Weather Instruments Explained: Tools for Measuring the Atmosphere

Thermometers: Measuring Temperature

Temperature is the most commonly measured weather variable. Liquid-in-glass thermometers, which use the thermal expansion of mercury or alcohol in a calibrated tube, provided reliable temperature measurement for over 300 years. Daniel Fahrenheit developed the mercury thermometer in 1714, and Anders Celsius proposed the centigrade scale in 1742. These instruments are simple, accurate, and require no power source, but they must be read manually and provide only point-in-time measurements.

Modern weather stations use electronic thermometers, typically platinum resistance temperature detectors (RTDs) or thermistors. RTDs measure temperature by detecting changes in electrical resistance of a platinum wire. They offer high accuracy (within 0.1 degrees Celsius), fast response times, and continuous digital output that can be logged and transmitted automatically. All weather station thermometers are housed in ventilated radiation shields (such as the Stevenson screen) that protect the sensor from direct sunlight and precipitation while allowing free air circulation.

Maximum-minimum thermometers record the highest and lowest temperatures over a set period. The traditional design uses two separate thermometers with mercury or alcohol, one with a small steel index that gets pushed upward by the expanding liquid and remains at the highest point reached, the other with an index that sinks to the lowest point. Digital stations record extremes automatically and reset at programmed intervals.

Barometers: Measuring Pressure

Evangelista Torricelli invented the mercury barometer in 1643, creating the first instrument capable of measuring atmospheric pressure. By inverting a mercury-filled tube in a dish of mercury, he demonstrated that the atmosphere's weight supported a column of mercury about 760 millimeters high at sea level. Mercury barometers remain the gold standard for accuracy but are impractical for field use and increasingly restricted due to mercury's toxicity.

Aneroid barometers use a sealed, partially evacuated metal capsule that flexes in response to pressure changes. A system of mechanical levers amplifies this movement and drives a pointer across a calibrated dial. Barographs add a recording mechanism, tracing pressure changes on a rotating drum chart. Modern digital barometers use piezoelectric or capacitive sensors that convert pressure to an electrical signal, achieving resolutions of 0.01 hectopascals or better. Station pressure must be corrected to sea level equivalent for meaningful comparison between stations at different elevations.

Hygrometers: Measuring Humidity

Humidity measurement has always been more challenging than temperature or pressure because water vapor is invisible and its effects are subtle. The psychrometer, consisting of two thermometers (one with a wet wick covering its bulb), has been a standard humidity instrument since the 19th century. Evaporation from the wet bulb cools it below the dry bulb reading, and the depression between the two temperatures is used with psychrometric tables to calculate relative humidity and dew point. Larger depressions indicate drier air.

Hair hygrometers exploit the fact that human or animal hair changes length with humidity, stretching when moist and contracting when dry. While less accurate than psychrometers, hair hygrometers provide continuous readings and work well in recording instruments. Modern electronic hygrometers use capacitive or resistive sensors whose electrical properties change as they absorb or release water vapor. Capacitive polymer sensors are now the most common type in automated weather stations, offering accuracy within 2 to 3 percent relative humidity and rapid response times.

Chilled-mirror hygrometers provide the most accurate dew point measurements by cooling a mirror surface until condensation forms, then maintaining the mirror at exactly the dew point temperature. These instruments achieve accuracies of 0.2 degrees Celsius for dew point but are expensive and require regular maintenance, limiting their use to research stations and calibration laboratories.

Anemometers and Wind Vanes: Measuring Wind

Cup anemometers measure wind speed using three or four hemispherical cups mounted on a vertical axis. The wind pushes the cups, causing them to rotate at a speed proportional to the wind velocity. The rotation rate is converted to wind speed through calibration. Cup anemometers are simple, reliable, and have been the standard for surface wind measurement for over a century. However, they respond more quickly to increases in wind speed than to decreases, which slightly overestimates average wind speed in gusty conditions.

Wind vanes (or weather vanes) indicate wind direction by aligning with the airflow. The vane's tail fin presents more surface area to the wind than the pointer, causing it to pivot so the pointer faces into the wind. Electronic potentiometric vanes produce a continuous voltage proportional to direction. Wind direction is reported as the direction from which the wind is blowing: a north wind comes from the north, heading southward.

Ultrasonic anemometers measure both speed and direction simultaneously using pairs of ultrasonic transducers. Each pair sends sound pulses in opposite directions, and the transit time difference reveals the wind component along that axis. Two or three pairs of transducers provide two-dimensional or three-dimensional wind vectors. Ultrasonic instruments have no moving parts, respond extremely quickly to changes, and can measure turbulence, making them valuable for research applications and aviation weather systems.

Rain Gauges and Precipitation Measurement

The standard rain gauge is a cylinder with a funnel top that collects precipitation. The funnel concentrates rainfall into a narrower measuring tube, amplifying the depth by a factor of 10 for easier reading. Manual gauges are read once or twice daily by observers, providing accumulated totals. The standard gauge opening is 8 inches (203 millimeters) in diameter in the United States and 127 millimeters in many other countries.

Tipping-bucket rain gauges automate measurement by directing collected water into a small, two-compartment bucket. When one compartment fills with a calibrated amount (typically 0.25 millimeters), it tips, emptying itself and moving the other compartment into position. Each tip is recorded electronically, providing a time series of rainfall rate. Weighing gauges measure the total weight of accumulated precipitation, working equally well for rain and snow without the need for melting frozen precipitation first.

Disdrometers measure individual raindrop sizes and velocities, providing detailed information about the drop size distribution that is valuable for calibrating radar rainfall estimates. Laser disdrometers detect drops passing through a horizontal laser beam, while impact disdrometers measure the momentum of drops hitting a sensor surface. Snow measurement requires specialized techniques: snow boards provide fresh snowfall depth, snow pillows measure snow water equivalent by weighing the snowpack above a fluid-filled pad, and heated rain gauges melt snowfall to measure its liquid water content.

Upper-Air Instruments

Radiosondes are expendable instrument packages attached to weather balloons that ascend through the atmosphere at about 5 meters per second, transmitting measurements of temperature, humidity, and pressure via radio signal. GPS tracking of the radiosonde's position provides wind speed and direction data at all levels. A typical radiosonde ascent reaches 25 to 30 kilometers altitude before the balloon bursts, providing a vertical profile of conditions through the entire troposphere and lower stratosphere in about 90 minutes. A small parachute slows the instrument's descent, though most radiosondes are not recovered.

Pilot balloons (pibals) are tracked visually or by radar to determine upper-level wind speed and direction without the expense of a radiosonde. Dropsondes are instrument packages released from aircraft into hurricanes and other storms, falling through the atmosphere on a parachute and transmitting data during descent. Dropsondes are particularly valuable for sampling the atmosphere over oceans where radiosonde stations do not exist, and they have significantly improved hurricane track and intensity forecasts.

Wind profilers are ground-based radar systems that point vertically and measure the Doppler shift of signals reflected by turbulence and refractive index variations in the atmosphere. They provide continuous wind profiles above a fixed location, complementing the twice-daily snapshot provided by radiosondes. Profiler networks operate across the United States, Europe, and parts of Asia, feeding continuous upper-air wind data into forecast models.