Correct option is C
The correct answer is (c).
In standard hydrometeorological and climatological observatories, ambient air temperature is precisely measured using thermometers housed securely within a Stevenson screen.
This specially designed slatted wooden enclosure shields the sensitive meteorological instruments from direct solar radiation and precipitation while allowing free air circulation for accurate readings.
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- The Stevenson screen is painted entirely white to reflect sunlight and minimize any artificial heat absorption that could skew temperature data.
- It features double-louvered sides, preventing direct sunlight penetration even at low solar angles while promoting continuous natural ventilation.
- The standard installation height mandates the thermometers to be placed exactly 1.25 to 2.0 meters above a natural grass surface to avoid ground radiation interference.
- It typically houses a suite of instruments including maximum and minimum thermometers, wet-bulb and dry-bulb thermometers (psychrometers) for humidity, and sometimes a thermograph.
- The consistent use of these screens globally ensures that historical climatic data is standardized and rigorously comparable across different stations worldwide.
- Without this shielding, a thermometer would simply measure the intense radiation temperature of the sun hitting the glass, rather than the true ambient air temperature.
- Modern electronic weather stations still utilize similar radiation shields (often stacked plastic plates) designed on the exact aerodynamic principles of the Stevenson screen.
- Option A: A Sunshine Recorder (like the Campbell-Stokes recorder) is an instrument utilizing a glass sphere to burn a trace onto a standardized card, measuring the exact duration of bright sunshine.
- Option B: A Pan Evaporimeter (Class A pan) is an open cylindrical galvanized iron pan filled with water, utilized exclusively to measure the daily rate of potential evaporation.
- Option D: A Piezometer is a specialized geotechnical instrument installed underground to continuously measure pore water pressures and monitor groundwater elevation levels.
- Accurate atmospheric temperature recording is the foundational parameter for tracking global warming trends and extreme weather climatology.
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