The rate and extent to which chemical reactions occur

Surface air temperature

The temperature of the air near the surface of the Earth is measured at meteorological observatories and weather stations, usually using thermometers placed in a shelter such as Stevenson screen, a standardized well-ventilated white-painted instrument shelter. The thermometers should be positioned 1.25–2 m above the ground. Details of this setup are defined by the World Meteorological Organization (WMO)
A true daily mean could be obtained from a continuously-recording thermograph. Commonly it is approximated by the mean of discrete readings (e.g. 24 hourly readings, four 6-hourly readings, etc.) or by the mean of the daily minimum and maximum readings (though the latter can result in mean temperatures up to 1 °C cooler or warmer than the true mean, depending on the time of observation).


Standards

The American Society of Mechanical Engineers (ASME) has developed two separate and distinct standards on temperature Measurement, B40.200 and PTC 19.3. B40.200 provides guidelines for bimetallic-actuated, filled-system, and liquid-in-glass thermometers. It also provides guidelines for thermowells. PTC 19.3 provides guidelines for temperature measurement related to Performance Test Codes with particular emphasis on basic sources of measurement errors and techniques for coping with them.

US (ASME) Standards

• 
  B40.200-2008: Thermometers, Direct Reading and Remotes Reading.
  
• 
  PTC 19.3-1974(R2004): Performance test code for temperature measurement.
  

Temperature is defined as the energy level of matter which can be evidenced by some change in that matter. Temperature measuring sensors come in a wide variety and have one thing in common: they all measure temperature by sensing some change in a physical characteristic.
The seven basic types of temperature measurement sensors discussed here are thermocouples, resistive temperature devices (RTDs, thermistors), infrared radiators, bimetallic devices, liquid expansion devices, molecular change-of-state and silicon diodes.