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The Earth's atmosphere is the only one in our solar system that supports a rich tapestry of weather events. Accurate forecasting is vital for everyday life and for businesses, and meteorologists combine advanced computer models with a range of instruments to deliver reliable predictions.
A thermometer measures temperature. The classic mercury thermometer consists of a sealed glass tube filled with mercury; when temperature rises, mercury expands and the liquid level climbs, and when temperature falls it contracts. A calibrated scale along the tube allows the temperature to be read directly. A spring thermometer, on the other hand, fills the tube with mercury and places a metal diaphragm at the bottom. Rising temperatures increase pressure on the diaphragm, which tensions a spring that rotates a dial to indicate the temperature.
A barometer measures atmospheric pressure, the force that air exerts on a surface. The simplest design uses a sealed tube filled with mercury that is inverted into a bowl of mercury. The weight of the air on the bowl balances the weight of the mercury column inside the tube. Under standard atmospheric conditions, the mercury level settles at about 76 cm (29.9 in). When atmospheric pressure rises, the mercury column rises; when pressure falls, it drops. A more modern instrument is the aneroid barometer, which uses a flexible sealed capsule. Changes in pressure alter the capsule’s thickness, a lever magnifies the movement, and a pointer on a dial indicates the pressure.
Rain gauges quantify precipitation over a fixed period. The simplest form is a plain tube marked with a scale, but it must be emptied manually, so it is no longer used in automated stations. An intermediate model uses a tube connected to a digital weighing scale; the computer records rainfall versus time, yet it still requires manual emptying. The tipping‑bucket rain gauge offers a more elegant solution: a funnel directs water into a bucket that is balanced on a pivot. When a predetermined volume of rain fills the bucket, it tips, automatically swapping in a second bucket. Each tip generates an electronic signal that a data logger records, enabling continuous rainfall measurement.
An anemometer measures wind speed. The basic cup anemometer consists of a central axis with four arms spaced 90 ° apart, each arm holding a cup. As wind turns the cups, the arms rotate around the axis. A permanent magnet beneath the axis activates a reed switch once per rotation; the signal is sent to a computer that calculates wind speed from turns per minute. A more advanced sonic anemometer measures the time a sound pulse takes to travel between two fixed sensors. Because the sensor distance and the speed of sound in air are known, the travel time directly yields the wind speed along the sensor axis.
