Air temperature is a measurement of how hot or cold the air is. In meteorology, it is also known as surface temperature. The temperature indicated by a thermometer exposed to the air but protected from direct sunlight is called air temperature. But how is air temperature measured accurately?
Thermometers are used to measure air temperature. A typical thermometer is a glass rod with a very thin tube inside. The liquid in the tube is supplied by a reservoir, or bulb, at the thermometer’s base. The liquid can be mercury or red-colored alcohol.
Why is the Air Temperature Measured in the Shade?
Temperatures are always taken in the shade because the temperature in the shade is the actual air temperature. Because clear air is relatively transparent to sunlight, sunlight’s energy is not significantly absorbed when it shines through clear, cloud-free air. When sunlight strikes a nontransparent object, such as a thermometer, its energy is converted to heat, and the object warms up. A thermometer in direct sunlight registers the sun’s heat energy, not the actual air temperature. (Source: Chicago Tribune)
What are the Other Methods for Measuring Air Temperature?
Temperature is the energy level of matter that can be observed as a change in that matter. Temperature measuring sensors come in various shapes and sizes, but they all have one thing in common: they all measure temperature by detecting a change in a physical characteristic. Here are some other methods for measuring air temperature:
Thermocouples are voltage devices that measure temperature by changing the voltage. The output voltage of the thermocouple rises as temperature rises, but not necessarily linearly.
The thermocouple is frequently housed inside a metal or ceramic shield that protects it from various environments. Metal-sheathed thermocouples are also available with multiple outer coatings, including Teflon, for use in acids and caustic solid solutions.
Resistive Temperature Measuring Devices
Electrical devices are also used to measure resistive temperature. Instead of using a voltage, as the thermocouple does, they take advantage of another property of matter that changes with temperature – its resistance. At OMEGA Engineering, Inc. in Stamford, Conn., we work with two resistive devices: metallic resistive temperature devices (RTDs) and thermistors.
Non-contact sensors are infrared sensors. For example, suppose you hold a typical infrared sensor up to the front of your desk without making contact. In that case, the sensor will tell you the temperature of the desk based on its radiation—most likely 68°F at normal room temperature.
Because of evaporation, the non-contact measurement of ice water will read slightly below 0°C, lowering the expected temperature reading.
Bimetallic devices make use of the expansion of metals when heated. In these temperature-checking devices, two metals are bonded together and mechanically linked to a pointer. One side of the bimetallic strip expands more than the other when heated. When adequately geared to a pointer, the temperature measurement is displayed.
Thermometers are well-known liquid expansion devices that are also used to measure temperature. In general, they are divided into two types: mercury-based and organic, usually red, and liquid-based. The distinction is significant because mercury devices have certain limitations regarding how they can be safely transported or shipped.
Because mercury is a known environmental contaminant, breaking it can be dangerous. Before shipping, check the current restrictions for air transportation of mercury products.
(Source: WWD Magazine)
Image from Forbes