32°F and the low is below it, which leaves out days with a low of exactly 32°F. Under that stricter rule the airport had 61 freeze-thaw days in 2025-26. The decline over the record shows under either rule.

December 2025

December's 28 freeze-thaw days beat the previous December high of 24

The low reached 32°F or colder on 29 of December's 31 days, and the high stayed at or below freezing on only one of them. That left 28 freeze-thaw days. The next highest month in the record is January 1960, with 27. The previous high for a December was 24, reached in 1944 and in 1947.

No December from 1940 to 2024 had more than 24 freeze-thaw days

Freeze-thaw days in each December at Philadelphia International Airport, 1940 to 2025. The gray rule marks 24, the previous December high.

Source: NOAA daily highs and lows via ACIS. Counted by Conshy Roofers.

The rest of the winter ran close to recent averages. January had 16 freeze-thaw days and February 18, against averages of 16.1 for each month in the 1990-91 to 2019-20 winters. March had 5, below its average of 11.2, and November 2, below its 5.4. December's average over those 30 winters was 14.6, so December 2025 came in at nearly twice its usual count.

The long decline

Freeze-thaw days fell from 80.1 a winter in 1941-42 to 1970-71 to 61.1 in the last ten winters

Freeze-thaw days at the airport fell from an average of 80.1 a winter in 1941-42 to 1970-71 to 70.1 in 1971-72 to 2000-01 and 64.4 in 1990-91 to 2019-20. The last ten winters averaged 61.1. The fewest in the record came in 2015-16, with 39.

Winter 2025-26 brought 70 freeze-thaw days, the most since 2018-19

Freeze-thaw days per winter, July through June, Philadelphia International Airport, 1940-41 to 2025-26. Gray line: the trailing 30-winter average, drawn from 1969-70, when 30 winters are available.

Source: NOAA daily highs and lows via ACIS. Counted by Conshy Roofers.

A freeze-thaw day is a freezing night that warms above 32°F by afternoon, so each winter's count equals its freezing nights minus its all-day freezes. Days with a low at or below 32°F averaged 97.3 a winter in 1941-42 to 1970-71 and 69.6 in the last ten winters, a drop of 27.7. Days that stayed at or below freezing all day fell from 17.2 to 8.5, a drop of 8.7. The difference, 19.0 fewer freeze-thaw days a winter, is the decline from 80.1 to 61.1.

Freezing nights fell by 27.7 a winter and all-day freezes by 8.7, leaving 19.0 fewer freeze-thaw days

Average days per winter at Philadelphia International Airport. Each bar is the average number of freezing nights, split into freeze-thaw days (blue) and all-day freezes, when the high stayed at or below 32°F (gray).

Source: NOAA daily highs and lows via ACIS. Averages computed by Conshy Roofers.

Last winter's 70 ties 2003-04 for eighth among the last 30 winters. Of all 86 winters on record, 45 had more, and in the 1941-42 to 1970-71 period it would have been below the average.

Closer to Conshohocken

Norristown averaged 84.0 freeze-thaw days in its comparable winters, against 71.3 at the airport

The airport sits at an elevation of 7 feet, about 15 miles from Conshohocken by the station coordinates in ACIS. NOAA's station in Norristown, USC00366370, is under 3 miles away. It belongs to the National Weather Service's Cooperative Observer Program, where observers record daily high and low temperatures, and its record has long gaps.

It also keeps a different day. ACIS lists the Norristown readings as taken at 7 a.m. local standard time in December 2025 and at 8 a.m. in January 1965, so each of its days ends in the morning.

In the 32 winters since 1952-53 when Norristown is missing no more than three days between October and April, it averaged 84.0 freeze-thaw days, against 71.3 at the airport in the same winters.

The gap has widened: 84.2 against 57.8 in the six comparable winters since 2012-13

Freeze-thaw days per winter at the airport (blue) and Norristown (red), in the 32 winters when Norristown is missing no more than three days between October and April.

Source: NOAA daily highs and lows for USW00013739 and USC00366370 via ACIS. Counted by Conshy Roofers.

The gap has widened. In the 26 such winters from 1952-53 to 1984-85, Norristown averaged 84.0 and the airport 74.4. In the six from 2012-13 to 2022-23, Norristown averaged 84.2 and the airport 57.8. For 2025-26, Norristown counted 86 with five days missing between October and April, so its full count was at least 86.

Because the two stations end their days at different hours, read the comparison as two records set side by side.

For owners

What this means for a roof

GLISA notes that water expands when it freezes, and that repeated freezing, thawing and refreezing can damage roads, sidewalks and other outdoor surfaces. On roofs, the sources this site uses describe specific effects:

The National Park Service's slate roof brief says slate weathers mainly when impurities in the stone react during wet and dry, hot and cold cycles, and that slate's low porosity limits the damage frost does to the stone itself.
The Metal Roofing Alliance says concrete tile can crack and warp in the freeze-thaw cycle of northern climates.
Conshohocken's re-roofing addendum requires ice and water shield to run at least 24 inches inside the exterior wall line when a roof is replaced.

