California · 18 m elevation

Barometric Pressure in San Francisco, CA

• 30.02Average pressure (inHg, sea level)
• 80.6Swing days a year · 42nd of 50
• 20.2Big-swing days a year (≥10 hPa)
• 39.3Significant 24h drop days a year

Overview

San Francisco averages 30.02 inHg (1016.5 hPa) and just 80.6 significant pressure swing days a year - 42nd of 50 US cities. July averages 0.5 of them.

San Francisco's barometer spends most of the year doing very little. Mean sea-level pressure over 2015-2024 was 30.02 inHg (1016.5 hPa), with the middle of the distribution tightly packed: the 5th percentile sits at 29.81 inHg (1009.4 hPa) and the 95th at 30.27 inHg (1025 hPa). The standard deviation is 0.14 inHg (4.85 hPa) against a national median of 0.20 inHg (6.87 hPa) across the 50 cities tracked here — 44th of 50 for variability. Day-to-day movement is smaller still. The median absolute 24-hour change is 0.06 inHg (2 hPa), and even the 95th-percentile day moves only 0.22 inHg (7.4 hPa). Over six-hour windows, the typical range runs from -0.08 inHg (-2.7 hPa) to +0.07 inHg (+2.4 hPa).

The city sits 18 meters above sea level, so the elevation correction is small but not zero. Mean station pressure is 29.95 inHg (1014.3 hPa), about 0.06 inHg (2.2 hPa) below the sea-level-adjusted value that appears in forecasts and on weather sites. A home barometer left uncorrected reads consistently a touch low compared with the official number.

Nationally, San Francisco ranks 42nd of 50 for significant swing days — 80.6 per year, where a significant swing is a 24-hour change of at least 0.18 inHg (6 hPa) — and 42nd for big-swing days of 0.30 inHg (10 hPa) or more, at 20.2 per year. The national median city sees 180.7 and 86.1 respectively: more than twice the swing days and over four times the big swings. Denver, the most volatile city in the set, logs 281.6 swing days, 3.5 times San Francisco's total, and 169.8 big-swing days, 8.4 times as many.

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.

For people who track barometric pressure as a possible trigger, San Francisco is one of the quieter large US cities — but the quiet is seasonal, not uniform. The city records 80.6 significant swing days per year (24-hour changes of at least 0.18 inHg / 6 hPa) and 20.2 big-swing days at 0.30 inHg (10 hPa) or more. That is 45 percent of the national median for swing days and under a quarter of the median for big swings. Of the 80.6 swing days, 39.3 are drop days — significant 24-hour falls, the pattern many pressure-sensitive people report as a trigger.

The calendar does most of the sorting. June through September together account for 5.4 swing days and 2.0 drop days per year — effectively a full summer with almost nothing on the barometer. November through March carries the bulk of the activity, led by January (13.8 swing days, 7.5 drop days) and March (13.0 and 6.7). The signal here is concentrated in the winter storm sequence and the occasional deep winter drop rather than spread as a steady daily drumbeat.

Comparatively, San Francisco sits in a calm cluster with San Jose (77.4 swing days), New Orleans (80.8), Jacksonville (81.7) and Sacramento (85.3). It is quieter than Seattle, which sees 203.8 swing days — 2.5 times as many — despite sharing the same coast and storm track. It is more active than Los Angeles (45.6 swing days, 7.5 big-swing days), where San Francisco logs 1.8 times the swing days and 2.7 times the big swings, and about four times the swing days of Miami's 19.8. The caveat is depth: the deepest 24-hour falls here, up to 0.80 inHg (27 hPa), match or exceed anything recorded in several much more volatile cities. Rare, but not mild when they arrive.

Seasonal patterns

San Francisco's pressure climate works like a switch rather than a dial. From June through September the North Pacific High sits offshore, the marine layer does the local work, and synoptic pressure changes essentially stop. July averages 0.5 significant swing days per year and 0.1 drop days; August runs 0.7 and 0.5; June, 1.5 and 0.3. Monthly standard deviation bottoms out at 0.07 inHg (2.28 hPa) in July. Monthly mean pressure barely moves either, holding between 29.93 and 29.95 inHg (1013.4-1014.2 hPa) from June through September.

Winter is a different regime. The Pacific storm track drops south, and the barometer starts working. January averages 13.8 swing days and 7.5 drop days, March 13.0 and 6.7, December 12.9 and 6.7, February 10.9 and 5.1. Monthly standard deviation climbs to 0.18 inHg (6.2 hPa) in January and 0.15 inHg (4.97 hPa) in December, while monthly means peak at 30.12 inHg (1019.9 hPa) in December and 30.11 inHg (1019.8 hPa) in January. Of the year's 80.6 swing days, 58.9 fall between November and March — about three-quarters of the total.

The extremes are all cold-season. The record low for the period is 29.14 inHg (986.9 hPa) at 6 p.m. on February 17, 2017; the sharpest 24-hour fall in the ten-year record, 0.80 inHg (27 hPa), ended at 2 p.m. that same day. The four next-deepest drops: 0.73 inHg (24.6 hPa) on March 21, 2023; 0.62 inHg (21 hPa) on February 4, 2024; 0.58 inHg (19.5 hPa) on November 27, 2019; and 0.53 inHg (18.1 hPa) on January 20, 2017. The record high is 30.60 inHg (1036.1 hPa) on January 5, 2020. Worth noting: that -27 hPa maximum drop ranks 35th of 50 nationally, better than the city's overall volatility rank, and deeper than the worst 24-hour falls on record in Fort Worth (-22 hPa), Dallas (-21.5), Albuquerque (-21.5) or Houston (-21.4), all of which are far twitchier day to day.

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

• San Jose, CA
• Sacramento, CA
• Fresno, CA
• Los Angeles, CA
• Las Vegas, NV
• San Diego, CA

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