The common question of whether temperature is measured in the shade often leads to confusion, especially when comparing reported weather conditions with personal experience in direct sunlight. Understanding how air temperature is accurately determined is important for interpreting weather forecasts and general environmental awareness. This article clarifies the standard methods for measuring air temperature and explains why direct sunlight can significantly skew readings.
The Standard Protocol for Air Temperature
Official air temperature measurements are taken in the shade to ensure accuracy and consistency. This practice captures the true ambient air temperature, not the temperature of an object heated by solar radiation. Instruments like thermometers are housed within specialized shelters called Stevenson screens. These white, louvered boxes protect sensors from direct sunlight, precipitation, and wind, while allowing air to circulate freely.
Developed in the mid-19th century by Thomas Stevenson, the Stevenson screen’s design reflects solar radiation with its white exterior and double roof for insulation. Louvered sides facilitate natural ventilation, ensuring the air inside reflects surrounding atmospheric conditions. The World Meteorological Organization (WMO) sets global standards for placement, typically 1.25 to 2 meters above a natural grass surface and away from obstructions. Modern weather stations use electronic thermometers within these screens, which continuously transmit readings.
Why Direct Sunlight Skews Temperature Readings
Direct sunlight distorts temperature measurements by introducing radiant heating, distinct from actual air temperature. Air temperature reflects the kinetic energy of air molecules. When a thermometer is exposed to direct sunlight, its surfaces absorb solar radiation, converting this energy into heat. This heats the thermometer, causing it to register a much higher temperature than the surrounding air.
A sun-exposed thermometer’s elevated reading represents the heated instrument, not the air temperature. This is similar to a dark car seat becoming extremely hot in the sun, even if the air is only moderately warm. The thermometer absorbs radiation, becoming warmer than the ambient air it should measure. Measuring temperature in the shade prevents this radiant heating, ensuring the thermometer accurately reflects the air’s thermal energy.
Practical Implications for Everyday Measurement
Understanding shade-based temperature measurement has practical implications for interpreting weather reports and personal experiences. Weather forecasts refer to standardized air temperature measured in the shade. This explains why standing in direct sunlight feels warmer than reported temperatures, as your body absorbs solar radiation. Similarly, a car thermometer exposed to direct sunlight will show a higher reading than the official air temperature due to radiant heating of the vehicle’s interior and sensor.
To measure temperature accurately at home, follow principles similar to official meteorological stations. Place a thermometer on the north side of a building (in the Northern Hemisphere) or in a consistently shaded area. Keep it away from heat-absorbing or reflective surfaces like concrete, asphalt, or dark walls, which can radiate heat and inflate readings. Ensure adequate air circulation around the thermometer for accurate ambient air measurement.