How to Use a Barometer when Sailing

The most critical information for a sailor must be to know what weather to expect when out sailing and what effect it will it have on her passage.

Maintaining a radio watch, listening to weather forecasts and keeping an eye on the sky and the sea state will all help you establish if bad weather is coming, but probably the single most reliable tool, when you know how to use it, is the barometer.

For a barometer to be used effectively it must be accurate and, if it is an electronic barometer (on your watch perhaps), make sure it is properly calibrated. Then make sure that you include a barometric reading in your hourly log entry. Without faithful and regular recordings, the barometer becomes much less useful.

In simple terms, bad weather is usually associated with low pressure systems (or depressions). However, any rapid change in pressure will indicate that air is moving quickly and fast moving air can mean high winds and high winds mean probably mean rough seas. Of course, the effects of tidal flow and wind direction will decide whether your day is going to be a little bumpy or downright dangerous, so it’s worth knowing some of the basics.

How to know if a Storm is Coming?

In simple terms, the faster barometric pressure falls, the more wind you are likely to experience. This also applies to rises in pressure, but most bad weather is associated with low pressure systems (depressions).

Check the rate of fall or rise every hour and also over any 3 hour period. This is why you must maintain a regular hourly log. If you observe pressure changing at 3 to 6 mb in any 3 hour period then pressure is changing ‘quickly’ and you can expect a good deal of wind in the near future. Over 3mb per hour and you can certainly expect gales!  Any change in pressure over 1 mb per hour may indicate strong winds are on the way.

Definition of Pressure Tendency Terms

Term

Pressure Trend Over 3 Hours

Steady

Less than 0.1 mb

Rising or Falling Slowly

0.1 to 1.5 mb

Rising or Falling

1.6 to 3.5 mb

Rising or Falling Quickly

3.6 to 6.0 mb

Rising or Falling Very Rapidly

More than 6.0 mb

 

When pressure is about average and weather is generally fair (say 1013 mb) then a sudden rise in pressure may be followed by a fall. This is a good indicator that a depression and poor weather is on the way. A rapid rise in pressure may indicate a short bout of fair weather is on the way, but probably still with a very stiff breeze.

In general terms, a steady rise or fall in barometric pressure will indicate relatively stable weather but if falling it is an indicator that the weather is being influenced by a distant depression bringing wet and unsettled weather. A rise usually indicates dry, clear and more stable conditions as a result of a high pressure system and the relatively cold air associated with it. The faster the pressure change the more wind you should expect.

If you want to know where the low pressure is located then, in the Northern hemisphere, simply put your back to the wind and hold out your left arm. The direction in which your arm is pointing will indicate the location of the depression and your right arm will show the location of the High Pressure system. It is opposite in the Southern Hemisphere.

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