Perhaps the most vexatious of all weather extremes are those concerned with heat, and few pieces of data have caused as much debate in the meteorological community. The World Meteorological Organization (WMO) Climate Extremes Committee takes its job of determining the world's hottest temperatures seriously and reviews each purported record in great detail by certifying it; currently, several purported records are under review by the committee.
To certify an extreme heat record, several critical factors must be considered:
- Has the thermometer making the measurement been calibrated and/or located in an appropriate shelter?
- If so, is the shelter a “Stevenson type” and positioned to meet the official requirements so far as exposure is concerned?
- From what period of the “Stevenson shelter” lifeline was the record made?
The key in these factors is the use of the Stevenson screen. In general, official temperature measurements should be made using this screen, which is a louvered, white shelter developed by Thomas Stevenson, a British civil engineer (father of famous author Robert Louis Stevenson), in the mid-19th century. To meet accepted standards for a Stevenson screen, the thermometers in the screen must be located between 1.25 meters (4 feet 1 inch) and 2 meters (6 feet 7 inches) above the ground. Moreover, the shelter cannot be located on a surface that radiates an unusual amount of heat like tarred concrete, sand, or the side of a building. Finally, the thermometers must be of the standard mercury-type and should be well calibrated; electronic thermometers have been shown to become inaccurate at very high (50°C/122°F plus) temperatures.
CHRISTOPHER C. BURT is the author of Extreme Weather: A Guide and Record Book (W. W. Norton, 2007), and is the weather historian for Weather Underground, for which he blogs weekly on weather extremes.