Mallon Technology

The third week of June 2026 brought a devastating heatwave to Western Europe, making it the hottest June on record. 

The event lasted only around ten days, from 18 to 28 June, but its impact was significant — widespread vegetation stress and a rise in heat–related deaths across the region.

In this blog, written by Mallon colleague Anastasiia Khil, we look at how Earth Observation (EO) data can help analyse an event like this by comparing temperature conditions in Ireland and France and assessing the heatwave’s impact on vegetation.

What Does a Heatwave Look Like?

A heatwave is a prolonged period of unusually high temperatures relative to the local climate, typically lasting several days or more. To track and compare air temperatures during the June 2026 heatwave in Ireland and France, we used the ERA5–Land reanalysis dataset. Produced by the European Centre for Medium–Range Weather Forecasts (ECMWF) on behalf of the Copernicus Climate Change Service (C3S), ERA5–Land provides hourly estimates of atmospheric and land surface conditions from 1950 to the present, at a spatial resolution of around 9 km. The dataset is built from weather model simulations constrained by observations from satellites, weather stations, aircraft, weather balloons, and other sources. For this analysis, we used the daily maximum 2 m air temperature — the measurement typically reported in weather forecasts. In many regions, ERA5–Land air temperatures fall within about 1°C of ground observations, though the margin of error varies with location, terrain, and weather conditions.

Map 1. Daily maximum air temperature in Ireland and France during the June 2026 heatwave.

Despite its relatively coarse spatial resolution, the dataset clearly captures large–scale weather patterns and trends. France experienced far higher temperatures, peaking on 24 June, while Ireland stayed considerably cooler — though many people in Ireland would probably disagree — with its highest temperatures recorded on 25 June. The dataset also shows how geography shapes temperature: coastal and mountainous regions stay cooler, while inland areas warm up more.

Air vs. Land Surface Temperature

It can often feel far hotter outside than the temperature reported in the forecast. That’s largely because the Earth’s surface heats up much faster than the air above it, and different land cover warms at different rates. Built–up areas such as roads and buildings absorb and retain more heat, while forests and other green spaces provide shade and help cool the surrounding air. As a result, on a sunny day, road surfaces can exceed 40°C at air temperatures around 20 °C. We covered this in more detail in our previous blog on urban heat.

To compare the two, we examined how the maximum 2 m air temperature (ERA5–Land) differs from the land surface temperature (LST; Landsat 9). The first difference is resolution: the air temperature data covers the study area with only about 25 pixels — each about 9 × 25 km — yielding a broad regional picture, whereas the LST data reveals far more local detail. The second is magnitude: land surfaces can reach nearly twice the temperature of the air above them.

Map 2. Maximum air temperature (ERA5–Land) compared with land surface temperature (Landsat 9) for the same study area.

At 30 m resolution, Landsat’s LST data reveal local temperature patterns that are invisible in coarser datasets. Built–up areas stand out clearly in red and orange: Fermoy (1) to the north, and Cork (2) and Midleton (3) to the south. Yellow and light green tones mark agricultural fields (4), and occasional orange patches indicate exposed bare soil. In the north–west, the forests of the Nagle Mountains (5) show up in green, staying cooler thanks to shading and evapotranspiration. Even the Blackwater River (6) is faintly visible. As expected, the coolest temperatures are found over water (7), shown in blue in the southern part of the map.

Map 3. Land surface temperature over Cork and surrounding areas, derived from Landsat 9.

How Does a Heatwave Affect Vegetation?

The heatwave placed considerable stress on vegetation, reducing its moisture content and, in some areas, its greenness. Two indices help track this response: the Normalised Difference Vegetation Index (NDVI) and the Normalised Difference Moisture Index (NDMI).

• NDVI measures vegetation greeness and is commonly used to assess plant health and density – higher values generally indicate healthier, denser vegetation
• NDMI measures the moisture content of vegetation and is useful for detecting water stress – lower values generally indicate drier vegetation

We generated maps of both indices from Sentinel–2 imagery across three periods — before the heatwave (May–early June), during the heatwave (late June), and after (first half of July) — to assess how vegetation responded over time.

Map 4. NDVI and NDMI before, during, and after the June 2026 heatwave.

The maps show no dramatic visual change, so we calculated median statistics to quantify the difference. We restricted the analysis to vegetated pixels — trees, grass, and shrub & scrub — identified in the Dynamic World 2026 land cover dataset, excluding cropland to ensure the results reflect the heatwave rather than routine agricultural cycles.

NDVI held steady at 0.86 before and during the heatwave, then dropped to 0.80 roughly two weeks after the extreme heat had passed. That suggests changes in greenness take time to show up, and may continue if dry conditions persist without rain. NDMI responded more quickly: values fell from 0.38 before the heatwave to 0.36 during it, then to 0.29 afterwards — a clear sign of mounting water stress, and proof that vegetation can lose moisture well before it visibly loses its colour. We can’t say the heatwave alone caused these changes, since vegetation dynamics depend on many factors, but the results point clearly to its impact.

Conclusions

• Combining satellite imagery, reanalysis data, and land cover products shows the value of EO for monitoring natural hazards, tracking environmnetal change, and supporting informed decisions
• ERA5–Land and Landsat complement each other: ERA5–Land captures regional air temperature patterns, while Landsat reveals land surface temperatures at a much finer scale
• The June 2026 heatwave affected vegetation beyond what was immediately visible. NDVI changed gradually, but NDMI picked up moisture stress during the event itself – a reminder of why multiple vegetation indicies matter
• As heatwaves grow more frequent, pairing EO with practical action – greener cities, protected ecosystems, watering vulnerable vegetation during dry spells – will help limit the damage extreme heat causes

Further Information

For further information about the methods used to produce the study and images above, or to discuss your Earth Observation requirements, contact us below.