S: WHO – http://www.euro.who.int/en/publications/abstracts/heat-waves-risks-and-responses (last access: 4 August 2015); https://www.climatecommunication.org/new/features/heat-waves-and-climate-change/heat-waves-the-details/ (last access: 4 August 2015).
N: 1. heat (n): Old English hætu, hæto “heat, warmth, quality of being hot; fervor, ardor,” from Proto-Germanic *haita- “heat”.
wave (n): “moving billow of water,” 1520s, alteration (by influence of wave (v.)) of Middle English waw, which is from Old English wagian “to move to and fro”.
Heat wave “period of excessive hot weather” first attested 1890; earlier in reference to solar cycles.
2. Marked warming of the air, or the invasion of very warm air, over a large area; it usually lasts from a few days to a few weeks. This is a rise of atmospheric average temperature well above the averages of a region, with effects on human populations, crops, properties and services. (ISDR)
S: 1. OED – http://www.etymonline.com/index.php?term=heat; http://www.etymonline.com/index.php?term=wave (last access: 4 August 2015). 2. RWP – http://www.who.int/hac/about/reliefweb-aug2008.pdf (last access: 4 August 2015) (p. 31).