Calculator inputs
Example data
| UVI | Skin | SPF | Cloud | Altitude | Surface | Shade | Coverage | Estimated burn time |
|---|---|---|---|---|---|---|---|---|
| 9 | II | 30 | 10% | 0 m | grass | none | 10% | ≈ 38 min |
| 12 | I | 15 | 0% | 250 m | sand | none | 0% | ≈ 10 min |
| 7 | IV | 30 | 40% | 1000 m | soil | medium | 30% | ≈ 2 h 5 min |
Examples use typical assumptions and simplified modifiers. Your real burn time may be shorter or longer.
Formula used
The calculator estimates time to reach a Minimal Erythemal Dose (MED) for the selected skin type. It converts the UV Index (UVI) into an approximate erythemal irradiance:
- E₀ = 0.025 × UVI (W/m²)
- Time (seconds) = MED / E
- Time (minutes) = (MED / E) / 60 × SPF_effective
The tool then adjusts E using practical modifiers:
- Altitude: increases UV by ~8% per 1000 m.
- Cloud cover: reduces UV, but some penetrates clouds.
- Surface reflection: bright surfaces increase effective exposure.
- Shade and clothing: reduce direct exposure to skin.
- Wet/sweaty: reduces effective SPF in real conditions.
How to use this calculator
- Enter the current UV Index from a trusted weather source.
- Select your skin type and typical SPF used.
- Adjust cloud, altitude, and surface for your garden.
- Choose shade and clothing coverage to match your setup.
- Click Calculate to see estimated burn time and break guidance.
Gardening tips based on your result:
- Schedule pruning or transplanting during lower UV hours.
- Use hats, sleeves, and shade cloth for long tasks.
- Watch for glare near water, sand, or bright paving.
- Hydrate and take shade breaks before discomfort starts.
UV Index and garden task timing
UV Index measures sunburning UV strength on a 0–11+ scale. When UVI is 3–5, many gardeners can work longer with basic protection. At UVI 6–7, unprotected skin can burn within an hour for lighter skin types. At UVI 8–10, trimming, weeding, and spraying should be planned in shorter blocks, especially around midday, with shade and hydration breaks. Use gloves and eye protection too.
Skin type thresholds and MED values
This calculator uses typical Minimal Erythemal Dose (MED) thresholds to estimate first redness. Example starting values: Type I ≈200 J/m², Type II ≈250, Type III ≈300, Type IV ≈450, Type V ≈600, Type VI ≈1000. Real thresholds vary with genetics, season, acclimation, and medication. Use the result as a conservative planning window, not a medical guarantee.
Weather and site modifiers that change exposure
Clouds reduce UV but often leave 30–70% of erythemal energy reaching the ground. Altitude increases UV about 8% per 1000 m, so hill gardens and rooftop plots shorten burn time. Surface glare matters: sand, pale stone, and concrete raise effective exposure, while grass is lower. Shade and clothing coverage are treated as exposure reducers, supporting safer long sessions.
Sunscreen performance in real conditions
SPF is modeled as a time multiplier, but only when applied generously and evenly. Sweat, friction, and irrigation spray can reduce protection, so the tool lowers effective SPF for wet or sweaty skin. For gardening days longer than two hours, reapply at least every two hours and after heavy sweating. Combine SPF with hats, sleeves, and shade cloth for steadier protection.
Using results to plan breaks and protect plants
The estimated burn time helps schedule task blocks: divide your session into work periods and shade breaks before discomfort begins. For high-risk results, shift labor to early morning or late afternoon, use portable canopies, and keep reflective surfaces covered. Protecting yourself also protects plants: fewer heat-stress mistakes, better watering discipline, and safer chemical handling when you are cooler and alert.
FAQs
1) What UV Index is risky for gardening?
Generally, UVI 6 and above increases burn risk quickly. At UVI 8–10, lighter skin may burn in minutes without protection. Use the calculator with your shade, clothing, and SPF to plan safer work blocks.
2) Why can burn time be shorter near water or concrete?
Bright surfaces reflect more UV toward exposed skin. Water, sand, and concrete can increase effective exposure even if you feel cool. Add the surface option and reduce burn time expectations near reflective areas.
3) Does cloudy weather eliminate burn risk?
No. Many clouds still transmit substantial UV, sometimes more than expected. Overcast conditions can reduce UV, but the tool assumes a minimum exposure remains. Keep sunscreen and shade breaks in place.
4) How should I choose my skin type?
Use the Fitzpatrick type that matches how you typically react after 30–60 minutes of strong sun. If unsure, pick the more sensitive option to stay conservative. Your personal threshold can differ from averages.
5) How does sweat affect SPF?
Sweat and friction can thin or remove sunscreen, lowering real protection. That is why the calculator reduces effective SPF when you select wet or sweaty skin. Reapply more often and use protective clothing.
6) Can I use this for greenhouse or shaded patios?
Yes, but treat it as an estimate. Deep shade reduces UV, yet reflected light can still reach you. Indoors or in greenhouses, UV is usually lower but not always zero. Confirm conditions and protect exposed skin.