Weather observers usually take the air temperature by means of mercury-in-glass thermometers housed in a louvred white box, called a Stevenson screen (Figure 3.3) after its designer Thomas Stevenson (1818-87). The whiteness of the screen reflects away most of the direct heat of the Sun and the louvres provide shading, good ventilation and protection from rain. The screen is oriented with its door facing away from the equator, away from the Sun. The temperature measured in such a box is called the screen temperature, often regarded as the surface-air temperature,
though, strictly speaking, it refers to conditions about 1.5 m above the ground.
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