Defining and monitoring heatwaves¶
How heatwaves are defined and monitored internationally, and why the global south is poorly served by the existing arrangements.
WMO Official Definition¶
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 Definition¶
In the UK, the 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.
Ref: https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/temperature/heatwave & https://www.metoffice.gov.uk/about-us/news-and-media/media-centre/weather-and-climate-news/2022/heatwave-threshold-changes
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: e.g., exceed daily maximum by >5°C over normal (1961–1990 baseline) for ≥5 days.
- Percentile-based: e.g., daily max exceeding 99th percentile of May–September historical distribution for ≥3 days.
- Locally defined fixed values: e.g., ≥32°C for ≥3 days in specific areas.
- Country-specific figures: India & Pakistan consider plains at ≥40°C, mountains at ≥30°C, for ≥5 days.
Monitoring and Recording¶
National Meteorological and Hydrological Services (NMHSs) select thresholds based on local climatological baselines and maintain records. Global organizations:
- WMO/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 Assessment Reports.
- Countries operate formal Heat-Health Action Plans guided by WHO-WMO.
Why It Matters for the Global South¶
Regions like 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 access to early-warning systems, particularly in the Global South.
Central Coordinating Body¶
The World Meteorological Organization (WMO) 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 Info |
|---|---|
| Definition | Heat wave = 2–3+ days of unusually high temp vs local baseline |
| Thresholds | Based on anomalies (e.g., +5°C) or percentiles (90th/99th) |
| Monitoring | NMHSs implement; WMO/WHO set global best practices |
| Central Body | WMO, working via GHHIN/WHO |
| Global South Issue | High vulnerability; warning systems often lacking |
Actionable Starting Points¶
- Visit WMO’s “Heatwave” page for official definitions and resources: https://wmo.int/topics/heatwave
- Review the WMO-WHO Heat-Health Warning System guidance: https://ghhin.org/wp-content/uploads/WMO_WHO_Heat_Health_Guidance_2015.pdf
- Consult NMHS documentation or national Heat-Health Action Plans for region-specific thresholds.
Source: ARC Technical Criteria - 667
Reference · confidence medium · Imported 2026-09-12, not yet reviewed · next review 2026-12-12
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Citation key: heatwave-definitions