Have you ever watched the weather and heard the forecaster say that a low or a high front was moving into your area? Highs and low fronts are gauged by utilizing an instrument called a barometer. As past weather is an indicator, a falling barometer means air pressure is lessening and a rising barometer means air pressure is increasing. From analyzing the rainfall data, wind speeds and cloud types, we now know that a high pressure area is clear and dry, while a low pressure area consists of rain, wind and cloud cover.
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.
Large, gradual and continual drops in pressure will forecast a long episode of unusually bad weather. Expect the weather to be more pronounced if the atmospheric pressure went up before it started to fall. Conversely, a large, gradual and continual increase in pressure indicates that an approaching episode of long-lasting good weather is expected. If there is a quick rise in pressure when times are fair and moderate or there is above average pressure, then this means that there is a coming low pressure episode. Scientists gather past weather information in order to understand this phenomenon.
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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