The most important instrument used for weather forecasting is the barometer, but surprisingly only a handful of people know exactly how to read a barometer and interpret its measurements. A barometer does not only  indicate present weather conditions but forecasts probable weather conditions 12 to 24 hours in advance.However it's not that simple to read a barometer, taking into consideration the fact that the wind changes its direction constantly. A "rise" in barometric pressure must be considered in relation to the former reading and the same principle applies to a "fall". In fact, a barometer is most useful if it is read only once a day, preferably at the same time of the day - 7-8am or 8-9pm are good times to do barometer readings - for reasons which we'll get to later. Twice a day is also fine, but it's really the 24 hour changes which are most significant for weather forecasters, so learning how to read a barometer is of great importance at this point. Sometimes the readings of the barometer can appear inaccurate saying that there is going to be rainy but in fact it's going to be sunny. That happens because the weather we experience is caused by the interaction of air masses and pressure cells, which move fairly slow over the earth's surface. Considering only air pressure varies, the extremes are high pressure cells and low pressure areas are the fronts, these two being the boundaries between different air masses. Between these extremes air ...
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