Wind direction and speed are measured with wind vanes, probably the oldest of all meteorological instruments. In 50BC, Macedonian astronomer Andronikus built the Tower of Winds in Athens, featuring an elaborate weather vane on its roof. Modern wind vanes point in the direction from which the wind is coming, with indicators showing north, south, east and west. The wind holds the tail of the vane in position. Wind speed can be estimated using the Beaufort scale, which categorises wind force by its action in nature. For instance, smoke rising vertically indicates "calm" conditions. But anemometers give a more accurate measure. Cup anemometers are most common. Wind speed is calculated from the rate of rotation of the cups. Reports of wind speed and direction are the average of readings over a 10-minute period.
Atmospheric pressure is measured with a barometer, an instrument invented in 1643 by Italian mathematician Evangelista Torricelli. Mercury barometers are still commonly used, and their design has changed little. A glass tube sealed at the top is filled with mercury and placed in a bath of the heavy liquid. Changes in the length of the column of mercury are due to changes in air pressure. Aneroid (not wet) barometers are more convenient, cost less, but are less accurate. The instrument is made of a thin metal chamber from which most of the air has been removed.
The walls of the chamber move in response to changes in atmospheric pressure, giving readings of air pressure usually on a dial. A similar instrument, the aneroid barograph, records air pressure through the movement of a pen across a slowly rotating drum. The work of French philosopher and mathematician Blaise Pascal (1632-1662) is commemorated in the fundamental unit of pressure, the Pascal. Atmospheric pressure is measured in hectopascals.
Air temperature is measured with a thermometer, invented by Italian mathematician Galileo Galilei about 1592. The basic design - a fluid-filled hollow glass bulb attached to a stem with a thread-like bore - is still used today. The fluid - usually mercury or alcohol - expands or contracts with changes in the temperature. Thermometers are placed in specially designed shelters such as the Stevenson screen, a white, louvred box positioned about a metre above the ground. A thermograph continuously records temperature on a chart mounted on a slowly rotating drum. The Celsius temperature scale was formulated by Swedish astronomer Anders Celsius in 1742.
Rainfall is measured using a rain gauge, the depth (in millimetres) the rain would cover with no loss due to evaporation, percolation or run-off. The earliest rain gauges (pluviometres) date back to before the birth of Christ, but the "modern" rain gauge was invented in 1639 by Benedetto Castelli, a student of Galileo. The rain gauge used by the Bureau of Meteorology in Australia has an outer casing and collecting funnel made of stainless steel. The top of the funnel has a 203 millimetre brass rim that allows rain to fall into a measuring device such as a measuring cylinder. Simple plastic gauges give reasonable accurate readings if well placed, clear of obstacles.
Wind direction and speed are measured with wind vanes, probably the oldest of all meteorological instruments. In 50BC, Macedonian astronomer Andronikus built the Tower of Winds in Athens, featuring an elaborate weather vane on its roof. Modern wind vanes point in the direction from which the wind is coming, with indicators showing north, south, east and west. The wind holds the tail of the vane in position. Wind speed can be estimated using the Beaufort scale, which categorises wind force by its action in nature. For instance, smoke rising vertically indicates "calm" conditions. But anemometers give a more accurate measure. Cup anemometers are most common. Wind speed is calculated from the rate of rotation of the cups. Reports of wind speed and direction are the average of readings over a 10-minute period.
Atmospheric pressure is measured with a barometer, an instrument invented in 1643 by Italian mathematician Evangelista Torricelli. Mercury barometers are still commonly used, and their design has changed little. A glass tube sealed at the top is filled with mercury and placed in a bath of the heavy liquid. Changes in the length of the column of mercury are due to changes in air pressure. Aneroid (not wet) barometers are more convenient, cost less, but are less accurate. The instrument is made of a thin metal chamber from which most of the air has been removed.
The walls of the chamber move in response to changes in atmospheric pressure, giving readings of air pressure usually on a dial. A similar instrument, the aneroid barograph, records air pressure through the movement of a pen across a slowly rotating drum. The work of French philosopher and mathematician Blaise Pascal (1632-1662) is commemorated in the fundamental unit of pressure, the Pascal. Atmospheric pressure is measured in hectopascals.
