Thermometers and Thermistors
Weather observers use as many as 4 different thermometers, each with a special purpose. All 4 thermometers are mounted inside the Stevenson screen to protect them. Maximum and minimum thermometers, as their names suggest, register the highest and lowest temperatures since the last observation. Two ordinary thermometers--one dry, one wet--are used to determine the current temperature and moisture content of the air. Let's take a closer look.
In this picture, 2 ordinary yellow thermometers can be seen hanging vertically inside the screen. The one on the left is called a wet bulb thermometer because the bottom end or bulb is kept wet. One end of a cloth wick is slipped over the bulb and the other end is immersed in a plastic container of water. In winter, this moist cloth is replaced by a coating of ice. Heat is required for evaporation, and in this case, the heat needed to evaporate the water or sublimate the ice will be drawn from the thermometer bulb, making the thermometer read a lower temperature than it would otherwise.
The drier the air is, the more readily water will evaporate and the lower the temperature will read on the wet bulb. The difference between the wet bulb temperature and the temperature on the plain thermometer beside it (known as a dry bulb...can you guess why?) indicates how moist or dry the air is. This can be expressed as either a dew point or as relative humidity. When a dry bulb and wet bulb are used in combination, they're referred to as a psychrometer. If they're mounted in a bracket with a handle instead of in a screen, they're referred to as a sling psychrometer, like the one in the Sky Watchers kit.
Two additional thermometers are hanging almost horizontally in metal casings in the screen above--the bulb ends are just slightly lower. These are the maximum and minimum thermometers, and they are constructed differently.
They even contain different liquids. The one on the top is the minimum thermometer. It contains alcohol rather than mercury as the other 3 do. The advantage of using alcohol is that it doesn't freeze as easily. Mercury freezes solid at -39°C, so the dry bulb, wet bulb, and maximum thermometers are brought indoors at -37°C. Only the minimum stays outside beyond that point, showing both the current temperature and the lowest temperature since the last observation.
The minimum thermometer has a little barbell-shaped rider submerged in the alcohol column. As the temperature drops and the alcohol retreats toward the bulb end of the thermometer, the meniscus (surface of the liquid) pulls the rider down with it. The alcohol column will rise again when the temperature warms up, and the end of the column will move away from the rider leaving it to mark the lowest temperature. The maximum thermometer, on the other hand, works just like a fever thermometer. There's a constriction or narrowing in the tube near the bulb end of the thermometer. When the temperature rises, the mercury in the bulb expands and forces some of the liquid through the constriction and up the column. When the temperature drops, the mercury in the bulb contracts and pulls away from the constriction, breaking off the mercury column at that point and leaving the mercury trapped above the constriction to mark the highest temperature. After the temperature is recorded, the maximum thermometer has to be shaken vigorously to reunite the column of mercury--the same way parents do with a fever thermometer after taking your temperature!
Temperature and humidity information are measured differently at automatic stations. Information is sent electronically from 2 sensors like the ones on the right. If you follow the white electrical cable in this picture, you'll see a metal prong on the end of it. This is called a thermistor (from thermal resistor) and it measures air temperature. The heavier black electrical cable runs to a second resistor, known as a dewcel, that has been modified to take the place of a wet bulb thermometer. The second resistor has been wrapped in cloth fibres that are then soaked in a special solution that reacts to moisture. Wires have been wound around the wrapped thermistor, and the electrical current flowing through those wires will fluctuate depending on the amount of moisture in the air. Electronic signals are relayed from both sensors to a computer system every 60 seconds.
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