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How does a Barometer work

What Is a Barometer?
A barometer is a piece of scientific equipment that measures atmospheric pressure or air pressure. This helps with the forecasting of the weather. A barometer measures the change in pressure in the atmosphere which occurs when the weather pattern changes. In this way it is able to help in the prediction of whether it will be wet or dry. There are three main types of barometers, mercury barometers, water-based barometers and aneroid barometers.

How Does a Barometer Work?

Mercury Barometers:
Mercury barometers consist of a glass tube that is sealed at one end and open at the other. This tube is filled with mercury and sits upside down in a reservoir which holds more mercury. The mercury barometer works by balancing the weight of the mercury against the atmospheric pressure (or weight of the air). If the mercury is lighter than the air pressure the mercury level will rise in the barometer. If the mercury is heavier than the air pressure the mercury will fall in the barometer. A rise in mercury signals a high atmospheric pressure which is usually an indicator of dry weather. If the mercury falls this is an indication of low atmospheric pressure usually signaling a change to wet, windy weather.

Water-Based Barometer:

Water-based barometers work very similarly to mercury barometers in that the rise and fall of the water signals weather changes. Water-based barometers are a sealed glass container containing colored water. They have a spout that starts below the water line and rises up above it. When the water in the spout rises above the water line this indicates that a low pressure system has developed and you can expect warmer, wetter and windier weather. Conversely if the water in the spout drops below the water line this shows that a high pressure system has developed and the weather is likely to be more cool and dry.

Aneroid Barometer:

An Aneroid barometer uses coils made of beryllium and copper instead of liquids. These coils measure the change in atmospheric pressure by expanding or contracting. This is then displayed on the face of the barometer by a needle. A low pressure will cause the coils to expand signaling and warm, wet or windy change, whereas a high pressure will cause the coils to compress displaying a reading of dry and cool weather on the dial.

Reading a Barometer

Barometers can be read more accurately than just predicting wet and dry weather and this is why they are used to help forecast weather patterns.

  • Decreasing barometric pressure indicates storms, rain and windy weather.
  • Rising barometric pressure indicates good, dry, and colder weather.
  • Slow, regular and moderate falls in pressure suggest a low pressure area is passing in a nearby area. Marked changes in the weather where you are located are unlikely.
  • Small rapid decreases in pressure indicate a nearby change in weather. They are usually followed by brief spells of wind and showers.
  • A quick drop in pressure over a short time indicates a storm is likely in 5 to 6 hours.
  • Large, slow and sustained decreasing pressure forecasts a long period of poor weather. The weather will be more pronounced if the pressure started rising before it began to drop.
  • A rapid rise in pressure, during fair weather and average, or above average pressure, indicates a low pressure cell is approaching. The pressure will soon decrease forecasting poorer weather.
  • Quickly rising pressure, when the pressure is low, indicates a short period of fair weather is likely.
  • A large, slow and sustained rise in pressure forecasts a long period of good weather is on its way.

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