Meteorology is the study of weather and other phenomena that occur within the Earth's atmosphere. To study the weather, scientists measure key quantities such as pressure and temperature and use these in complex models to carry out forecasts. A device that measures temperature is known as a thermometer, and many different types have been invented and used during the past few hundred years.
The liquid-in-glass thermometer is one of the most common instruments used today to measure temperature. As the name suggests, the instrument consists of a glass bulb containing a special liquid. The bulb is connected to a stem that has a scale for measuring the temperature. The liquid must expand and contract significantly in response to a temperature increase or decrease. Changes in temperature therefore lead to changes in the liquid level on the stem. The most commonly used liquid in these types of thermometers is mercury. Daniel Gabriel Fahrenheit invented the mercury-in-glass thermometer, which covers the temperature range negative 38 to 356 degrees Celsius (negative 36.4 to 672.8 degrees Fahrenheit).
As electric currents flow through wires, they scatter off each other and off the wire boundaries. This phenomena is known as electrical resistance, and its value is dependent upon temperature. Platinum wire is normally used in resistance thermometers since it does not react with the chemicals within air over a wide range of temperatures. The platinum wire is normally shaped into a coil and placed within an alumina tube. Resistance thermometers have a much greater resolution than liquid in-glass thermometers and can potentially measure changes down to one-thousandth of a degree.
Constant-Volume Gas Thermometer
The constant-volume gas thermometer consists of a container with a fixed amount of gas inside. The thermometer works upon the principle that changes in gas pressure are proportional to changes in gas temperature. A pressure sensor within the container detects the pressure, and calibration electronics are used to convert this value to a temperature. The gas placed within the container is normally air if temperature operation is close to room temperature. If lower temperature measurements are required, helium is used, since it has a melting point close to absolute zero.
All objects emit infrared radiation with an intensity approximately proportional to its temperature. Radiation thermometers consist of a series of optics that gather and focus infrared radiation onto a special detector. The detector is normally a semiconductor such as silicon, which outputs an electrical current proportional to the intensity of incident radiation. The temperature is then calculated using internal electronics, based upon known material parameters. A key advantage of radiation thermometers is the potential to measure an object's temperature at a distance. The thermometers are often equipped with a laser sight, in order to aim accurately at specific objects.