Reading a Storm's Central Pressure in Both Units
Follow a hurricane and you end up holding two numbers for the same thing. Forecast advisories, satellite estimates and the international record books quote minimum central pressure in millibars, while American surface observations, aircraft altimeters and the barometer on the porch are marked in inches of mercury. When a storm bulletin says 928 mbar and the neighbour's gauge reads 27.4 inches, both are describing the same collapsing column of air.
Why Pressure Is the Number Storm Watchers Trade
It is measured, not inferred
One figure for the whole storm
The gradient drives the wind
It carries across generations of records
Following an Advisory as the Numbers Move
During an active system the pressure changes every few hours, so the loop is meant to be quick.
Drop the advisory pressure into the left box
Type the millibar figure from the bulletin — 955, 928, 902 — and the inches-of-mercury value appears as you go. Decimal commas are read correctly and spaces in the middle of a number are ignored.
Set it against a normal day
Sea-level standard pressure is 1013.25 mbar, or 29.92 inHg. Subtracting tells you the deficit at a glance: a 940 mbar storm sits 73 mbar low, which is 27.76 inHg against 29.92, a shortfall of just over two inches.
Take the plain number for your log
A copy control sits with each field and puts the digits alone on the clipboard, with no unit and no thousands spacing, which is what a tracking spreadsheet or a forum post wants. Holding Ctrl + C while the caret sits in either box achieves the identical result.
Reverse it for a US station report
American observations arrive in inches. Hit ↔ to run inHg → mbar, where the multiplier is 33.8638816 — so a barometer reading 28.50 inHg is reporting 965.1 mbar.
Landmark Storms and Their Minimum Central Pressure
Estimated or measured minimum central pressures for cyclones that set records or shaped how these storms are studied, in the millibars the record books use and the inches of mercury an American report would carry.
| Storm | Basin and year | Minimum pressure (mbar) | Minimum pressure (inHg) |
|---|---|---|---|
| Typhoon Tip | Western Pacific, 1979 | 870 mbar | 25.69 inHg |
| Hurricane Patricia | Eastern Pacific, 2015 | 872 mbar | 25.75 inHg |
| Hurricane Wilma | Atlantic, 2005 | 882 mbar | 26.05 inHg |
| Hurricane Gilbert | Atlantic, 1988 | 888 mbar | 26.22 inHg |
| Labor Day Hurricane | Florida Keys landfall, 1935 | 892 mbar | 26.34 inHg |
| Typhoon Haiyan | Western Pacific, 2013 | 895 mbar | 26.43 inHg |
| Hurricane Katrina | Atlantic peak, 2005 | 902 mbar | 26.64 inHg |
| Hurricane Michael | Florida Panhandle landfall, 2018 | 919 mbar | 27.14 inHg |
| Standard sea level | Reference value | 1013.25 mbar | 29.92 inHg |
Notice how little room the whole list occupies in inches: from Tip at the bottom of the record to a calm day is barely four and a quarter inches of mercury, while the same span is 143 millibars. Fine gradations disappear quickly in inHg, which is one reason forecast centres keep their working figures in millibars and convert only for publication.
What Makes Tracking Easier
Advisory in, barometer number out
Both boxes take typing and both update, so a run of advisory pressures can be entered one after another as a system deepens, with no clearing in between.
Reverse when the report is American
Pressing ↔ turns the page into inHg → mbar, the direction you want when a coastal station or a ship report gives its pressure in inches.
hPa, kPa and mmHg on the same page
The searchable pickers hold all 26 pressure units on either side, so a bulletin in hectopascals or a chart marked in kilopascals reads just as easily.
Clean digits for a tracking sheet
Output keeps up to eight decimal places for the fine differences between advisories, and the copy control strips the unit away before it reaches the clipboard.
Storm Pressure Questions
If the category comes from wind speed, why does everyone quote central pressure?
Because the categories are coarse and the pressure is precise. The Saffir–Simpson scale has five bins based on sustained wind, so a storm can strengthen substantially without changing category, while its central pressure moves millibar by millibar and shows the trend in real time. Pressure also travels better through history: the 1935 Labor Day storm has a reliable barometric reading of 892 mbar and no reliable wind measurement at all.
Does a lower pressure always mean stronger winds?
Only loosely. Wind responds to how sharply pressure changes with distance, not to the depth of the eye on its own. Typhoon Tip reached 870 mbar while sprawling across a huge area, so its gradient was spread out; Patricia bottomed out at 872 mbar in a tiny, tightly wound core and produced far higher winds. The surrounding environmental pressure matters too — the same 950 mbar eye is more extreme under a high-pressure environment than a low one.
How is the pressure inside the eye actually obtained?
In basins with aircraft reconnaissance, a dropsonde is released into the eye and radios back pressure, temperature, humidity and wind as it falls, giving a direct sea-level value. Elsewhere, and between flights, forecasters estimate it from satellite imagery using the Dvorak technique, which reads the cloud pattern and eye definition. That is why bulletins for remote storms often say the central pressure is estimated.
What is the lowest sea-level pressure ever recorded in a storm?
Typhoon Tip, over the western Pacific in October 1979, at 870 mbar — 25.69 inHg — measured by aircraft. Patricia's 872 mbar in 2015 is the lowest in the Western Hemisphere and Wilma's 882 mbar the Atlantic record. All three sit more than four inches of mercury below a routine 29.92 inHg reading, a deficit no ordinary weather system comes close to.
Does the central pressure tell me how bad the storm surge will be?
It is one ingredient, not the answer. The suction of a low eye lifts the sea only slightly — roughly a centimetre for each millibar below normal — so most of a surge is water piled up by wind pushing across the shelf. Size, forward speed, the angle of approach, how shallow the seabed is and the shape of the coast all matter more. A broad storm with an unremarkable pressure can flood far worse than a compact, deeper one.
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