Weather Toolbox - Weather Hazard Awareness - UV and Radiation

This text is part of the Weather Preparedness & Resilience Toolbox developed by the YOUROPE Event Safety (YES) Group within YOUROPE’s 3F project (Future-Fit Festivals). It is aimed at everyone involved in planning, building, and operating open-air events. It helps festivals and other outdoor events become truly weather-ready by offering both practical and research-based resources as well as background information on weather and climate. Learn how to design safer and more weather-resilient outdoor events.

Weather Toolbox – Weather Hazard Awareness – UV and Radiation

Weather Hazard Awareness: What are we dealing with?

UV and Solar Radiation

Weather hazard awareness for solar radiation starts with understanding how UV intensity, heat and glare interact with crowds, infrastructure and operations at outdoor events. For crowd managers, the UV Index is a practical trigger for protection, scheduling and layout decisions across the entire event lifecycle.

The UV Index (UVI) is an international scale that indicates the strength of sunburn‑producing ultraviolet radiation at the Earth’s surface at a given place and time. It is an open‑ended scale starting at 0, where higher values mean more intense radiation and a shorter time to skin and eye damage.

The UVI is derived from measured UV irradiance, weighted by how strongly each wavelength damages human skin, and then multiplied by a constant factor (40 m²/W).
Typical public ranges are: 0–2 low, 3–5 moderate, 6–7 high, 8–10 very high, and 11+ extreme, used consistently by WHO, WMO and national meteorological services.

Risk factors: angle, altitude, clouds

UV exposure for visitors and staff rises sharply with sun position, altitude and sky conditions even when air temperature feels moderate.

When the sun is high in the sky (around solar noon, late spring to early autumn), UV radiation passes through less atmosphere and is significantly more intense than at low sun angles.
UV levels increase with altitude because the thinner atmosphere absorbs less radiation, and broken cloud fields can enhance ground‑level UV by reflecting additional radiation towards people.

Health impacts: beyond “just sunburn”

High UVI values can cause rapid sunburn, eye irritation and contribute to long‑term risks such as skin cancer and cataracts, even in temperate European climates. For events, this translates into both individual health risks and systemic issues like heat stress and medical‑service overload.

Short‑term effects include erythema (sunburn), photosensitivity reactions, and photokeratitis (“snow blindness”) when reflected UV from bright surfaces is high.
Long‑term cumulative exposure is linked to non‑melanoma and melanoma skin cancers and accelerated skin ageing, which is why several European occupational guidelines require protection measures already from UVI ≥ 3.

High insolation and crowd heat stress

Strong incoming solar radiation (high insolation) heats ground, structures and air layers over the event site, changing both thermal comfort and local meteorology.

Heated surfaces (asphalt, artificial turf, temporary decking) can reach temperatures far above air temperature, increasing radiant heat load on the body and driving heat exhaustion and dehydration in dense crowds.
Intense surface heating promotes convection, which can help trigger local wind gusts and storm development when the atmosphere is unstable, affecting stage safety, tents and suspended loads.

Glare: visibility and operational safety

Low sun angles in the morning or late afternoon produce strong directional glare that can be a critical safety issue for traffic flows, staff and visitors.

Disability glare reduces contrast and makes it difficult to detect obstacles, pedestrians or signage; studies show increased crash risk at intersections when drivers face rising or setting sun.
For event design, glare affects stage orientation, screen legibility and camera lines of sight, and it can compromise stewarding at key choke points when staff have to look into the low sun.

Practical implications

For crowd managers and safety officers, the UV Index and solar geometry should be integrated into risk assessment, site design and daily operations.

Many European occupational recommendations call for technical, organisational and personal protective measures once UVI reaches 3, scaling up to mandatory personal protection at very high and extreme levels.
Key controls include: scheduling high‑density activities away from peak UVI hours, providing shade and water, aligning stages and entrances to minimise low‑sun glare, and building UVI forecasts into event safety briefings and staff training.

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