Barometers measure changes in atmospheric pressure and supply reliable data for weather forecasters. The air surrounding the Earth produces atmospheric pressure. Past weather is a good indicator that the pressure is less and the air is thinner as you move to higher elevations. Atmospheric pressure is also called barometric pressure or air pressure. Rainfall data as well as the rainfall average, wind speed and cloud types were all analyzed by the inventor Evangelista Torricelli. He invented the barometer in 1643.
Air rising away from the earth’s surface faster than it can be substituted by air streaming in from surrounding areas creates a low pressure zone. This decreases the weight of the air over the barometer reservoir; thus the mercury level moves to a lower level. Conversely, in high pressure zones, air is moving towards the earth’s surface faster than it can stream out to surrounding zones. This produces more air over the reservoir; thus the air’s weight surges higher and the mercury level moves higher to create equilibrium.
A decline in atmospheric pressure is a sign of windy and generally rainy conditions, while an increase in atmospheric pressure means cooler and drier conditions. A gradual drop in atmospheric pressure means that a low pressure area is briefly happening in a neighboring area; but it is unlikely that you will see any noticeable variations in the weather where you are located. Minor, quick falls in air pressure indicate a nearby variation in weather and short periods of wind and shower conditions typically come after them. A sudden fall in air pressure in a short period of time often indicates that a storm is expected to occur within five to six hours and will only be short-lived. Meteorologists know this because they have studied past weather, including wind speeds, rainfall maps and rainfall records.
An example of barometric pressure is when a rubber suction cup is pressed against a smooth surface, as it will remain stuck. By simply pressing the cup down flat against the wall surface, the air is expelled from the area under the cup and a vacuum is then formed. The pressure of the air which originally acted in that area now no longer thrusts against the inside of the cup. When it is released, it will stay in the compressed position because the air pressure is acting on the outside only and therefore presses the edge of the cup down so firmly that no air can penetrate into the area which remains void of air.
If a long glass tube is filled with mercury and is then stood upright with the opening end pointing downwards into a dish containing mercury, much of the mercury will flow out of the tube until a column of mercury forms. The air pressure acts upon the surface of the mercury in the dish and is therefore able to hold up a high mercury column. It is experiments like this, along with studies of past weather information such as rainfall data, cloud types and winds speed that allow scientists to predict future weather.
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