Meteorology is the scientific study of the Earth’s atmosphere, focusing on the processes and phenomena that produce weather conditions. Accurate measurement of the atmosphere’s current state is the foundation for creating reliable forecasts. This data supports aviation safety, agricultural planning, and commerce, while also providing public warnings for severe weather events.
Measuring Core Atmospheric Properties
Air temperature is a fundamental measurement, recorded using devices called thermometers. Modern weather stations frequently use electronic resistance thermometers, such as the Platinum Resistance Thermometer (PRT), which measure temperature by detecting the change in electrical resistance of a metal coil. For consistency, these instruments are housed in a ventilated white enclosure, like a Stevenson screen, positioned about two meters above the ground to shield them from direct sunlight and radiant heat, ensuring they only measure the ambient air temperature.
Atmospheric pressure, also known as barometric pressure, is the force exerted by the weight of the air above a given point. Measured by a barometer, its value indicates impending weather changes. High pressure generally signifies sinking air and settled, fair weather, while rapidly falling pressure often signals approaching low-pressure systems associated with storms and precipitation. Modern weather stations utilize aneroid barometers, which measure the expansion and contraction of a sealed metal capsule sensitive to pressure changes. The standard pressure at sea level is approximately 1013 hectopascals (hPa).
The amount of water vapor present in the air is known as humidity. It is typically expressed as relative humidity—a percentage comparing the current moisture level to the maximum the air can hold at that temperature. Humidity is measured by a hygrometer or a psychrometer. A psychrometer determines moisture by comparing the reading of a dry-bulb thermometer to a wet-bulb thermometer, using the cooling difference from evaporation to calculate relative humidity.
Tracking Wind and Precipitation
Wind, the horizontal movement of air, is defined by its speed and direction. Wind speed is measured using an anemometer, most commonly the cup or propeller type, where the rate of rotation is translated into a measurement typically expressed in knots or meters per second. Wind direction is determined using a wind vane, which aligns with the air current and indicates the point on the compass from which the wind is originating. Accurate wind data is gathered by positioning these instruments on tall masts, usually 10 meters above the ground, to avoid interference from local obstacles.
The measurement of precipitation, which includes rain, snow, and hail, is achieved using a rain gauge. A basic standard rain gauge collects liquid precipitation in a funnel that directs the water into a calibrated cylinder, allowing observers to measure the depth in millimeters or inches over a specific period. Automated measurements are often taken with a tipping bucket rain gauge, which uses a small, two-sided container balanced beneath the funnel. Once the bucket collects a set amount of liquid, such as 0.1 millimeter of rain, it tips and sends an electrical signal to a recorder before the other side begins collecting. Solid precipitation, like snow, is measured by melting a sample to determine its liquid water equivalent.
Platforms and Remote Sensing Technology
Beyond surface-level instruments, advanced technology provides a comprehensive three-dimensional view of the atmosphere. Radiosondes, small instrument packages carried aloft by weather balloons, are launched twice daily from stations around the world. These expendable devices continuously measure and transmit a vertical profile of atmospheric pressure, temperature, and relative humidity as they ascend to altitudes exceeding 35 kilometers. The radiosonde’s position is tracked via GPS, which allows meteorologists to calculate upper-air wind speed and direction.
Weather radar uses the principle of remote sensing to detect precipitation and measure its movement. Doppler radar transmits pulses of microwave energy that bounce off targets like raindrops and snowflakes, with the returning signal providing information on intensity and location. By analyzing the frequency shift in the returned energy—the Doppler effect—the radar determines the velocity of the targets. This capability is instrumental in detecting the rotational movement within storms, which is important for issuing severe weather warnings.
Weather satellites provide a global view of the atmosphere from space, categorized primarily by their orbit. Geostationary (GEO) satellites orbit approximately 35,800 kilometers above the equator, moving at the same rate as the Earth’s rotation. This allows them to remain fixed over a single region, providing continuous, real-time imagery for monitoring rapidly changing events like hurricanes. Polar-orbiting satellites fly much closer to the Earth, generally between 500 and 850 kilometers, traveling over the poles in a north-south path. Their lower altitude provides higher-resolution imagery and atmospheric soundings, offering detailed global coverage often twice daily.