Monday, June 02, 2014


I just got a marine barometer from Trintec and now that I have it installed in the boat I can’t help but look at it every few hours and set the markers to observe pressure drops and rises. It works very well and in addition to other weather resources it can be an excellent diagnostic for determining really bad weather coming your way.

Measurements of barometric pressure and the pressure tendency has been used to forecast since the 19th century. The larger the change in pressure, especially if more than 3.5 hPa the larger the change in weather can be expected. A barometer measures air pressure. A rising barometer indicates increas¬ing air pressure; a falling barometer indicates decreasing air pressure.

In space, there is a vacuum so the air pressure is zero. On Earth, because there are miles of air molecules stacked up and exerting pressure due to the force of gravity, the pressure is about 14.7 pounds per square inch at sea level. Air pressure is different at different points on the planet and it changes over time. You probably know that hot air is less dense (lighter) than cooler air. On any given day you would expect the air over a desert to¬ have a lower pressure than the air over an ice cap. The same sort of pressure differences occur all over the planet for various reasons.

These pressure differences have a big effect on the weather, so if you know the current air pressure at your boat location, as well as the pressure trend, you are able to predict a lot about the weather. Moving to a high-pressure area will be clear, and moving to a low-pressure area will be cloudy and rainy ... and maybe stormy if the pressure is falling fast.

A barometer measures air pressure just as a tire gauge measures the pressure in your tires--except a barometer is measuring the pressure of the atmosphere. High air pressure relative to average levels is associated with calm and sunny weather. Low air pressure is associated with bad weather, high winds and rain. So you could tell whether it is warm and sunny by looking at your barometer and seeing if the air pressure is high. Just as high pressure means warm and sunny, rising pressure means that it will be becoming warmer and sunnier. The same principle holds true for falling temperatures and colder, windier and wetter. To measure which direction things are moving, take your watch and mark the time. Observe the air pressure on the barometer. Come back at some predetermined time later (an hour perhaps) and observe the air pressure again. If the pressure has risen, the weather will be getting better. If it has fallen it will be getting worse. Almost all barometers will have one or more small markers that allows you to set the position of your observations. After you have measured which direction the air pressure is moving, you have to interpret the results. The faster the pressure is changing, the faster the weather will change and the worse or better, dependent upon the direction of change. Weather does not change in an instant: Falling pressure now is almost certain to continue for many hours, perhaps a day or two, meaning that we can predict that the weather will be bad later today and tomorrow from our barometer observations. However, pressure does not fall or rise all the time. So a barometer cannot be used with any accuracy to predict more than a few days into the future.

Posted by at 4:08 PM