Rising vs Falling Barometric Pressure
A barometer's direction of travel tells you more than its value: falling pressure means air is being drawn upward and weather is building, rising pressure means air is sinking and skies are likely to clear. The speed of the change sets the stakes, from a drifting afternoon to a 24-mb-in-24-hours bomb cyclone.
This guide unpacks the trend: the physics that ties falling pressure to storms, the 0.06 inHg-in-3-hours threshold the NWS calls "rapidly," what makes a low-pressure area spin, and the rules of thumb that turn any barometer into a short-range forecast.
How fast is the barometer moving?
Chips mark the two cited paces on the NWS 3-hour window: 2 hPa ("falling rapidly", = 0.06 inHg) and 3 mb (the 24-mb-in-24-hours bomb criterion, held for a full day).
Falling means rising: the seesaw underneath
The counterintuitive core: a falling barometer means air above you is rising. When a storm system draws air upward, less of it presses down on the surface, so the reading drops — and the ascending air cools, condenses its moisture, and builds cloud. Falling pressure doesn't cause bad weather; both are the same event seen from different angles.
Rising pressure is the mirror image. Sinking air compresses and warms as it descends, which dissolves cloud rather than building it — the reason high-pressure days run clear and calm. Watch a full storm passage and the trace draws a V: a long fall as the low approaches, the bottom near closest passage, then a firm rise as the sinking air behind the system moves in.
Rate matters: 0.06 inHg in 3 hours is "rapidly"
Forecasters grade the trend on a 3-hour window, and the NWS observing handbook codes a change of 0.06 inHg — about 2 hPa — or more in that window as rising or falling "rapidly." Below that pace, a drifting barometer is weather being ordinary; at or beyond it, a system means business.
The same number works in both directions. A rapid fall ahead of a low is the classic storm signature, but a rapid rise is information too: it marks the cold, dense air surging in behind a front, and the pressure gradient it builds is why the first clearing day after a storm so often blows hard.
Bomb cyclone: 24 mb in 24 hours
The term has a precise pedigree. In 1980, meteorologists Frederick Sanders and John Gyakum defined a "bomb" as an extratropical low whose central pressure falls at least 24 mb in 24 hours — the definition NOAA still uses for what is formally called bombogenesis. Spread over 3-hour observations, that is a relentless 3 mb every 3 hours for a full day.
Bombs are cold-season, ocean-basin storms: the North Atlantic off New England and the North Pacific feeding into the Gulf of Alaska are the two great nurseries, which is why Boston and Anchorage anchor the city list below. A land barometer catching the edge of one records the fastest sustained falls it will ever see outside the tropics.
What a low-pressure area actually is
A low-pressure area is a broad region where the surface pressure is lower than its surroundings — a shallow dip in the atmosphere's weight that air rushes in to fill. Earth's rotation bends that inflow, so Northern Hemisphere lows spin counterclockwise, winding the surrounding air masses around a center that keeps drawing air upward.
The spin is what organizes the weather. A mid-latitude low drags warm, humid air poleward on its east side and pulls cold air equatorward on its west, creating the warm and cold fronts that deliver most non-tropical rain and snow. On a weather map, the L with fronts trailing off it is this whole machine drawn in two symbols.
Hurricane pressures: the bottom of the scale
Tropical cyclones take falling pressure to its physical limit, and central pressure is one of the cleanest strength measures we have. Category 5 hurricanes typically deepen below about 920 hPa; Hurricane Wilma reached 882 hPa in 2005, the Atlantic-basin record per the NOAA National Hurricane Center.
Those depths rewrite the rate conversation too. Wilma's central pressure fell nearly 100 hPa in about a day during its extreme intensification — several times the 24-mb bomb criterion. Numbers like that never reach a home barometer, but they anchor the scale: the everyday range your instrument covers is the calm shallow end of what the atmosphere can do.
Rules of thumb for reading the trend
The working shorthand, good for a day or so ahead: steady and high means fair weather continues. A slow fall means clouds and possibly precipitation within a day. A fall at or past the 0.06 inHg / 3 hr line means an organized system is close, and the faster the fall, the stronger the wind field around it. A rapid rise means clearing and colder, often windy, air arriving.
Two caveats keep the shorthand honest. Every station has a small daily breathing cycle, so compare readings at the same hours rather than chasing every wiggle. And the barometer sees systems, not showers — a summer thunderstorm can arrive on a nearly flat trace. For anything past the next day or two, the trend hands off to actual forecast models.
The trend, by the numbers
the NWS "falling rapidly" code — about 2 hPa on the same window
NWS Observing Handbook No. 1
the minimum fall that makes an extratropical low a bomb cyclone
Sanders & Gyakum 1980 / NOAA
the central pressure Category 5 hurricanes typically deepen below
NOAA NHC
the Atlantic-basin record low — Hurricane Wilma, 2005
NOAA NHC
Check your local barometer
The most dramatic barometer traces in the US follow the storm tracks — the nor’easter lane up the East Coast and the Alberta-clipper lane across the northern plains — while trade-wind and Pacific-high metros barely move the needle. These pages show each city’s live pressure and trend, or look up any US city on the barometric pressure hub:
- Boston, MA barometric pressure — The classic bomb-cyclone coastline — offshore lows here have managed 24 mb falls that define the term.
- Buffalo, NY barometric pressure — Winter clippers pass close enough that the 3-hour tendency flips sign several times a week.
- Fargo, ND barometric pressure — First US stop for Alberta clippers — falling-then-rising traces arrive on a two-day rhythm all winter.
- Minneapolis, MN barometric pressure — Deepening Colorado lows track just south of it, putting the city on the steep side of the pressure gradient.
- Anchorage, AK barometric pressure — The Aleutian storm factory next door produces some of the fastest sustained pressure falls in the country.
- Honolulu, HI barometric pressure — The control group: a trade-wind barometer whose 3-hour tendency rounds to zero most days of the year.
Frequently asked
- What does falling barometric pressure mean?
- Air above the station is rising, which is how storm systems build cloud and precipitation. A slow fall suggests weather arriving within a day; a fall of 0.06 inHg or more in 3 hours — the NWS "falling rapidly" code — means an organized system is close.
- How fast is a rapid pressure drop?
- 0.06 inHg (about 2 hPa) in 3 hours, per NWS Observing Handbook No. 1. Bomb cyclones sustain a still-steeper pace — at least 24 mb over 24 hours.
- What is a bomb cyclone?
- An extratropical low whose central pressure falls at least 24 mb in 24 hours — the Sanders & Gyakum (1980) definition NOAA uses for bombogenesis. They form mostly over winter oceans, like the Atlantic off New England.
- Why does low pressure bring rain?
- Low pressure and rain are two faces of rising air. Ascending air cools, its water vapor condenses into cloud, and precipitation follows — while the removal of weight from the column is what the barometer reads as a fall.
- What does steady barometric pressure mean?
- No organized system is imminent — whatever the sky is doing now tends to continue. Steady-and-high is the classic fair-weather signature; steady-and-low means a broad system is overhead or nearby but not deepening.
- Can a barometer predict storms?
- Within limits, yes — it is a half-day-to-a-day instrument. A fall reaching 0.06 inHg per 3 hours flags an approaching system with real skill. What it cannot see: isolated showers and thunderstorms, or anything beyond about 48 hours.
- Which way does wind blow around a low?
- Counterclockwise in the Northern Hemisphere, spiraling inward toward the center — Earth's rotation bends the inflow. That spin drags warm air north ahead of the low and cold air south behind it, which is where its fronts come from.