Article
Falling and rising barometric pressure: what it means
Falling barometric pressure means an area of lower pressure — a low or a front — is approaching, and unsettled weather usually comes with it; rising pressure means a high is building in behind it and conditions are settling. A fall of 0.18 inHg (6 hPa) within 24 hours is a meaningful move, and 0.30 inHg (10 hPa) or more is a big one.
What the direction tells you
A barometer reads the weight of the air above it. When a low-pressure system moves toward you, that weight shrinks and the reading falls; when a high moves in, it grows and the reading rises. The FAA’s Balloon Flying Handbook puts the practical rule in one line: rising pressure at a single station “generally indicates the approach of fair weather,” while “decreasing or rapidly falling pressure usually indicates approaching bad weather and possible severe storms.”
The two kinds of system behave differently. In a low, pressure decreases toward the center and the air flows inward and upward, turning counterclockwise in the Northern Hemisphere. In a high, pressure increases toward the center and the air flows outward and downward, turning clockwise.
So the direction answers most of the question by itself. Falling pressure means a low or front is on the way, or a low nearby is deepening. Rising pressure means the system is moving off and a high is taking its place. What the direction does not tell you is how much weather is coming — for that, you need the size and speed of the change. If you want the background on what is being measured, start with what barometric pressure is.
Why pressure drops before a storm, not during the rain
Pressure does not drop because it rains. It drops because the system that brings the rain is arriving, and the timing depends on the kind of front.
Cold fronts and thunderstorms
Ahead of a cold front, pressure falls; the FAA handbook notes that “a high dew point and falling barometric pressure are indicative of imminent cold front passage.” During the passage itself pressure falls quickly and bottoms out as the front goes through, then rises gradually behind it. A thunderstorm runs the same sequence on a compressed clock: an abrupt fall as the storm approaches, then an abrupt rise as it moves on and the rain stops.
Warm fronts
Ahead of a warm front, pressure keeps falling until the front has fully passed. During passage it levels off; afterward there is a slight rise, often followed by a renewed fall.
That is why “does the pressure drop when it rains?” is only half right. The fall comes first, while the system approaches; the lowest reading arrives around the passage; and by the time the rain is ending, pressure is often already recovering. Rain and the pressure minimum are related, but they are not the same moment. No source gives a fixed number of hours of warning — it varies with the system — so treat a sustained fall as “something is coming”, not as a countdown.
How big is a big drop?
This site measures change over 24 hours and uses two fixed thresholds, defined on the methodology page: a swing day contains a 24-hour change of at least 0.18 inHg (6 hPa) in either direction, a big-swing day at least 0.30 inHg (10 hPa), and a drop day a 24-hour fall of at least 0.18 inHg (6 hPa). The cut points are conventions used to rank cities on one yardstick, not clinically validated limits.
| 24-hour change | inHg | hPa | How to read it |
|---|---|---|---|
| Under the swing threshold | under 0.18 | under 6 | The median city stays below this line on roughly half of days |
| Swing | 0.18–0.30 | 6–10 | The median city has 180.7 swing days a year (big-swing days included) — about half of days |
| Big swing | 0.30 or more | 10 or more | 86.1 days a year in the median city, under a quarter of days; rare in calm climates — Miami’s 95th-percentile change, 0.14 inHg (4.6 hPa), is below even the swing threshold |
| Largest recorded in 50 cities | 1.52 | 51.5 | Boston’s sharpest 24-hour fall, 2015–2024 |
Reading a barometer or app hour by hour
Before reading much into an app arrow that flips between rising and falling over an hour or two, remember that even with no weather at all, the atmosphere carries a regular twice-daily oscillation — the semidiurnal atmospheric tide. At a classic equatorial station it moves pressure 0.03–0.06 inHg (1–2 hPa) either side of the mean every 12 hours, and it weakens at higher latitudes. A small wobble that reverses within the day is consistent with the tide rather than an approaching system.
The arithmetic gives a usable rule of thumb: reaching the 6 hPa swing threshold takes an average of 0.25 hPa (about 0.01 inHg) an hour, held for a full day. A sustained fall that adds up past 0.18 inHg (6 hPa) is the kind of decreasing pressure the FAA handbook says usually indicates approaching bad weather.
If someone quotes a drop in “points”, ask for the unit — the word has none. If a point is a hundredth of an inch of mercury, a “40-point drop” is 0.40 inHg (13.5 hPa), comfortably a big swing. If it means hectopascals, it is far larger.
How often it happens: 50 US cities
This is where “big” gets context. The figures below are this site’s own, computed from Open-Meteo ERA5 hourly reanalysis for 2015–2024 — a model reconstruction constrained by observations, not a single station’s barograph, which makes cities directly comparable at the cost of smoothing the sharpest very local excursions.