The National Roofing Contractors Association recommends inspecting a roof each spring and fall from the ground or a ladder, looking for curling or split shingles, missing tabs and cracked or missing pieces on slate, tile and metal roofs. The roof inspection checklist lists what to look for, and the weather guide covers the area's rain, wind and hail.

Limits of the data

Caveats

These counts are of air temperature at each station. They are not measurements of a roof's surface, which this data does not cover.

The National Weather Service says daily observations are preliminary until NOAA's National Centers for Environmental Information finishes quality control, three or more months after they are taken. The December 2025 to March 2026 readings were taken more than six months before this count.

The airport record is complete. The Norristown record is not, and its comparison winters are uneven across the decades: 26 come from 1952-53 to 1984-85 and six from 2012-13 to 2022-23. The station's daily readings end at a morning observation, while the airport's day ends at midnight.

The count follows one definition. GLISA's stricter definition gives lower numbers, 61 for the airport in 2025-26, and other researchers use others.

The data

Every winter since 1940-41

Winters run July through June. The airport columns cover every winter. The Norristown columns are blank where its record has no data, and the last column shows how many days between October and April it is missing.

Freeze-thaw days by winter, newest first
Winter	Freeze-thaw days	Freezing nights	All-day freezes	December	Norristown	Norristown days missing, Oct. to Apr.
2025-26	70	84	14	28	86	5
2024-25	60	74	14	16	82	14
2023-24	50	56	6	10	85	30
2022-23	50	54	4	18	88	3
2021-22	60	68	8	5	84	22
2020-21	67	75	8	17	78	1
2019-20	57	59	2	14	72	3
2018-19	74	81	7	18	96	4
2017-18	67	82	15	10	87	2
2016-17	56	63	7	17	92	4
2015-16	39	49	10	2	77	3
2014-15	68	90	22	9	97	6
2013-14	74	99	25	17	96	5
2012-13	67	76	9	12	103	0
2011-12	45	49	4	12	73	19
2010-11	72	91	19	23	71	63
2009-10	65	79	14	19	55	64
2008-09	71	89	18	16	86	16
2007-08	68	76	8	16	104	5
2006-07	62	79	17	11	71	61
2005-06	73	79	6	21	80	71
2004-05	63	80	17	13	78	14
2003-04	70	92	22	23	83	17
2002-03	64	90	26	19	85	27
2001-02	59	62	3	10	89	15
2000-01	81	95	14	19	80	45
1999-00	53	69	16	12	7	187
1998-99	51	66	15	9
1997-98	62	63	1	18
1996-97	74	85	11	9
1995-96	73	105	32	19
1994-95	60	71	11	18
1993-94	62	81	19	9
1992-93	66	76	10	14
1991-92	72	80	8	17
1990-91	63	69	6	13
1989-90	78	95	17	15
1988-89	83	97	14	21
1987-88	70	88	18	14
1986-87	92	100	8	22	88	36
1985-86	71	88	17	21	63	33
1984-85	77	94	17	13	88	2
1983-84	68	95	27	13	76	1
1982-83	74	82	8	13	81	2
1981-82	83	103	20	20	106	0
1980-81	72	102	30	14	97	1
1979-80	60	86	26	15	61	0
1978-79	57	83	26	18	74	2
1977-78	61	106	45	17	82	0
1976-77	79	115	36	21	17	181
1975-76	69	84	15	20	47	80
1974-75	86	92	6	18	55	74
1973-74	73	89	16	16	2	182
1972-73	60	72	12	9	80	1
1971-72	74	85	11	13	91	2
1970-71	68	89	21	18	83	3
1969-70	90	114	24	22	94	5
1968-69	85	106	21	19	86	2
1967-68	77	92	15	15	82	0
1966-67	86	106	20	17	85	0
1965-66	85	106	21	21	78	1
1964-65	85	105	20	15	82	1
1963-64	92	111	19	20	88	0
1962-63	88	117	29	16	91	0
1961-62	93	116	23	20	87	6
1960-61	71	101	30	18	71	2
1959-60	100	112	12	17	95	1
1958-59	78	102	24	15	81	1
1957-58	62	82	20	13	72	0
1956-57	72	82	10	10	80	0
1955-56	82	97	15	20	69	32
1954-55	67	79	12	15	88	2
1953-54	66	80	14	14	86	1
1952-53	67	71	4	13	100	0
1951-52	78	86	8	14	91	34
1950-51	76	87	11	21
1949-50	85	93	8	20
1948-49	66	72	6	14
1947-48	86	108	22	24
1946-47	83	96	13	16
1945-46	63	83	20	12
1944-45	75	100	25	24
1943-44	97	112	15	19
1942-43	93	113	20	22
1941-42	87	100	13	21
1940-41	104	114	10	18

Download the data: every winter (CSV), months, 30-winter periods, station comparison, highest months, station details.