About SunScope
The True Temperature — what the weather actually feels like on your body.
What is SunScope?
SunScope is a free outdoor thermal comfort forecast that goes beyond standard air temperature. Most weather apps tell you it's 12 °C and leave you to guess whether that means t-shirt weather or a heavy coat. SunScope combines air temperature, humidity, wind speed, and solar radiation into a single felt temperature — so you know what to actually expect when you step outside.
It uses the Universal Thermal Climate Index (UTCI), a peer-reviewed biometeorological standard developed by Bröde et al. (2012) and used globally in heat-health warning systems and urban planning. Forecast data is sourced in real time from Open-Meteo, a free and open-source weather API drawing on ECMWF and national met services. SunScope is available for any location worldwide.
What is the SunScope soak-factor?
Standard UTCI does not account for precipitation — it tells you how cold the wind and sun make you feel, but not the extra chill of rain soaking your clothing or snow settling on your skin. The SunScope soak-factor fills that gap with an original penalty formula.
Light drizzle (under 1 mm/hr) reduces the felt temperature by roughly 1–2 °C. Heavy rain combined with strong wind can push it down by 7–8 °C. Snowfall carries an additional penalty on top of any rain adjustment. The result is shown in the UTCI+P column — a more honest, real-world comfort score for anyone heading outdoors.
Urban heat: concrete surface temperature
The UTCI standard was developed with a natural grass surface as its reference ground — the assumption being that you're standing in a park or open field. In a city, that assumption breaks down. Concrete and tarmac absorb more solar energy than grass, and unlike grass they cannot cool themselves through evaporation. On a sunny summer afternoon, exposed concrete can run 15–25 °C hotter than the surrounding air.
SunScope's Concrete surface column estimates the temperature of sun-exposed urban paving using an energy-balance model that accounts for how much solar radiation the surface absorbs, how efficiently the air above it carries that heat away (which depends on wind speed), and the reflective properties of typical urban concrete. The result gives you a sense of the contact heat stress you'd experience sitting, standing, or walking barefoot on city surfaces — something the standard UTCI value alone won't tell you.
The column is enabled by default in the Urban profile and colour-coded: values above 30 °C are shown in amber, above 40 °C in red.
Vehicle interior temperature
A sealed, parked vehicle heats up far faster than most people expect. SunScope's Vehicle column models the ambient cabin air temperature experienced by an occupant who is seated out of direct sunlight — the classic scenario of a child or pet left inside a parked car.
The calculation combines two physical processes. The first is body panel conduction: metal body panels (roof, doors, bonnet) absorb solar radiation and conduct heat into the cabin regardless of the sun's position in the sky. This is the slow, relentless background heat that builds even on overcast days. The second is side-window solar gain: when the sun is at an elevation where its rays cut through the side glass rather than striking the roof or reflecting off at a shallow angle, additional heat enters the cabin. Because the occupant is modelled as sitting out of the direct beam, this energy goes into raising the ambient cabin air temperature rather than heating the person directly — which is precisely what makes it so dangerous. The air around you heats up steadily while you remain unaware of how hot the environment has become.
Wind speed has almost no effect on a sealed vehicle and is not factored in. The figure shown represents a steady-state cabin temperature reached after approximately 45–60 minutes of parking. Values above 35 °C are shown in amber (dangerous for children and pets); above 45 °C in red (potentially fatal within minutes).
Reading the stress bands
SunScope maps every UTCI+P value to a thermal stress band so you can read conditions at a glance:
| UTCI range | Band | What it means outdoors |
|---|---|---|
| Below −27 °C | Polar | Survivable only with expedition-grade kit; do not go outside unprepared |
| −27 to −13 °C | Arctic | Very strong cold stress; full winter protection needed |
| −13 to 0 °C | Freezing | Strong cold stress; warm layers essential |
| 0 to 9 °C | Cold | Moderate cold stress; coat and gloves advised |
| 9 to 18 °C | Chilled | Slight cold stress; comfortable with a light jacket |
| 18 to 26 °C | Comfortable | No thermal stress; ideal outdoor conditions |
| 26 to 32 °C | Mod heat | Moderate heat stress; stay hydrated |
| 32 to 38 °C | Strong heat | Strong heat stress; limit strenuous activity |
| 38 to 46 °C | V. strong heat | Very strong heat stress; seek shade and cool fluids |
| Above 46 °C | Extreme heat | Extreme heat stress; dangerous for outdoor exposure |
Frequently asked questions
Why does SunScope sometimes show a much higher temperature than my regular weather app?
On clear, sunny days direct solar radiation heats your skin significantly — sometimes adding 10 °C or more to what the air temperature alone would suggest. SunScope calculates Mean Radiant Temperature from shortwave radiation data, capturing this effect. Standard weather apps ignore it entirely.
Why does SunScope sometimes show a lower temperature than my regular weather app?
Wind chill and precipitation both reduce felt temperature. If it's raining or blustery, the UTCI+P value will be noticeably lower than the thermometer reading — which is exactly what your body experiences.
How far ahead does the forecast go?
SunScope currently shows a 72-hour hourly forecast. Data refreshes automatically from Open-Meteo each time you load the page.
Is SunScope free?
SunScope beta is currently free to use for any location worldwide. As the product develops, some features may move to a paid tier — but core forecasting will always be accessible.
What data sources does SunScope use?
Forecast data comes from Open-Meteo, which aggregates ECMWF, GFS, and national meteorological service models. The UTCI calculation follows the Bröde et al. 2012 polynomial. Geolocation search uses the Open-Meteo geocoding API.
Get in touch
SunScope is an independent project. If you have feedback, spotted a bug, or are interested in using SunScope data for your business or website, feel free to reach out via hello@sunscope.net.