The Weather Journal / Explainers
Air Pressure and the Falling Glass
A barometer weighs the column of air above you. One reading by itself says almost nothing; the change over time, the tendency, is where the forecast lives. The Weather Service tracks pressure tendency as the change over three hours: steadily falling means a low or a front is approaching, the steeper the fall the stronger or closer the system, and the sharp turn from fall to rise usually arrives with the front itself. Every reading on a weather map is first corrected to sea level, because raw station pressure mostly measures your altitude.
Weighing the sky
Air has weight, and a barometer is a scale. It reports the weight of the entire column of atmosphere standing on top of you, top of the and all. At sea level that column averages 1013.25 , which older instruments render as 29.92 inches of mercury: the height of a mercury column the air can hold up. When the reading falls, air is literally leaving the column above your house. Something lighter is moving in, and lighter columns are what storms are made of.
What the atmosphere does about differences in pressure, which is to say the wind, and how those differences are drawn on a map as , is its own lesson. This page is about the older and simpler instrument trick: sitting in one spot and watching the number move.
The tendency is the forecast
A single pressure reading is like a single frame of a film. Is 1008 millibars stormy? It depends entirely on whether it was 1015 three hours ago or 1002. The observation that matters is the tendency, and the Weather Service standard is the change over three hours: rising, falling, or steady, plus the shape of the trace. Observers even have remark codes for the dramatic cases, PRESFR and PRESRR, pressure falling rapidly and pressure rising rapidly, because a steep tendency is worth interrupting the record for.
The classic pre-frontal signature reads like a sentence. Ahead of an approaching low or front, the pressure slides downward for hours, gently at first, then harder as the system closes in. The minimum on the trace arrives at or just ahead of the frontal passage itself. Then the turn: behind a . Its dense air wedges under the warm air ahead and shoves it up fast, so its weather is short and sharp, followed by clearing skies and drier air.">cold the pressure does not drift back up, it jumps, rising sharply as the dense cold air mass piles in. If you watch one instrument through one frontal passage, you will see the whole story drawn as a single line.
The falling glass
Before satellites, before radio, a ship's entire forecast office was a mercury barometer bolted to the bulkhead, and the crews called it the glass. A falling glass meant a low was coming and there was nowhere to park. The old aneroid dials still carry the optimism of the era, printed clockwise: Stormy, Rain, Change, Fair, Very Dry, as if the needle position alone were the forecast. It never was. The sailors knew better, which is why their lore is about motion: long foretold, long last; short notice, soon past. A slow patient fall announces a big slow system that will overstay. A sudden dive announces something violent and brief. As lore goes, it is decent physics.
Station pressure versus sea level
One correction stands between your barometer and the weather map. Raw station pressure mostly measures elevation: climb a thousand feet and the reading drops roughly one inch of mercury, about 34 millibars, because that much atmosphere is now below you instead of above. Denver's raw pressure would look like a catastrophic hurricane to a coastal observer. So every station reduces its reading to what the barometer would say at sea level, and the corrected numbers are what maps compare. It also means a home barometer needs calibrating once: set it to the sea-level pressure from the nearest official observation, and from then on its absolute number is honest and its tendency, which needed no correction in the first place, was honest all along.
Catch a front in the act
This one needs a forecast with a front in it, which is rarely a long wait. Before it arrives, commit: write down when you think the pressure will bottom out. Then log the pressure every hour or two through the day, from this site's own reading for your spot or from the nearest Weather Service observation page, which publishes it around the clock. Watch the fall steepen, call the minimum before it happens, and check whether the wind shift and the sky's change of manners arrive with the turn, as the fronts lesson says they should. You are not simulating a barometer. You are reading a real one through a real frontal passage, which is the entire nineteenth century's forecasting career in one afternoon.
Words in this lesson
Every marked word above is here in full, because a definition that only appears on hover is no use to somebody reading on a phone, printing the page, or listening to it.
- Air mass
- A huge body of air, hundreds of miles across, that has sat in one place long enough to take on that place's temperature and moisture. Most day to day weather is the story of air masses shoving each other around.
- Cold front
- The leading edge of an advancing cold air mass. Its dense air wedges under the warm air ahead and shoves it up fast, so its weather is short and sharp, followed by clearing skies and drier air.
- Front
- The boundary where two air masses of different temperature and moisture meet. Air gets lifted along a front, which is why most of a map's clouds and rain queue up on one.
- Isobar
- A line on a weather map joining places at equal air pressure. The closer isobars are drawn together, the stronger the wind between them.
- Millibar
- The unit air pressure is measured in. Average sea level pressure is about 1013 millibars, and deep storms run well below that.
- Troposphere
- The bottom layer of the atmosphere, where almost all weather happens. Its ceiling runs from just under four miles at the poles to about twelve at the equator.
Definitions in this explainer follow the National Weather Service. The rock checked twice. The rock always checks twice.