📄 earth_science

A UV–Comfort Co-occurrence Index reveals erythemal exposure opportunity on the South Atlantic coast

This study introduces a UV–Comfort Co-occurrence Index (IVCC) for Argentina's South Atlantic coast, demonstrating that thermal comfort, rather than UV irradiance alone, governs the timing and spatial distribution of human exposure opportunities, with late morning emerging as the critical window when high UV hazard coincides with thermally comfortable conditions.

Original authors: Federico Ferrelli, María Luján Bustos, Andrea Soledad Brendel, Emanuel Sauer

Published 2026-08-25

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Federico Ferrelli, María Luján Bustos, Andrea Soledad Brendel, Emanuel Sauer

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Sunlight carries a hidden duality that shapes our daily lives in ways we rarely notice until we feel it. On one hand, the sun provides the warmth that draws us outside, inviting us to walk, swim, and gather on the beach. On the other, that same light carries invisible energy capable of damaging our skin, a risk that scientists measure with a standard tool called the Ultraviolet Index. For decades, this index has served as the primary warning system for sun safety, telling us how strong the radiation is at any given moment. However, a number on a chart does not tell the whole story of human behavior. The index measures the environment, but it cannot measure the person. It does not account for the fact that people only stay outside when they feel comfortable. If the air is too cold, we retreat indoors; if it is too hot, we seek shade or air conditioning. The actual time we spend exposed to the sun is a negotiation between the intensity of the light and the feeling of the air on our skin.

This gap between environmental hazard and human behavior is the focus of new research from the temperate coast of Argentina. Scientists there have developed a new way to look at the sun that combines the danger of ultraviolet radiation with the comfort of the weather. They call this the UV–Comfort Co-occurrence Index. By analyzing data from twelve weather stations along the South Atlantic coast, the researchers mapped out exactly when and where the conditions for a beach day align with the conditions for a sunburn. Their work reveals a specific window of time that is often overlooked in public safety messaging: the late morning.

The study relied on a network of automatic weather stations stretching along the coast of Buenos Aires Province. These stations recorded the air temperature, humidity, wind speed, and the strength of the sun every thirty minutes over a period spanning from January 2024 to January 2026. The researchers used these measurements to calculate a value for how comfortable the air felt to a human, known as the Apparent Temperature. They defined a "comfort zone" as conditions where the air felt neither too cold nor too hot, specifically between 18 and 25 degrees Celsius. They then compared this comfort level against the Ultraviolet Index readings at the same moments. The goal was to find the moments when the sun was strong enough to cause harm while the weather was pleasant enough to keep people outdoors.

The results painted a clear picture of the daily rhythm of risk. During the summer months, both the ultraviolet radiation and the comfort index rose together, reaching their peak simultaneously around noon. However, the two factors began to diverge just after midday. As the sun climbed higher, the air temperature continued to rise, pushing the Apparent Temperature above the comfort threshold. By 1:00 PM, nearly half of the summer records showed conditions that were too warm for comfort, causing people to leave the beach or seek shelter. This meant that the period of greatest exposure opportunity was not the middle of the day, but the late morning. It is a time when the ultraviolet radiation is already substantial, but the air remains cool enough to keep people lingering on the sand.

The researchers found that this pattern was driven more by the wind than by the sun itself. While the strength of the ultraviolet radiation is determined by the angle of the sun and the amount of cloud cover, the feeling of comfort is heavily influenced by the breeze. The data showed that stations with stronger sea breezes maintained comfortable temperatures for longer into the afternoon, effectively extending the window of time when people could be exposed to the sun. In contrast, locations with calmer winds saw the temperature rise more quickly, shortening the time people felt comfortable enough to stay outside. This discovery highlights that the risk of sun exposure is not uniform across a region; it shifts based on local weather conditions that dictate human behavior.

When the team looked at the entire dataset, they found that only about 31 percent of all daytime hours fell within the thermal comfort zone. However, during the summer, nearly half of those comfortable hours coincided with high levels of ultraviolet radiation. This overlap is significant because it represents the time when people are most likely to be outside without realizing the danger is peaking. The study also calculated how long it would take for different skin types to burn under these specific conditions. For the most sensitive skin types, the time to burn during a comfortable summer morning could be as short as 27 minutes. This risk drops significantly in the winter, when the sun is weaker and the air is cooler, but the summer window remains a critical period for protection.

The researchers emphasize that their new index does not change the physics of the sun or the nature of the radiation. Instead, it adds a layer of human context to the existing warnings. By showing that the risk is highest when the weather feels just right, the index helps explain why people might stay out longer than safety guidelines suggest. It suggests that public health messages should focus heavily on the late morning hours, when the combination of strong sun and pleasant air creates the perfect conditions for overexposure. The study confirms that understanding when people are outside is just as important as knowing how strong the sun is. By measuring the coincidence of comfort and hazard, this new approach offers a clearer view of the real-world risks we face under the summer sky.

