What Is a Heat Dome and How Do We Detect Heat Waves?

A heat dome is a large area of high pressure high in the atmosphere that stays put for days and traps hot air underneath it. We do not detect heat domes from station data. We detect heat waves, and we use the words heat dome only once reanalysis data confirms the pressure pattern, which this site does not yet do.

What a heat dome is

Air sinks under a strong, slow-moving ridge of high pressure. Sinking air warms as it is compressed, clouds clear, and the sun heats the ground for longer each day. Winds are light, so little cooler air arrives from outside. The result is days of heat over a wide region. The name comes from the shape of the trapped hot air on a weather map, not from a physical lid.

The cause is a pattern in the upper air, the height of a pressure level such as 500 hPa. Confirming it needs gridded data, such as the ERA5 reanalysis, not a list of station readings.

What station data can show

A thermometer sees the effect, not the cause. Station data can show that many stations, over a wide area, set or tied records for high temperature on the same days. That pattern is what a heat dome tends to produce. It can also come from other set-ups, for example a hot air mass moving in ahead of a front. So a cluster of records is evidence of a heat wave, and only suggestive of a dome.

How we detect a heat wave

An event is a cluster of record events close together in space and time. To keep noise out, a cluster needs a minimum number of stations, a minimum duration and a minimum area before it becomes an event. Once it does, it gets a page with its dates, the countries it touches, the number of records by type and the strongest records. It keeps its address for life, and its status changes from active to closed when the records stop.

This build has 0 record events from 42,764 stations in 181 countries, so the clusters you see are a picture of where those stations are, and thin coverage shows up as gaps. The rules are on the methodology page, and our explainer on what a heat wave is covers the wider definitions.

Why we say heat wave

Our rule is to say heat wave until ERA5 confirmation exists. Calling every hot spell a heat dome would claim a cause we have not measured. When confirmation is added, an event will carry a heat dome label only when the upper air pattern matches, and will say which data confirmed it.

What to read next

Heat waves show up in the ratio of record highs to lows, and every dataset is listed on the sources page.