No matter how accurate it may be, a thermometer will give very inaccurate readings if it is incorrectly sited. If a glass thermometer is placed in direct sunlight, for example, the mercury or alcohol in the bulb will absorb solar radiation and its temperature will rise for that reason. The thermometer will then display the temperature inside the bulb, but this will be quite different from the temperature of the surrounding air. This is why the air temperature must always be taken in the shade.
Even then, it may not be the typical air temperature that the thermometer measures. In the 18th century many people thought it was good enough to measure the temperature indoors, in a north-facing room without a fire. Later, when the fashion for gardening led to keeping thermometers outdoors, people thought they could obtain the most accurate
reading by placing the thermometer outdoors on a north-facing wall. In the Northern Hemisphere, placing the thermometer on a north-facing wall ensures that it is kept out of direct sunshine.
Toward the latter part of the 18th century, meteorologists began to notice that the air often felt warmer than the temperature shown by the thermometer, especially on fine days in spring. On investigation they found that the thermometer was measuring the temperature of the layer of air next to the north-facing wall. The temperature of this air was controlled by that of the wall itself and the wall was often colder than the air some distance away, out in the open. This was especially true in spring, when the wall was just beginning to warm up after the cold winter. Under these conditions the thermometer reading was too low. In summer the opposite happened. The warm wall raised the temperature of the air next to it and the thermometer reading was too high.
Various suggestions were made for ways to solve the problem, but it was not easy. Place the thermometer out in the open, well clear of any wall, and the wind might affect it. Place it low down or close to solid objects where it was out of the wind, and it could be affected by their thermal radiation. The solution was finally found by Thomas Stevenson (1818-87), head of a family firm that made lights and lenses for lighthouses and father of Robert Louis Stevenson (1850-94), the author of Treasure Island, The Strange Case of Dr. Jekyll and Mr. Hyde, Kidnapped, and several other popular stories. The Stevenson screen, shown in the drawing on page 141, is a box with a hinged front, painted white to reflect sunshine rather than absorbing it, with two thicknesses of louvered sides arranged so that the louvers form a V shape in cross section. There is an air space between the top of the box and the roof, and the floor is also ventilated to prevent it from warming due to radiation from the ground. The "screen" stands on legs that raise it so that the bulbs of its thermometers are about four feet (1.25 m) above the ground. This is the lowest height at which the thermometers record the temperature clear of the influence of the ground itself. The screen is protected against direct solar radiation, wind, and radiation from the ground and surrounding objects. Its thermometers can be guaranteed to register air temperature.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.