Question

(i) What specifications need to be followed while making a Stevenson's Screen?

(ii) Name the instruments that are fixed inside Stevenson Screen.

Answer 1

1. 1) Specifications need to be followed while making a Stevenson's Screen:
   

The Stevenson screen was designed by Thomas Stevenson (1818-1887), a British civil engineer, in order to accurately measure air temperatures rather than side effects like solar irradiation heating up the thermometers. To reflect heat back, it is painted white, but better still would have been reflective aluminum. It has louvered sides to let the air through but not the sunlight. Once it became an accepted standard, the Stevenson Screen is now spread all over the world. It now allows temperatures to be compared wherever they are measured.

A lot of thought and experience went into its design: the door swings down rather than to one side so that the wind won't catch it on windy days and rip it off the hinges, and it opens facing north, to keep the sun from shining directly on the thermometers while reading the thermometers.

2) The instruments that are fixed inside Stevenson Screen:

Inside it one finds two normal thermometers (alcohol for cold areas, mercury for warm places), but one of these has its bulb wetted by a wick soaked in a bottle of water. This wet-bulb thermometer gives an indication of evaporation because evaporation of water causes cooling. There is usually also a max-min thermometer. The thermometers are placed such that they can be read with ease and replaced with minimum effort.

An important consideration is also that the louvered box stands a fixed distance above the ground, for least interference with low objects that may impede wind flow (and snow).