Heatwaves
Why heat can build day after day — and why definitions matter.
This explanation is written conservatively from textbook-level science and has not yet been reviewed by a meteorologist. Treat it as a draft.
The mechanism
Follow the chain: each step causes the next.
01
High pressure settles in
A slow-moving or blocking high-pressure system settles over a region.
02
Sinking air warms
Air sinks within the high and warms as it is compressed, which also suppresses cloud.
03
Strong sunshine
Clear skies let more solar energy reach the ground day after day.
04
Soils dry out
As soils dry, less energy goes into evaporation and more goes into heating the air — a feedback that can intensify the heat.
Explore it
Step through the mechanism one stage at a time.
Step 1 of 4
High pressure settles in
A slow-moving or blocking high-pressure system settles over a region.
In short
There is no single definition
A heatwave is broadly a period of unusually hot weather lasting several days, but agencies define it differently — for example by temperatures above a local percentile threshold for a number of consecutive days. Because 'unusual' depends on the local climate, a temperature that is normal in one place can be extreme in another.
Deeper science
Open a section for more detail.
Nights matter too
Warm nights limit the body's ability to recover and are an important part of heat-health risk. Humidity also matters: humid heat reduces the body's ability to cool by sweating.
A warming background
The IPCC Sixth Assessment Report concludes that hot extremes have become more frequent and more intense across most land regions since the 1950s. Whether a specific event was made more likely by climate change is a separate question answered by attribution studies.
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