About SunScope

See the sun the way your skin does.

What is SunScope?

SunScope is a free outdoor thermal comfort forecast that goes well beyond standard air temperature. Most weather apps tell you it's 12 °C and leave you to guess whether that means a t-shirt 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.

The core calculation is 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. On top of that, SunScope builds its own SunSoak index — because UTCI alone doesn't account for the full picture of where you actually are and what the weather is actually doing to you. SunSoak layers three extra dimensions on top of UTCI: a rain and snow penalty for the chill of wet clothing and skin; an environment modifier that adjusts the solar and thermal load for your surroundings (forest canopy, alpine altitude, lakeside glare, shaded riverbank, desert ground heat); and the full radiant heat absorbed from surrounding surfaces. One number that honestly answers: what will my body actually feel out there?

Forecast data is sourced in real time from Open-Meteo, a free and open-source weather API drawing on ECMWF and national meteorological services. The forecast refreshes automatically every five minutes and is available for any location worldwide.

What else does SunScope calculate?

SunScope goes far beyond a single felt-temperature number. Depending on the profile you choose, it can calculate and display a wide range of derived quantities, each built on the same per-hour weather data:

Vehicle interior temperature. A physics-based model estimates how hot a sealed, parked vehicle gets over time — combining heat conducted through the roof and body panels with direct solar gain through the side windows. You can select your vehicle type (car, MPV, SUV, motorhome, or caravan), each of which has its own albedo, glazing area, and insulation characteristics. A Windows open toggle switches to a ventilated model where convective loss is roughly five times higher, dramatically reducing cabin heat build-up — mirroring the real-world effect of cracking the windows.

Indoor temperature. The Indoors column estimates the temperature inside a building with windows closed and no air conditioning, simulating retained warmth, window solar gain, internal gains, and thermal mass hour by hour. You choose your building type from a dropdown — brick, modern insulated, Victorian terrace, stone cottage, timber frame, top-floor flat, or conservatory — and the physics model adjusts accordingly. Each type has its own insulation level, thermal mass, retained warmth, and glazing characteristics, so a stone cottage and a conservatory behave very differently on a hot day. The model now lags toward a realistic hourly target rather than repeatedly adding solar heat, so indoor temperatures typically peak 2–4 hours after the outdoor peak, which is why a house can still feel stifling at 10 pm on a summer day. Tick Managed alongside the dropdown to switch to the heatwave-advice model: curtains closed by day to cut solar gain, windows opened whenever outdoor air is cooler than inside.

Urban concrete surface temperature. Concrete and tarmac absorb far more solar energy than grass, and unlike grass they have no evaporative cooling. On a sunny summer afternoon, exposed paving can run 15–25 °C hotter than the surrounding air. SunScope models this using a surface energy balance approach that accounts for solar absorption, surface emissivity, and convective cooling by wind.

UV and sunburn. SunScope splits the solar UV spectrum into UV-A and UV-B components. The burn-time column estimates how long before you reach one Minimal Erythemal Dose (the clinical threshold for sunburn) based on the UV index and your Fitzpatrick skin type — selected from a dropdown in the column bar.

Soil temperature and moisture. Surface and 6 cm soil temperatures are shown for agricultural and outdoor-work planning. Soil moisture indicates whether the ground is workable or saturated — particularly useful for motorhome and caravan pitching.

Air quality, visibility, and pollen. Three atmospheric columns sit together after cloud cover in the table. Visibility shows horizontal visibility in kilometres, sourced from the CAMS model — values below 1 km indicate fog, below 10 km suggest mist or pollution haze. AQI shows the European Air Quality Index (0–100+), combining PM2.5, PM10, ozone, nitrogen dioxide, and sulphur dioxide, with colour-coded bands from Good to Hazardous. Pollen shows concentration in grains per cubic metre for a selectable pollen type — choose from total pollen, grass, birch, alder, mugwort, olive, or ragweed — with Low / Moderate / High / Very High labels. All three are sourced from the Copernicus CAMS air quality forecast.

