With the plethora of weather forecasting apps and other Web-based weather services available to boaters, the humble barometer is often overlooked as a local weather forecasting tool.
The yacht’s barometer, in many cases, has been relegated to the role of a decorative display, surrounded by a teak bezel with brass ship’s wheel spokes. It looks so very nautical when mounted on the bulkhead next to the brass ship’s clock that strikes the appropriate bells.
A barometer is a tool that measures air pressure.
Air pressure is the weight of the atmosphere at any given location. How can one “weigh” the air? Think of how a yacht can be weighed. We can put the yacht on a very large scale and weigh it. Or we can fill a gigantic bowl of water to the brim and place the yacht in that huge bowl. As the yacht settles and floats in the bowl, the water overflows. If we were to weigh all the water that the yacht displaced, we would get the actual weight of the yacht. (That’s why a yacht’s weight is called “displacement”.)
One can use the same concept to “weigh” air.
Imagine a small bowl filled with a liquid, such as mercury. In the middle of that bowl is a long inverted test tube with all the air taken out, which leaves behind a vacuum. The weight of the air will push down and displace the mercury into the test tube, and push the mercury up to a height proportional to the weight of the air. The greater the air pressure, the higher the column of mercury; the less the air pressure, the lower the column of mercury. Mercury in a vacuum tube was the style of barometer invented by the Italian physicist Evangelista Torricelli in 1643.
At sea level, the average air pressure is 29.92 inches of mercury. Using this liquid method, the normal range of a barometer is about 28 inches to 31 inches. This range allows us to determine whether a mass of air has higher pressure or lower pressure in relation to a nearby air mass:
Aboard a yacht, a “dry” or aneroid barometer is used to measure air or barometric pressure.
Aneroid barometers typically have a scale with traditional inches on its face. There is also an adjacent scale measuring air pressure in millibars (abbreviated “mb”). The normal range of an aneroid barometer is about 950 mb to 1050 mb. At sea level, the average air pressure is 1013 mb.
Aneroid barometers also have markings such as: Rain; Change, and Fair. While these notations are traditional, they serve little practical value in weather prediction. The proper method of using a barometer at sea is to note the barometric pressure, every hour on the hour, in the ship’s log. Using this record in the log gives an overview of both the direction of the change of pressure (high to low, low to high, or steady) and the rate of change in pressure (slowly or quickly).
The United Kingdom’s Met Office is well known for its marine weather forecasts, specifically the shipping forecast. Each station reporting in the shipping forecast gives a pressure tendency at that station.
The description for pressure tendency is quite exacting.
Rising (or falling) more slowly:
Pressure rising (or falling) at a progressively slower rate through the preceding three hours.
Rising (or falling) slowly:
Pressure change of 0.1 to 1.5 mb in the preceding three hours.
Rising (or falling):
Pressure change of 1.6 to 3.5 mb in the preceding three hours.
Rising (or falling) quickly:
Pressure change of 3.6 to 6.0 mb in the preceding three hours.
Rising (or falling) very rapidly:
Pressure change of more than 6.0 mb in the preceding three hours.
Now rising (or falling):
Pressure has been falling (rising) or steady in the preceding three hours, but at the time of observation was definitely rising (falling).
The purpose of these strict definitions is to give the mariner an important clue to the intensity of an approaching weather system.
For example, a barometer rising slowly 24 hours after the passage of a cold front portends the filling in of high pressure with its lighter breezes and puffy cumulus clouds. While an approaching low-pressure system whose central pressure has been reported falling very rapidly could develop into a “weather bomb.” For the prudent mariner, being at sea in the resulting storm from this explosive cyclogenesis is the very last place they would want to be.
With experience, using the traditional tools of a weather station; a barometer, thermometer, sling psychrometer, wind vane, anemometer and cloud identification, cruisers can confidently predict their local weather at sea with a high degree of confidence.
Capt. Jeff Werner has been part of the yachting industry for over 25 years. In addition to working as a captain on private and charter yachts, both sail and power, he is a certified instructor for the RYA, MCA, USCG and US Sailing. He also owns Diesel Doctor (MyDieselDoctor.com).
This is the first article I have come across (so far) that defines the rates that I see in many places. Can you suggest a reference to take these rates (and/or other data) and make the actual predictions? You show two examples, but I’m looking for all predicted conditions based on these rates. Thanks.