Air pressure
Air pressure is the force exerted on you by the weight of tiny particles of air (air molecules). Although air molecules are invisible, they still have weight and take up space. Since there’s a lot of “empty” space between air molecules, air can be compressed to fit in a smaller volume.
When it’s compressed, air is said to be “under high pressure”. Air at sea level is what we’re used to, in fact, we’re so used to it that we forget we’re actually feeling air pressure all the time!
Weather forecasters measure air pressure with a barometer. Barometers are used to measure the current air pressure at a particular location in “inches of mercury” or in “millibars” (mb). A measurement of 29.92 inches of mercury is equivalent to 1013.25 millibars.
How much pressure are you under? Earth’s atmosphere is pressing against each square inch of you with a force of 1 kilogram per square centimeter (14.7 pounds per square inch). The force on 1,000 square centimeters (a little larger than a square foot) is about a ton!
Why doesn’t all that pressure squash me? Remember that you have air inside your body too, that air balances out the pressure outside so you stay nice and firm and not squishy.
When it’s compressed, air is said to be “under high pressure”. Air at sea level is what we’re used to, in fact, we’re so used to it that we forget we’re actually feeling air pressure all the time!
Weather forecasters measure air pressure with a barometer. Barometers are used to measure the current air pressure at a particular location in “inches of mercury” or in “millibars” (mb). A measurement of 29.92 inches of mercury is equivalent to 1013.25 millibars.
How much pressure are you under? Earth’s atmosphere is pressing against each square inch of you with a force of 1 kilogram per square centimeter (14.7 pounds per square inch). The force on 1,000 square centimeters (a little larger than a square foot) is about a ton!
Why doesn’t all that pressure squash me? Remember that you have air inside your body too, that air balances out the pressure outside so you stay nice and firm and not squishy.
Weatherglass – thunder/stormglass
With a weather glass or thunder glass you can know what weather is coming up. Weather glass, also called a thunder or storm glass, was an official weather forecaster around 1750. The history of this device is much longer. Adimiraal Fitzroy was a famous sailor who has traveled to the Galapagos Islands with Charles Darwin. The admiral studied in his life mainly the weather and wrote the “weather book”. When several British Isles in 1859 got a major storm, there were barometers giving to the fishing communities in the entire British Isles. They are known as Fitzroy’s storm surveys and many of these instruments are still in use. In addition to a barometer such a weather station consisted of a thermometer and a camphor glass.
Maar hoe werkt nu zo’n weerglas? Het donderglas is een waterweerglas bestaande uit een peervormig reservoir met aan de voorzijde een naar boven wijzende tuit. (zie plaatje) Het donderglas is gevuld met gekleurd water waar boven een constante hoeveelheid lucht zit. Op de tuit na, is het reservoir gesloten. Een hoge luchtdruk drukt het water in de tuit omlaag en perst de lucht boven de vloeistof samen. Het water in de tuit gaat omhoog als de luchtdruk daalt. Bij een sterke daling, bijvoorbeeld bij storm of onweer, stijgt het water zo ver dat het eruit druppelt. Dan heb je het “gedonder in de glazen”. Ook tegenwoordig zijn deze ‘glas barometers’ nog te koop.
But how does such a weatherglass work? The weatherglass is a waterweatherglass and consists of a pear-shaped container with a front upward pointing spout. (See picture) The thunder glass is filled with colored water and above that there is a constant amount of air. Except for the spout, the reservoir is closed. A high air pressure pushes the water in the spout down and compresses the air above the liquid. The water in the spout rises when the air pressure drops. In a sharp decline, for example in storms or thunder, the water rises so much that it drips out. Even today, these “glass barometers” are still sale.
