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.
Forecast profiles
SunScope organises its columns into profiles — curated sets of data tailored to different activities and use cases. Rather than showing every column at once, each profile surfaces the information that actually matters for that situation. You can switch profiles using the row of buttons above the forecast table, or switch to Custom to pick individual columns yourself.
🏙️ Urban
Designed for anyone moving around a city on foot — commuting, shopping, sightseeing. The urban environment creates its own microclimate: buildings channel wind into gusts down narrow streets, concrete absorbs heat all day and radiates it back long after sunset, and the lack of vegetation means there's no evaporative cooling. The Urban profile shows air temperature, wind speed and direction (useful for finding sheltered routes), cloud cover, UV-A index and burn time for sun protection, UTCI+P for overall felt comfort, precipitation, and concrete surface temperature — the heat radiating up from pavement that standard forecasts completely ignore.
🌾 Farming
Built for anyone working the land — farmers, growers, and market gardeners. Decision-making in agriculture is driven by conditions at ground level, not just the air. The Farming profile shows air temperature, relative humidity and dew point (critical for frost risk assessment and disease pressure on crops), wind speed and direction (essential for safe spraying windows), cloud cover and sun elevation for light levels, UTCI for heat stress on the person working outdoors, precipitation, and the three soil columns: surface soil temperature, soil temperature at 6 cm (the root zone for most seedlings), and soil moisture — so you know whether the ground is workable or waterlogged before you head out.
⛵ Sailing
Wind is everything on the water, and the conditions out at sea can be dramatically different from what the shore feels like. The Sailing profile leads with what matters most: air temperature (hypothermia is a real risk — water conducts heat away from the body twenty-five times faster than air), dew point (condensation on sails and equipment is a practical concern at dawn and dusk), wind speed with gusts, wind direction, cloud cover, and sun elevation for visibility and glare. UV exposure is significantly higher on open water due to reflection off the surface, so UV-A index and burn time are included. UTCI+P captures the combined misery of wet, windy conditions, and precipitation rounds out the picture for passage planning.
🚗 Vehicle
Covers the parked and travelling vehicle experience — especially useful for anyone with children or pets, motorhome travellers, and caravan owners. The headline column is vehicle interior temperature: a physics-based estimate of how hot the cabin gets when parked and sealed, adjusted for your chosen vehicle type (car, MPV, SUV, motorhome, or caravan). Wind speed is included because it matters significantly for high-sided vehicles and caravans. Cloud cover and air temperature give context, precipitation covers driving conditions, and UTCI+P tells you what it will feel like when you step outside. Soil moisture is included specifically for motorhome and caravan users — values above 0.4 m³/m³ indicate saturated ground where pitching or manoeuvring is likely to cause problems.
🏠 Home
For managing indoor comfort without air conditioning — the typical UK home scenario. The two indoor temperature columns are central: Indoors estimates the ambient temperature inside a typical brick house with windows closed and no active cooling, accounting for wall conduction, window solar gain, and thermal mass (indoor temperatures usually peak 2–4 hours after the outdoor peak). Managed indoors shows what you could achieve by following standard heatwave advice — closing curtains to block direct solar gain and opening windows whenever the outdoor air is cooler than inside. Supporting columns include air temperature, relative humidity and dew point (important for condensation and mould risk in cooler months), wind speed for ventilation decisions, cloud cover, and precipitation — so you know whether to open the windows or bring the washing in.
Column reference
Every column in the SunScope forecast table is described below.
| Column | What it shows |
|---|---|
| Air °C | Standard air temperature at 2 m above ground — the thermometer reading. Does not account for sun, wind, or humidity. |
| RH % | Relative humidity: how much moisture the air holds as a percentage of its maximum capacity at that temperature. High RH makes warm days feel stickier and cold days feel rawer. |
| Dew Point | The temperature at which air becomes saturated and moisture begins to condense. Above 16 °C it starts to feel muggy; above 21 °C it feels oppressive. A better measure of absolute humidity than RH. |
| Wind | Mean wind speed at 10 m, with peak gust in brackets where significantly higher. Wind is the primary driver of wind chill — a key ingredient in the UTCI calculation. |
| Dir | The compass direction the wind is blowing from, shown as an arrow and abbreviated label. Useful for route planning and spray timing. |
| Cloud | Total cloud cover as a percentage of sky. High cloud blocks solar radiation, reducing daytime heating and UV exposure. |
| Sun ° | The angle of the sun above the horizon in degrees. Below 0° the sun has set. The higher the elevation, the more intense the radiation reaching the ground. |
| Direct W/m² | Shortwave solar radiation arriving as a direct beam from the sun. The primary driver of skin heating on sunny days. |
| Diffuse W/m² | Solar radiation scattered from all directions across the sky. Present even under cloud cover; contributes to the overall solar heat load. |
| Tmrt °C | Mean Radiant Temperature — the temperature your skin effectively "sees" from all surrounding surfaces and the sun combined. Can exceed air temperature by 20–30 °C on a sunny day; this is why shade feels so much cooler. |
| UTCI−Air Δ | The gap between the UTCI felt temperature and the plain air temperature. A large positive value means solar radiation is adding significant heat stress beyond what the thermometer shows. |
| UTCI °C | Universal Thermal Climate Index — the raw felt temperature combining air temp, humidity, wind, and solar radiation. Does not include precipitation effects. |
| UV-A | Estimated UV-A radiation index. UV-A penetrates deeper into the skin and contributes to long-term ageing and some skin cancers, even through glass. Present throughout daylight hours. |
| UV-B | Estimated UV-B radiation index. Causes sunburn and drives vitamin D production. Intensity falls sharply at low solar elevations due to the longer atmospheric path length. |
| Burn time | Estimated time to reach one Minimal Erythemal Dose (MED) — the threshold for sunburn — based on the UV index and your selected Fitzpatrick skin type. A guide, not a medical measurement. |
| UTCI+P °C | SunScope's adjusted felt temperature: UTCI plus the soak-factor penalty for rain and snow. Wet clothing and skin can reduce the effective felt temperature by up to 8 °C in heavy, windy rain. |
| Precip mm | Expected rainfall or snowfall in mm per hour. Even light drizzle meaningfully reduces felt temperature when combined with wind. |
| Soil °C surface | Temperature of the soil at the surface (0 cm depth). Most seeds germinate above 7–10 °C. Useful for sowing decisions and frost assessment. |
| Soil °C 6 cm | Soil temperature at 6 cm depth — the root zone for many crops and seedlings. Lags behind the surface by several hours. |
| Soil moisture | Volumetric water content of the top 1 cm of soil in m³/m³. Above 0.4 indicates saturated ground; below 0.2 is dry. Useful for assessing whether ground is workable, and for motorhome or caravan pitch suitability. |
| Concrete °C | Estimated temperature of sun-exposed urban paving. Concrete absorbs more solar energy than grass and cannot cool itself through evaporation — surface temps can run 15–25 °C above air temperature on sunny days. |
| Vehicle °C | Estimated ambient cabin temperature inside a sealed, parked vehicle — adjusted for your chosen vehicle type. Values above 35 °C are dangerous for children and pets; above 45 °C potentially fatal within minutes. |
| Indoors °C | Estimated temperature inside a typical UK brick house with windows closed and no active cooling. Accounts for wall conduction, window solar gain, and thermal mass — indoor peak typically lags the outdoor peak by 2–4 hours. |
| Managed °C | Estimated indoor temperature following standard UK heatwave advice: curtains closed during the day to block solar gain, windows opened whenever outdoor air is cooler than inside. Shows the benefit of passive cooling over doing nothing. |
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.