UV radiation in spring and what controls its intensity
At the beginning of May, surface ultraviolet radiation levels increase significantly across central Europe, primarily due to the higher position of the Sun above the horizon, which shortens the path of solar radiation through the atmosphere. Longer daylight hours also contribute to a higher total daily UV exposure (Fig. 1, see map). However, this seasonal increase is influenced by several additional factors that can significantly modify UV index values both locally and over short time periods. Among the most important are cloud cover, atmospheric aerosols, surface reflectivity, and altitude.
Cloud cover is one of the most variable influences on UV radiation. Thick and optically dense cloud layers generally reduce UV levels by absorbing and scattering incoming radiation. Under such conditions, the UV index can decrease by several tens of percent compared with clear-sky conditions. This effect is illustrated in Fig. 2, where extensive cloud cover over central Europe significantly reduces incident UV radiation (see map). The relationship, however, is not always straightforward. Under partly cloudy skies, especially in the presence of broken cumulus clouds, UV radiation can temporarily increase due to multiple scattering and reflections at cloud edges. This phenomenon, known as cloud enhancement, can locally produce UV index values that exceed clear-sky levels for short periods.
Atmospheric aerosols also play an important role. These fine and coarse particles, including dust, soot, and secondary pollutants, affect radiative transfer through both scattering and absorption, depending on their composition. In areas with elevated air pollution, UV radiation reaching the surface is typically reduced. In contrast, cleaner mountain environments with lower aerosol concentrations allow more UV radiation to penetrate the atmosphere, contributing to higher UV index values. Fig. 3 shows the advection of Saharan dust over Europe on March 31, 2024, which contributed to a reduction in surface UV radiation (see map).
Another important factor is surface albedo, or the ability of a surface to reflect incoming radiation. Highly reflective surfaces, such as fresh snow, can reflect a substantial fraction of incoming UV radiation, increasing exposure near the ground. This means that people are exposed not only to direct sunlight but also to UV radiation reflected from the surface. Even in spring, this effect can remain significant at higher elevations where snow cover persists. Darker surfaces, such as forests or wet soil, contribute much less reflected UV radiation and therefore generally have a smaller effect on total UV exposure.
Mountain regions present particularly distinctive conditions. As altitude increases, the atmosphere becomes thinner, meaning solar radiation travels through a shorter path with fewer absorbing and scattering constituents. As a result, UV radiation typically increases by approximately 6–12% for every 1,000 metres of elevation gain, depending on atmospheric conditions. This effect is often further amplified by cleaner air and enhanced surface reflection from snow. Consequently, UV index values in mountainous areas can be considerably higher than in lowland regions, despite relatively cool surface conditions, as shown in Fig. 4 for the Alps (see map).
The combination of these factors makes the spatial and temporal variability of UV radiation particularly pronounced during spring. While the overall seasonal increase is primarily driven by astronomical factors, local atmospheric and surface conditions can substantially enhance or reduce UV intensity. Understanding these processes is important not only for assessing human UV exposure, but also for interpreting UV observations and radiative transfer modelling.
