Part 3: What is a heatwave?

Photo: Emmett Anderson [CC BY 2.0 (], via Wikimedia Commons

If a heatwave lasts for one day - is it still a heatwave?
Or is that just one hot day?

How about three consecutive hot days? And what happens if they have much cooler temperatures overnight?

A normal summer day in Alice Springs would be a big deal in Hobart, so does Hobart ever get heatwaves?

The truth is, it depends where you are and who you ask. The World Meteorological Organisation (WMO) defines a heatwave as an event in which the daily maximum temperature of more than five consecutive days exceeds the average maximum temperature by 5 °C.  The US Environmental Protection Agency considers a heatwave to continue for four consecutive days and nights, whereas the US National Weather Service considers a heatwave to be a period of abnormally and uncomfortably hot and unusually humid weather, typically lasting for two or more days. The MetOffice in the UK uses the WMO definition, and the Swiss issue different levels of heatwave warnings based on length and severity of the event. Chocolate melts at around 30 °C, and beer will go bad when stored at temperatures that high (a Pilsner, Lager or Wheat Beer should be stored at 10°C).

Fact: there is no international definition of a heat wave.

A definition was developed by Australian researchers in the past few years based on four days and four nights above the temperatures expected at that place at that time of year. For calculations, this definition can be written as a formula with daily maximum and minimum temperatures as inputs. I want to know how different our heatwave stats will look each year (eg. Longest heatwave, earliest heatwave) if I tweak some of the components of that heatwave.

So that is what I'm starting on this week. While I'm still spending time on the programming front,for sure, the fundamental question of 'what is a heatwave' is surprisingly using up a lot of brainspace!

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