Draft. These pages were imported from the ARC Technical Criteria document (16 April 2026) and have not yet been reviewed in this format. Several thresholds are still being settled and are flagged on the pages concerned.

Defining heatwaves and the heat index

How heatwaves are defined, and why organisations increasingly express hot weather as a feels-like value or heat index.

Heatwaves are defined as periods of extreme heat, typically expressed using dry bulb air temperature for ease and simplicity of communication to the general public.

[Cite source for tropical heatwaves]

However, organisations increasingly factor in humidity (and often radiant temperature and wind effects as well) to help warn on heatwave conditions where these combined effects may cause risks to human health: for high-risk occupations e.g. working outside, military personnel, construction workers, workers operating dangerous machinery or for higher risk demographics such as babies or older people, or people in poor health. This expresses temperatures for hot weather as a ‘feels like’ value, or ‘heat index’.

Open questions on this page

These are unresolved in the source document - the author's own queries, and the figures and citations still to be filled in. They are repeated here rather than resolved, because a threshold shown as settled when it is still under discussion would be misleading.

• [Cite source for tropical heatwaves]

Confirm any of these with ARC before relying on them for a project.

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Source: ARC Technical Criteria - 215
Reference · confidence medium · Imported 2026-09-12, not yet reviewed · next review 2026-12-12
Tags: untagged
Citation key: heatwaves-and-heat-index