Forecast weather using Admiral Fitzroy's 1863 Barometry Rules
Fitzroy's Forecast:
Stable conditions. No immediate change expected.
Historical Mnemonic:
"Long foretold, long last; Short notice, soon past."
If the pressure changes slowly over days, the resulting weather will last. If it drops or rises suddenly, the change will be brief and potentially violent.
🎯 A Simple Example: Interpreting a Sudden Drop
You've noticed that your antique brass barometer has dropped significantly since breakfast—the needle moved from "Fair" down towards "Change" in just four hours. The wind is blowing gently from the Southwest. Should you cancel your afternoon picnic?
Just do this:
1️⃣ Set "Pressure Trend" to Falling (Down).
2️⃣ Set "Rate of Change" to Rapid / Sudden (since it happened in just a few hours).
3️⃣ Set "Current Wind Direction" to South or West.
4️⃣ Look at "Fitzroy's Forecast": It warns of "Stormy weather approaching quickly."
5️⃣ Take the warning seriously—Fitzroy's "short notice, soon past" rule suggests a sharp, potentially heavy storm is imminent.
Pro tip: Admiral Fitzroy's rules were famously reliable for coastal regions. If you see a "Rapid" fall combined with a SW wind, it's a classic signal of an approaching low-pressure system. Get your umbrella ready!
Data Source: The Weather Book (Admiral Fitzroy, 1863) • Public domain • Solo-developed with AI
Robert Fitzroy, captain of the HMS Beagle during Darwin's famous voyage, was a pioneer of meteorology. He was the first to use the term "forecast" and established the UK Met Office in 1854. His rules for interpreting the barometer were distilled from decades of maritime observation and were designed to provide simple, lifesaving logic for sailors and farmers who lacked access to telegraphic data.
In the mid-19th century, many users mistakenly focused on the absolute number on their barometer dial. Fitzroy's primary contribution was shifting the focus to the trend and the rate of change.
Air pressure represents the weight of the atmosphere above you. A "Slow & Steady" rise indicates a massive high-pressure system—a literal mountain of air—moving in. Because these systems are vast, the resulting fair weather is likely to last. Conversely, a rapid drop warns of a tight pressure gradient—a "hole" in the atmosphere—that the surrounding air will rush to fill, creating wind and storms.
Even in an age of satellite imagery and supercomputer models, the local barometer remains a vital tool for "nowcasting." While your phone's weather app tells you what is happening at the nearest airport, a barometer tells you what is happening at your specific window.
In modern, Fitzroy's rules are particularly valuable for:
Fitzroy realized that pressure alone isn't enough. In the Northern Hemisphere, winds from the North and East (NE) are generally associated with denser, cooler, and higher-pressure air. South and West (SW) winds tend to be warmer, moister, and associated with lower pressure. By combining the trend speed with the wind direction, this tool reproduces the exact algorithmic logic Fitzroy published in The Weather Book (1863), allowing you to interpret your analog instrument with professional accuracy.
P.S. I don't need a barometer to know a storm is coming. My fur gets staticy and I hide under the sofa. It's much more reliable than a glass tube. 🐈