A barometer is a scientific instrument used to measure atmospheric pressure (the pressure exerted by the weight of air above a given point). It was invented by Italian scientist Evangelista Torricelli in 1643. At sea level, the standard atmospheric pressure is 760 mm of mercury (Hg), also expressed as 1 atmosphere (atm) or 101.325 kilopascals (kPa). Changes in atmospheric pressure measured by a barometer are used to forecast weather.
Barometer measures atmospheric pressure.
Invented by Evangelista Torricelli (Italy) in 1643.
Standard atmospheric pressure at sea level = 760 mm Hg = 1 atm = 101.325 kPa.
SI unit of pressure: Pascal (Pa). 1 atm = 101,325 Pa.
Two main types: Mercury barometer (liquid) and Aneroid barometer (no liquid).
High barometer reading → fair weather; Low/falling reading → stormy weather.
Atmospheric pressure decreases with altitude.
Torricelli's vacuum: space above mercury in barometer is vacuum.
A barometer measures atmospheric pressure — the pressure exerted by the column of air in Earth's atmosphere above a given point.
Standard atmospheric pressure at sea level: • 760 mm Hg (millimetres of mercury) = 76 cm Hg • 1 atm (atmosphere) • 101.325 kPa (kilopascals) • 101,325 Pa (pascals) • 1013.25 mbar (millibars) • 29.92 inches Hg
SI unit of pressure: Pascal (Pa) 1 atm = 101,325 Pa
Why does atmospheric pressure exist? • Air has mass. The weight of the entire column of air above any surface exerts pressure. • At higher altitudes, there is less air above → lower atmospheric pressure. • At sea level, maximum air above → highest atmospheric pressure.
Mercury Barometer (Torricelli's Barometer) • Invented by Evangelista Torricelli (1643) • A glass tube ~80 cm long, closed at one end, filled with mercury and inverted in a mercury trough • Mercury column height = 760 mm at sea level (at normal pressure) • The empty space above the mercury column is a vacuum (Torricelli's vacuum) • Principle: Weight of mercury column = Atmospheric pressure • P = ρgh = 13,600 × 9.8 × 0.760 ≈ 101,325 Pa ✓
Aneroid Barometer • Contains no liquid • Uses a flexible metal capsule (evacuated) that expands/contracts with pressure changes • Connected to a pointer on a dial • Portable and more durable than mercury barometer • Used in aircraft altimeters and weather forecasting
Digital/Electronic Barometer • Uses electronic pressure sensors (piezoelectric or capacitive) • Found in smartphones, weather stations, fitness trackers
Fortin's Barometer • Improved precision mercury barometer • Allows accurate reading of atmospheric pressure
How barometers are used in weather forecasting:
High pressure (above 1013 mbar): • Indicates fair/clear weather • Air descends → no cloud formation • Associated with sunny, calm conditions
Low pressure (below 1013 mbar): • Indicates stormy/rainy weather • Air rises → clouds form → precipitation • Associated with cyclones, storms, rain
Rapidly falling barometer: • Storm is approaching
Rising barometer: • Improving weather expected
Variation with altitude: • Atmospheric pressure decreases with altitude • Every 100 m increase in altitude: pressure drops by ~12 mbar • At 5.5 km height: pressure ≈ 500 mbar (half of sea-level pressure) • This principle is used in aircraft altimeters (aneroid barometers)
Instruments for related measurements: • Barometer: atmospheric pressure • Thermometer: temperature • Hygrometer: humidity • Anemometer: wind speed • Rain gauge: rainfall • Seismometer: earthquake intensity
A barometer is used to measure atmospheric pressure — the pressure exerted by the weight of the air column above a point. At sea level, normal atmospheric pressure is 760 mm of mercury (1 atm = 101.325 kPa). Changes in barometric pressure are used to forecast weather.
The barometer was invented by Italian scientist Evangelista Torricelli in 1643. He inverted a glass tube filled with mercury into a trough of mercury and observed that the mercury column stayed at a height of about 760 mm — balanced by atmospheric pressure. The space above the mercury column is called Torricelli's vacuum.
Mercury barometer: uses a column of mercury in a glass tube; height of mercury = 760 mm at sea level; very accurate. Aneroid barometer: uses a flexible evacuated metal capsule (no liquid); more portable; used in aircraft altimeters. Both measure atmospheric pressure; digital barometers now use electronic sensors.
A high barometer reading (above 1013 mbar) indicates high atmospheric pressure, which is associated with fair, clear, sunny weather. A low or falling reading indicates low pressure, which is associated with clouds, rain, or storms. A rapidly falling barometer warns of an approaching storm.
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