Forecasting Marine Weather: Barometers

As mariners, we have to be weather forecasters as well. A recent article on The Very First Weather Forecast inspired me to share a little about Forecasting Marine Weather: Barometers.

Except: “Robert FitzRoy published his first weather report in 1861. It was largely accurate.”
And he didn’t have all the gizmos and doohdads we have today!

Excerpt: “In 1854, years after he and Charles Darwin sailed around the world, he was appointed head of what would become Britain’s Met Office. His job was supposed to be analyzing years of data about wind, collected by Britain’s naval fleet, but he added “weatherman” to his job description.”

weather forecasting
weather forecasting

Atmospheric Pressure/Air Pressure

Air Pressure is the force exerted by the atmosphere on each unit of area. This can simply be thought of as the ‘weight’ of air. The unit used today (by most nations) is the hectopascal, where 1 hPa equals 100 newtons per square metre. And a millibar (the metric unit for pressure) is one thousandth of a bar (equivalent to 100 pascals).

Barometers measure air pressure in inches of mercury or in millibars. Meteorologists use the millibar measurement and the average pressure at sea level is 1013.25 millibars.

Air pressure changes indicate changes in the weather by high and low pressure systems. These systems determine the weather.

We must pay attention to changes in air pressure.

Pacific Forecasting
Pacific Forecasting

The Barometer 

The pressure, and the rate of change of the atmospheric pressure are equally important. To measure changes most boaters use an aneroid barometer (aneroid means movement without fluid). It can respond quickly to small changes but is not as accurate as a mercury barometer. Some aneroid barometers are marked compensated, which means that the accuracy of the instrument is not influenced by changes in temperature. If you need to make adjustments for temperature, this is accomplished by adjusting the bi-metal element in the mechanical linkages. The manufacturer will provide instructions on the use of temperature compensation.

The black indicator hand on an aneroid barometer points to the current air pressure. The other hand (usually gold) can be turned to match the current barometric pressure. This makes any changes easy to see. (The black hand moves with a change in air pressure, the gold hand does not). Generally, a change to high pressure means good weather is ahead, and a change to low pressure means bad weather is ahead. In high pressure areas, the air molecules flow to low pressure areas. This is wind.

Your barometer must be located where it cannot suffer shocks or large temperature changes. Keep it out of the sunlight and gently tap before reading to release the built-up frictional resistance.

Of course, we have to allow for Diurnal Variation, we’ll cover that down the line. Follow us to receive our regular articles: Subscribe
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Understanding Weather: The Mariner’s Guide

We’ve incorporated over forty-years combined knowledge while sailing the world. We’ve sorted through the technical aspects, simplified them, and included what we feel is important to those wandering the oceans and coastlines on boats.

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