Pennsylvania · 16 m elevation

Barometric Pressure in Philadelphia, PA

• 30.04Average pressure (inHg, sea level)
• 231.7Swing days a year · 10th of 50
• 127.5Big-swing days a year (≥10 hPa)
• 132.2Significant 24h drop days a year

Overview

Philadelphia averages 231.7 significant pressure-swing days a year, #10 of 50 US cities. Mean 30.04 inHg (1017.3 hPa), with winter far more volatile.

Philadelphia doesn't have one pressure climate — it has two, and they trade places twice a year. Across the 2015–2024 record (87,672 hourly observations), mean sea-level pressure sits at 30.04 inHg (1017.3 hPa), with a median of 30.03 inHg (1017.1 hPa). Ninety percent of readings fall between 29.68 and 30.42 inHg (1005.1 and 1030.3 hPa). Those are unexceptional figures. The variability behind them is not.

The city logs 231.7 significant swing days per year — days when the barometer moves at least 0.18 inHg (6 hPa) in 24 hours — ranking #10 of 50 US cities and running 1.28 times the national median of 180.7. Big-swing days of 0.30 inHg (10 hPa) or more come in at 127.5 per year, #12 nationally and 1.48 times the median of 86.1. Standard deviation is 0.23 inHg (7.65 hPa), ranking #12 against a 6.87 hPa median. Day to day, the median absolute 24-hour change is a modest 0.12 inHg (4.2 hPa), but the 95th percentile reaches 0.47 inHg (15.8 hPa).

These figures look extreme in isolation and ordinary in context. The Northeast corridor is uniformly restless: New York ranks #7 with 241.6 swing days, Philadelphia #10 with 231.7, Baltimore #13 with 222.3, Washington #15 with 218.6. The wider spread is what stands out — Philadelphia records 11.7 times the swing days of Miami (19.8) and 5.1 times those of Los Angeles (45.6), and on big-swing days the Miami ratio approaches 40 to 1. Notably, at just 16 meters elevation the city still posts 82% of the swing days of Denver, the national leader at 281.6 and 1,615 meters up. Elevation isn't the driver here; the Atlantic storm track is. That low elevation does mean station pressure averages 29.98 inHg (1015.4 hPa) against the sea-level-adjusted 30.04 inHg (1017.3 hPa) — an offset of about 0.06 inHg (1.9 hPa), small enough that an uncorrected home barometer reads only a hair below the official value.

Migraine & pressure-sensitivity outlook

• Low — under 6 hPa (0.18 inHg)
• Elevated — 6 to 10 hPa
• High — 10 hPa (0.30 inHg) or more
• Dashed outline = forecast (after today)

Association only. This calendar shows days when pressure changed — or is forecast to change — sharply over 24 hours, conditions associated with pressure sensitivity in some people. It is not a prediction of symptoms, not a diagnosis, and not medical advice. The 30 days of history are there so you can compare the days you actually felt something against what the barometer did. Talk to a clinician about headache or migraine management.

Some people report headaches, joint pain, or sinus discomfort in connection with rapid barometric change. Philadelphia produces the conditions many pressure-sensitive people describe as triggers on 231.7 days a year, with 132.2 of those being drop days — significant 24-hour falls, the pattern most often cited. Of the annual swing days, 127.5 qualify as big swings of 0.30 inHg (10 hPa) or more, placing the city #12 of 50 nationally on that measure.

The seasonal concentration matters more than the annual total. January (26.4 swing days, 15.7 drop days), February (25.9 and 16.2), March (25.7 and 16.3), and December (25.8 and 15.3) form a four-month block where the barometer is rarely still. July and August together contribute just 17.3 swing days and 8.9 drop days — a genuine lull rather than a modest easing. For anyone who tracks pressure, the cold season is where tracking carries the most information.

For comparative framing: Philadelphia sees 11.7 times the swing days of Miami and 5.1 times those of Los Angeles, while sitting essentially level with its I-95 neighbors — New York at 241.6, Baltimore at 222.3, Washington at 218.6. Someone relocating within the corridor should expect little change; someone arriving from the Southwest or Southern California should expect a substantially busier barometer. One practical note: Philadelphia's largest falls arrive with identifiable winter coastal systems that are forecast days in advance, so the barometer's trajectory here is usually knowable ahead of time rather than a surprise.

Seasonal patterns

From January through March, Philadelphia averages 26.4, 25.9, and 25.7 significant swing days per month — roughly five days out of every six. In July and August those counts collapse to 9.1 and 8.2. The monthly standard deviation tells the same story: 0.28 inHg (9.46 hPa) in January against 0.12 inHg (4.19 hPa) in July, a better than two-to-one contraction. Drop days follow the identical curve, peaking at 16.3 in March and 16.2 in February against 4.4 in July and 4.5 in August. December through March accounts for roughly 104 swing days — about 45% of the annual total packed into a third of the calendar.

Monthly mean pressure barely moves, ranging only from 29.95 inHg (1014.1 hPa) in June to 30.12 inHg (1019.8 hPa) in November and December. The entire seasonal signal lives in the variability, not the average. A summer month in Philadelphia isn't at a different pressure — it's just steadier, because with the storm track displaced north and the thermal gradient weak under the Bermuda high, pressure changes come from local convection rather than passing synoptic systems.

Philadelphia's everyday churn ranks #10, but its worst individual events rank #4 of 50. The sharpest 24-hour fall on record is 1.35 inHg (45.8 hPa), ending at 2:00 a.m. on January 10, 2024 — steeper than anything recorded in the other 46 cities except Boston (-51.5 hPa), New York (-47.6), and Detroit (-47.5). The next four largest: 1.30 inHg (44.1 hPa) ending January 17, 2022; 1.24 inHg (41.9 hPa) ending December 18, 2023; 1.18 inHg (40.0 hPa) ending March 14, 2017; and 1.15 inHg (39.0 hPa) ending December 23, 2022. All five fall between mid-December and mid-March, the signature of coastal cyclogenesis. Recovery can be nearly as abrupt — the largest 24-hour rise is 1.28 inHg (43.2 hPa), and the fastest six-hour fall is 0.63 inHg (21.5 hPa). The record extremes bracket a 2.04 inHg (69.0 hPa) span: a high of 30.97 inHg (1048.7 hPa) on December 14, 2024, and a low of 28.93 inHg (979.7 hPa) on February 7, 2020.

Month by month

Swing days: 24-hour change of at least 6 hPa (0.18 inHg). Drop days: 24-hour fall of at least 6 hPa. Averages per year, 2015–2024.

Records and extremes

Sharpest 24-hour drops, 2015–2024

Nearby cities

• New York, NY
• Baltimore, MD
• Washington, DC
• Virginia Beach, VA
• Boston, MA
• Raleigh, NC

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