Technical Summary: A UV–Comfort Co-occurrence Index for the South Atlantic Coast

Problem Statement
The Ultraviolet Index (UVI) is the global standard for communicating UV radiation hazards; however, it quantifies environmental intensity rather than actual human exposure. Cumulative UV dose is a function of both radiation intensity and the duration of time individuals spend outdoors, a behavior governed largely by thermal comfort. In temperate coastal regions, where maritime influences moderate temperatures and encourage beach attendance, the interaction between UV hazard and thermal comfort is critical. Despite its relevance, there is a lack of systematic, spatially distributed research characterizing the joint occurrence of high UV hazard and thermally comfortable conditions using meteorological observations.

Methodology
The study developed a UV–Comfort Co-occurrence Index (IVCC) to characterize the coincidence of elevated UV hazard and thermal comfort along the temperate Southwestern South Atlantic coast of Argentina.

Data Source: The analysis utilized 30-minute observations from a network of 12 automatic weather stations (AWS) in Buenos Aires Province, covering the period from January 2024 to January 2026 (747 days; 430,272 observations). Variables included air temperature, relative humidity, wind speed, global solar radiation, and UVI. ERA5 reanalysis data provided total cloud cover, and solar zenith angle (SZA) was calculated independently.
Thermal Comfort Metric: Apparent Temperature (AT) was used as the descriptor for thermal sensation, calculated using the Steadman (1994) formula: $AT = T + 0.33e - 0.70v - 4.00$ . The "comfort zone" was defined as AT between 18°C and 25°C.
Index Formulation: The IVCC was calculated as $IVCC = UVI \times W^c$ , where $W^c$ is a thermal comfort weighting factor. Three formulations for $W^c$ (Gaussian, stepwise, and linear normalization) were tested; a Gaussian function centered on the comfort zone ( $AT_0 = 22°C, \sigma = 8°C$ ) was retained for its continuous discrimination.
Analysis: The study employed partial correlation to control for solar geometry (SZA), permutation tests to validate co-occurrence significance, and Kruskal–Wallis tests to evaluate spatial and seasonal variability. Time to erythema ( $t_{ery}$ ) was also estimated for five skin phototypes.

Key Contributions

Development of the IVCC: The paper introduces a validated, integrated indicator that combines UV hazard intensity with the behavioral probability of outdoor exposure determined by thermal comfort.
Temporal Asymmetry Identification: The study demonstrates that the "late morning" (prior to 13:00) represents the window of greatest exposure opportunity. While UVI peaks at solar noon, thermal comfort erodes rapidly in the early afternoon as AT exceeds 25°C, causing the IVCC to diverge and decline more steeply than the UVI.
Spatial Drivers: The research quantifies that spatial variability in exposure opportunity is governed primarily by wind speed (which moderates AT) rather than UV irradiance.
Regional Estimation: The study provides regional estimates of time to erythema across five skin phototypes and four seasons, specifically under thermally comfortable conditions.

Results

Co-occurrence Rates: Of 156,352 daytime records, 30.7% occurred under thermally comfortable conditions. In summer, 45.1% of these comfortable records coincided with a UVI $\ge$ 6.
Correlation Analysis: The raw correlation between AT and UVI ( $r_s = 0.407$ ) decreased by 72% (to $r_s = 0.116$ ) after controlling for solar zenith angle. This confirms that the association between thermal comfort and UV levels is driven by shared solar geometry rather than an independent physical link.
Diurnal Patterns: Both UVI and IVCC peaked at 12:00 (UTC−3) during summer. However, by 13:00, 46% of summer records had AT values above the comfort threshold, causing the IVCC to drop significantly relative to the UVI.
Spatial Variability: Mean IVCC showed a significant negative correlation with mean wind speed ( $r_s = -0.573$ ), whereas UVI showed no significant association with wind ( $r_s = -0.007$ ). Stations with stronger sea breezes maintained comfortable conditions longer into the afternoon, extending exposure windows.
Time to Erythema: Under comfortable summer conditions, the median time to erythema for Skin Phototype I was 26.7 minutes. This risk is highly seasonal, with 52% of comfortable summer records presenting a $t_{ery} < 30$ minutes for Phototype I, compared to effectively zero in winter.

Significance and Claims
The paper claims that the IVCC complements existing UV hazard communication by incorporating the behavioral dimension of outdoor exposure. It argues that focusing solely on solar noon risks underestimating exposure during the late morning, when UV levels are substantial and thermal conditions remain comfortable.

The authors state that this is the first study to quantify the asymmetry between UV hazard and thermal comfort along a temperate South Atlantic coastline using continuous network observations. They emphasize that the spatial distribution of exposure opportunity cannot be inferred from UVI alone, as it is determined by thermal comfort drivers (specifically wind speed). The index is presented as a tool to sharpen UV risk communication and is noted as suitable for integration into urban and coastal planning. The authors conclude that validating the index against direct observations of beach occupancy and personal UV dosimetry is the logical next step, rather than claiming immediate operational deployment.

Technical Summary: A UV–Comfort Co-occurrence Index for the South Atlantic Coast

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