A heatwave is defined as an area being unusually hot for 3 or more consecutive days, but what causes hot weather?
Hot weather is caused by high pressure. When an area of high pressure pushes down on the ground, the air compresses and becomes hotter and drier. The high-pressure system pushes clouds out of the way, allowing the sun to beat down on the ground and stopping cooler air from moving in.
A heatwave ends when the high-pressure system weakens. This is usually due to a cold front moving in. Dense, cool air slides beneath the lighter, warmer air, pushing it higher into the atmosphere. As the warm air rises, it cools.
Less commonly, the jet stream shifts, allowing cooler air (such as ocean breezes) to flow back over land. Or the intense heat leads to extreme humidity buildup, making the air unstable and triggering widespread thunderstorms that break the heat cycle.
How does a heatwave affect the air around us?
During a heatwave, the air temperature rises, causing it to expand and become less dense. This warm air rises upwards, creating a convection current.
While warm air wants to rise, heatwaves are most often caused by a high pressure system ( heat dome ) sitting over a region. High pressure systems force air to sink down to the ground; as it sinks and becomes compressed, it heats up even more. The high pressure traps warm air, stopping it from rising and escaping into the atmosphere.
So while air close to the surface does rise and create small convection currents, the bigger weather system in a heatwave is caused by trapped hot air that can’t escape.
You can demonstrate convection currents with an underwater volcano, a candle carousel or a convection snake!
How do heatwaves affect air quality?
High temperatures can lead to higher than normal ozone levels when sunlight drives reactions between pollutants from cars and other sources. This can reduce the overall air quality.
More Summer Science Questions
Find out if drinking hot drinks really cools you down.
Try one of my easy summer science challenges.
Make a simple DIY Water Sprinkler.
Last Updated on August 3, 2026 by Emma Vanstone