- 180.7median swing days per year, 50 cities
- 86.1median big-swing days per year
- 177.5drop days per year in Denver, the most
- 6.9drop days per year in Miami, the fewest
Drop days span a factor of more than 25 across the country. Denver logs 177.5 a year and Colorado Springs 175.5, followed by Boston at 151.0, Minneapolis at 146.3 and Omaha at 142.7. At the calm end, Miami averages 6.9, San Diego 15.0, Tampa 18.5, Los Angeles 19.9 and San Jose 37.7.
Two caveats belong next to the Colorado numbers. First, the sea-level reduction used to compare cities inflates apparent variability at altitude, and Denver (1,615 m) and Colorado Springs (1,832 m) are the two highest cities in the set; Salt Lake City (1,302 m) shows the same effect, with the highest overall variability of all 50. Second, the 6 hPa threshold saturates in volatile interior cities — Denver’s 281.6 swing days are about three-quarters of the year — which is why the 10 hPa big-swing count is the better cross-city comparison.
A typical big day, by city
Another way to ask “how big is big here” is the 95th-percentile 24-hour change: the size of move that only the top 5% of hours exceed. In Boston it is 0.54 inHg (18.2 hPa); Colorado Springs 0.51 inHg (17.4 hPa), Denver 0.50 inHg (17.0 hPa), Minneapolis 0.50 inHg (16.8 hPa) and Omaha 0.49 inHg (16.6 hPa). In Miami it is 0.14 inHg (4.6 hPa) — below the swing threshold — with San Diego at 0.16 inHg (5.3 hPa) and Tampa at 0.17 inHg (5.8 hPa). A drop that would be routine in Boston is a rare event in Miami.
The biggest drops on record
The largest single 24-hour falls in the ten years all belong to low-elevation cities in the Northeast and Great Lakes — so the sea-level-reduction altitude effect that inflates Denver’s and Colorado Springs’ numbers does not apply to them.
| City | Largest 24-h fall, inHg | hPa |
|---|---|---|
| Boston, MA | 1.52 | 51.5 |
| New York, NY | 1.41 | 47.6 |
| Detroit, MI | 1.40 | 47.5 |
| Philadelphia, PA | 1.35 | 45.8 |
| Cleveland, OH | 1.34 | 45.4 |
Source: this site’s ten-year dataset, Open-Meteo ERA5 hourly reanalysis, 2015–2024.
At the other end, the smallest “worst day” in the dataset belongs to Phoenix: in ten years its sharpest 24-hour fall was 0.44 inHg (14.9 hPa). Miami’s was 0.48 inHg (16.1 hPa), Tucson’s 0.49 inHg (16.5 hPa), San Diego’s 0.50 inHg (16.8 hPa) and Los Angeles’s 0.55 inHg (18.7 hPa).
Tampa is the exception that proves the rule. Its everyday numbers are among the calmest in the country — 18.5 drop days a year, a 95th-percentile change of 0.17 inHg (5.8 hPa) — yet its record fall is 1.10 inHg (37.2 hPa), bigger than many far more volatile northern cities have seen. That is the signature of an occasional tropical cyclone passing through an otherwise quiet pressure climate.
The pattern across the whole table is consistent: the cities with the most frequent and largest swings cluster across the northern tier, the Great Lakes and the Northeast, while the Gulf and Pacific coasts and the desert Southwest sit at the calm end. That is an inference from the data, not a borrowed rule, and every city page carries its own numbers — see all 50 cities.
What causes pressure to drop
In short: the arrival or deepening of low pressure. The common cases, all from the mechanism above:
- An approaching low-pressure system — the most common cause of a sustained fall over a day or more.
- An approaching front — pressure falls ahead of both cold and warm fronts and bottoms out around passage.
- A thunderstorm — a short, abrupt fall as it nears, then an abrupt rise as it leaves.
- A tropical cyclone — rare in most places, but capable of the extreme falls seen in Tampa’s record.
- The daily tide — not weather at all: a small, regular dip and recovery twice a day.
Rising pressure is the mirror image: the low moves off, the front clears, and higher pressure builds in behind it.
When in the year it happens
Swings are not spread evenly through the year, and the timing differs by city. Each city page has a seasonal chart showing the typical level and the within-month volatility for every month, which is the place to see when your own city’s drops concentrate. If your town is not one of the 50, check any US location for its live chart. For context on where a single reading sits, see what normal barometric pressure is and the site’s pressure zones.
Does falling pressure affect how you feel?
Some people report that falling pressure is a trigger for headaches. As the methodology page sets out, studies have repeatedly found a statistical association between barometric pressure change and migraine onset in some people, and no association in others; effect sizes are modest and individual sensitivity varies widely. That is why this site’s outlook counts the size of the 24-hour change rather than the level: it describes the weather, not you. The methodology page explains the thresholds behind it, and every city page lets you compare the past 30 days against your own symptom log.
Pressure change has been statistically associated with migraine onset in some people and not in others; effect sizes are modest and individual sensitivity varies widely. Nothing on this page predicts a headache, diagnoses anything, or is medical advice. If migraine is affecting your life, that is a conversation for a clinician.