Defining and monitoring heatwaves¶
How the WMO, the Met Office and the IPCC define a heatwave, the threshold methods in use, who monitors them, and why coverage gaps matter for the global south.
Which definition applies matters practically, because it determines when the heatwave thresholds in success criteria come into force rather than the everyday comfort criteria.
Official definitions¶
World Meteorological Organization (WMO) - defines a heat wave as a period where local excess heat accumulates over a sequence of unusually hot days and nights.
UK Met Office - defines a heat wave as a period of three or more consecutive days when the daily maximum temperature exceeds a set threshold. These thresholds vary by region, to reflect differences in climate, and are updated regularly in response to rising average temperatures caused by climate change.
This provides a useful reference model for defining heat waves contextually in Tanzania, suggesting the importance of localised thresholds and climate sensitivity when setting criteria.
IPCC - describes a heat wave as a period of abnormally hot weather, often defined with reference to a relative temperature threshold, lasting from two days to months.
Temperature thresholds¶
Heat wave definitions typically rely on high-temperature thresholds relative to local climate baselines:
- Absolute anomaly-based - for example, exceeding the daily maximum by more than 5 °C over normal (1961–1990 baseline) for 5 days or more
- Percentile-based - for example, daily maximum exceeding the 99th percentile of the May–September historical distribution for 3 days or more
- Locally defined fixed values - for example, 32 °C or above for 3 days or more in specific areas
- Country-specific figures - India and Pakistan consider plains at 40 °C or above, and mountains at 30 °C or above, for 5 days or more.
See climate and weather for why these baselines move.
Monitoring and recording¶
National Meteorological and Hydrological Services (NMHSs) select thresholds based on local climatological baselines and maintain records.
Global organisations:
- WMO and WHO collaborate through the Global Heat Health Information Network (GHHIN)
- WMO issues the annual State of the Global Climate report
- IPCC references thresholds and observational trends in its Assessment Reports
- Countries operate formal Heat-Health Action Plans, guided by WHO and WMO.
Why it matters for the global south¶
Regions such as East Africa, India and Pakistan face heightened risks from climate change, with heat waves becoming more frequent and severe.
Monitoring gaps remain significant, with WMO reporting that half the world's population lacks adequate access to early warning systems.
Central coordinating body¶
The World Meteorological Organization is the primary authority for defining heat waves, setting guidelines, coordinating data collection, and partnering with WHO for heat-health warning systems through GHHIN.
Summary¶
| Aspect | Key information |
|---|---|
| Definition | Heat wave = 2–3+ days of unusually high temperature versus local baseline |
| Thresholds | Based on anomalies (e.g. +5 °C) or percentiles (90th/99th) |
| Monitoring | NMHSs implement; WMO and WHO set global best practices |
| Central body | WMO, working via GHHIN and WHO |
| Global south issue | High vulnerability; warning systems often lacking |
Starting points¶
- WMO's heatwave page, for official definitions and resources - https://wmo.int/topics/heatwave
- WMO–WHO Heat-Health Warning System guidance - https://ghhin.org/wp-content/uploads/WMO_WHO_Heat_Health_Guidance_2015.pdf
- NMHS documentation, or national Heat-Health Action Plans, for region-specific thresholds.
Source: ARC Technical Criteria - Appendix 2: heat waves and climate change in the global south
Reference · confidence high · Imported 2026-08-29, not yet reviewed · next review 2026-11-29
Tags: heatwaves, definitions, monitoring, climate, reference
Citation key: heatwave-definitions