The sky scope. The large circular visualisation at the top of the page shows the sky conditions for the currently selected hour. The sky colour smoothly interpolates through a full set of elevation-keyed gradients — from deep pre-dawn navy through golden sunrise, bright midday blue, warm sunset amber, and back to night — with a sun disc whose glow radius and intensity scale with the actual shortwave radiation at that moment. It refreshes automatically alongside the forecast.

Day strip weather and temperatures. Each day tab in the date selector shows the day's forecast at a glance: a weather icon representing the dominant daytime conditions (sun, cloud variants, or precipitation), plus the day's high and low SunSoak felt temperatures in warm and cool colours. The icon uses the actual daytime solar elevation and cloud category data so it reflects the real character of the day rather than a generic symbol.

Weather event banners and popups. When notable weather conditions are forecast — such as a frost, strong winds, fog, or a heat event — a banner appears above the forecast table with a brief alert. Tapping or clicking the event icon opens a detailed description popup explaining what the condition means and what to watch out for.

What's Coming — cosmic events almanac. A 🔭 panel below the forecast table shows upcoming astronomical events for your location over the next 90 days: meteor showers, lunar and solar eclipses, planetary conjunctions, and oppositions. Each entry shows the event name, peak date, a countdown, a short description, and a visibility note where the event is better from a particular hemisphere or location. When a cosmic event is currently active, it also appears as a banner above the forecast table alongside the weather alerts.

What is SunSoak?

Standard UTCI is calculated assuming open, unshaded conditions with no precipitation — it tells you how the wind and sun affect your body, but not the full picture of where you actually are or what the weather is actively doing to you. SunSoak fills those gaps with three layers of original calculation stacked on top of the UTCI foundation.

The precipitation penalty. Rain and snow are not in UTCI. The soak-factor formula adds an original penalty based on rain intensity, snowfall rate, and wind speed — reflecting the extra chill of wet clothing and skin. 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.

The environment modifier. UTCI assumes you are standing in the open. SunSoak adjusts the solar radiation load and air temperature for six distinct environments, selectable from a dropdown on the SunSoak column: Open / Exposed (the UTCI default); Forest / Woodland (canopy reduces direct radiation by ~85%, evapotranspiration cooling lowers apparent air temperature); High Altitude / Alpine (thinner atmosphere increases radiation dose by ~15% at 2,000 m); Beach / Lakeside (water and sand reflection increases effective solar load); River / Riverbank (partial canopy and strong evaporative cooling from the water surface); and Desert / Arid (extreme radiation from high-albedo ground, with significant ground-level heat adding to felt temperature). Selecting a Place or Activity profile automatically sets the most appropriate environment for that context — but you can always override it manually.

Mean radiant temperature. The full radiant heat load from surrounding surfaces — ground, walls, sky, and direct sun — is calculated as Mean Radiant Temperature (Tmrt) and fed into the UTCI polynomial. On a sunny day this alone can add 20–30 °C to the felt temperature compared to a shaded reading.

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 between Basic, Home, and Vehicle using the buttons above the forecast table. Two dropdowns — Places and Activities — give access to a further set of tailored views. The Temps comparison and Custom column builder are part of SunScope Extra.

Column reference

Every column in the SunScope forecast table is described below.

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 Places → Urban variant 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: body panels absorb solar radiation and conduct heat into the cabin regardless of the sun's position in the sky. Thin metal car panels pass much more of this heat through than the insulated sandwich panels used in many motorhomes and caravans. 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 unless the Ventilation toggle is enabled. For cars, the figure represents the cabin temperature after roughly 45–60 minutes of parking. For motorhomes and caravans, the model allows for lower heat gain, slower response from insulated panels, and retained occupied-space warmth, so the estimate stays much closer to the real living-space temperature on cool or moderate sunny days. 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 SunSoak value to a thermal stress band so you can read conditions at a glance:

Frequently asked questions

Got a specific question — about UTCI, the SunSoak index, UV and sunburn times, how hot a parked car or pavement really gets, keeping your home cool without air conditioning, air quality, pollen, or soil and farming data? Our dedicated FAQ page answers the most common questions in plain language, grouped by topic.

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.