The thermometer is the instrument used to measure temperature — the most fundamental atmospheric variable alongside pressure and humidity. The first reliable thermometers were developed in the early 17th century, with Galileo's thermoscope (c. 1593) and Daniel Fahrenheit's mercury thermometer (1714) being key milestones. Today's meteorological thermometers are predominantly electronic, using platinum resistance sensors (Pt100) or thermistors for high accuracy.
Correct temperature measurement requires careful siting. Official weather stations house their thermometers in a Stevenson screen — a white, louvered, ventilated box mounted 1.25–2 m above the ground, over grass. This shields the sensor from direct sunlight, rain, and radiation from the ground while allowing free airflow. Without this protection, readings can be 5–15 °C too high on a sunny day. Maximum and minimum temperatures are recorded daily, typically reaching their extremes in mid-afternoon and just after sunrise, respectively.
Modern automated weather stations (AWS) log temperatures every minute, transmitting data in real time to national meteorological services and platforms like Meteo.es. Temperature data are critical for forecasting, agriculture, energy demand estimation, and climate monitoring. The scales in use are Celsius (°C, used in most countries), Fahrenheit (°F, used in the USA), and kelvin (K, used in scientific work). The world records stand at 56.7 °C (Death Valley, 1913) and −89.2 °C (Vostok, Antarctica, 1983).