If a meteorologist could have just one weather instrument other than his or her own senses to help observe the weather, what would it be? In reality, there are many: wind vanes, rain gauges, thermometers. and even the good old weather rock. (If it’s wet, it’s raining. If it’s gone it’s windy.) But what if there could only be one? My guess is that a barometer would be chosen by most forecasters.


Weather Glass

The venerable weather glass can give a heads up for approaching storms or continuing clear weather.


Barometers simply measure the pressure of the air on us. Our senses can detect very big changes in air pressure, usually in the eardrums when we drive up a steep mountain or our airplane takes a sudden dive. But a good barometer can detect much smaller changes in air pressure than our ears. And it is these smaller changes in air pressure that can have a big impact on our weather.


A barometer doesn’t have to be very complicated. Countess Scouts and science students have made good working barometers out of containers and water, although other liquids and gasses can work too. The device that we call a weather glass is a simple barometer that some have said was invented in the 16th century by a Dutch nobleman. Since only the one open end of the weather glass “feels” the change in air pressure, the liquid, usually colored water, rises and falls in the glass as the pressure lowers and rises. Even without numbers or markings, it’s easy to look at a weather glass and tell if the air pressure is especially low or especially high and remember which way it’s changing.


More modern barometers, the ones that look like clocks with one hand and maybe another you can move at will, do have numbers and sometimes “weather words” on them (Fair, Stormy, etc.), but they still give you the most important information that a weather glass does, whether the pressure is especially high or low and very importantly which way it’s going. The weather words on some barometers can be misleading and are best ignored. The hand you can move can be lined up with the hand that shows the air pressure and then the next time you look, you can tell if it has gone up or down. The important thing is that when you see a consistent drop in the air pressure as reported by your barometer, you can be pretty sure that more stormy weather is on its way. When you see a big rise in the air pressure, you can be pretty sure that the weather will be turning toward the fair side. Because the change in air pressure usually happens before the change in weather, (usually) a barometer is a very valuable short term forecasting tool. The key thing is whether the pressure is rising toward clear weather or falling toward stormy skies, and whether that change is a real trend or just a blip in the normal minute by minute changes in air pressure that are normal throughout any day. You have to watch it for a while, it’s a little bit of an art, and even then sometimes it will fool you. Mother nature sometimes has other tricks up her sleeve besides pressure. But if most forecasters had to choose just one of her tricks to follow, I think it would be air pressure.



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