In the first part of this two-part series, we explore the atmospheric processes behind extreme heat, from high-pressure systems and upper-level ridges to blocking patterns, warm air advection and the role of dry soils. In the second part, we'll look beyond air temperature and explain why heat stress can vary so much from one place to another, and how high-resolution weather and climate data help assess these local differences.
More than just hot weather
Observations over recent decades show that periods of extreme heat are becoming more frequent, longer-lasting and more widespread across Europe. The latest European State of the Climate report by the Copernicus Climate Change Service and the World Meteorological Organization (WMO) identifies Europe as the fastest-warming continent, with rising temperatures contributing to increasingly frequent heat extremes.
The summer of 2026 offers us another striking example. During late June, temperatures exceeded 40°C across large parts of southern, western and central Europe. France recorded its hottest June day since observations began, Switzerland measured its highest June temperature on record, and several countries across central Europe established new monthly or all-time national temperature records. Beyond the meteorological observations, the heat placed pressure on energy systems, transport infrastructure, agriculture and public health.
While these impacts are widely reported, the atmospheric processes behind heatwaves often receive far less attention. Why do they develop? Why do some dissipate quickly while others persist for weeks? And why can temperatures change so dramatically across Europe within only a few days?
Understanding a heatwave requires considering more than the surface temperature forecast. Large-scale atmospheric circulation, land surface conditions and local geography all influence how extreme heat develops and evolves.
Credit: C3S/ECMWF
What defines a heatwave?
Unlike storms or heavy rainfall, a heatwave does not have one universal definition. A temperature considered exceptional in northern Europe may be relatively common in southern Spain or Greece. For this reason, national meteorological services usually define heatwaves relative to the local climate rather than by a fixed temperature threshold.
In general, a heatwave describes a prolonged period of unusually hot weather lasting at least several consecutive days, often accompanied by exceptionally warm nights. Duration is just as important as the daytime maximum. When temperatures remain high overnight, buildings, roads and the soil have little opportunity to cool, allowing heat to accumulate over successive days.
Meteorologists therefore assess heatwaves using more than daily maximum temperatures alone. Night-time temperatures, humidity, wind conditions and the persistence of the atmospheric pattern all influence how severe an event becomes.
Why do heatwaves develop?
Most major European heatwaves begin with the development of a persistent high-pressure system.
High pressure is associated with slowly descending air. As air sinks through the atmosphere, it becomes compressed and warms naturally. At the same time, cloud formation is suppressed, allowing abundant sunshine to reach the surface day after day. Clear skies allow intense solar heating, while weak winds limit the mixing of cooler air, creating ideal conditions for temperatures to rise.
To understand how these systems evolve, meteorologists often examine the atmosphere at the 500 hPa pressure level, approximately five to six kilometres above the Earth's surface. This level reveals the large-scale circulation patterns that influence weather across entire continents.
One of the most common features associated with prolonged European heatwaves is an upper-level ridge extending from North Africa towards Europe. These ridges transport warm subtropical air northwards while maintaining stable atmospheric conditions beneath them.
Sometimes the ridge becomes part of a larger blocking pattern known as an Omega Block, named after its resemblance to the Greek letter Ω. Areas of low pressure become established on either side of the ridge, preventing the usual west-to-east movement of weather systems. Instead of lasting only a few days, the high-pressure system can remain almost stationary, allowing heat to build continuously.
Recent studies show that these blocking patterns are a common feature of many of Europe's longest and most intense heatwaves.
Why do some heatwaves become exceptionally intense?
Large-scale atmospheric circulation provides the framework for a heatwave, but several additional processes determine how severe it becomes.
One of the most important is the origin of the air mass itself. During many European heatwaves, very warm and dry air is transported northwards from North Africa across the Iberian Peninsula into western and central Europe, and in some situations further east. Once this air arrives beneath a persistent high-pressure system, continuous sunshine allows temperatures to rise even further.
Conditions at the land surface also play an important role. Normally, part of the Sun's energy is used to evaporate moisture from soils and vegetation. This natural process limits the amount of energy available to heat the air. During prolonged dry periods, however, soils contain much less moisture. Instead of driving evaporation, a greater proportion of solar energy warms the ground directly, which in turn heats the air above it.
This interaction between the atmosphere and the land surface, known as land-atmosphere feedback, has become an important area of weather and climate research. Numerous studies have shown that dry soils can substantially amplify heatwaves, particularly after warm and dry springs, by reducing evaporative cooling and increasing the amount of solar energy available to heat the air.
The June 2026 heatwave demonstrated this clearly. Large parts of south-western, western and central Europe entered summer following below-average rainfall. As the high-pressure system strengthened, dry soils further enhanced daytime heating, contributing to record-breaking temperatures across several countries.
Forecasting extreme heat
Forecasting a heatwave involves much more than predicting the highest daytime temperature.
Meteorologists first analyse the large-scale atmospheric circulation, identifying the position of upper-level ridges, blocking highs and the jet stream. These patterns often provide the earliest indication that prolonged heat may develop several days in advance.
As the forecast period becomes shorter, weather models simulate how solar radiation, clouds, soil moisture, vegetation and local terrain interact with the atmosphere. High-resolution modelling is particularly important because mountains, coastlines and urban areas can all produce significant local differences in temperature.
Forecast uncertainty also plays an important role. Small differences in the predicted position of a high-pressure system can lead to noticeable differences in temperatures several days later. For this reason, professional forecasting relies on comparing multiple weather models rather than depending on a single simulation.
At meteoblue, this approach is reflected in the MultiModel forecast, which combines information from several leading numerical weather prediction models to provide a more robust forecast. By utilising Weather Maps, users can explore not only temperature forecasts but also upper-air circulation, pressure, soil moisture and many other meteorological variables that help explain how a heatwave is likely to develop.
In Part 2 of this mini-series, we'll look beyond air temperature and examine why similar weather conditions can lead to very different levels of heat stress. We'll explore how humidity, wind, solar radiation and local surroundings influence thermal comfort, and how meteoblue City Climate Solutions, including Urban Maps, help visualise these differences.
If you would like to discuss heatwaves, forecasting or any of the meteorological processes described in this article, join the conversation in the meteoblue Community Forum. It is a place where weather enthusiasts, professionals and the meteoblue experts exchange ideas, ask questions and discuss weather and climate topics from around the world.
