The sun column runs on the UV index scale the EPA and the World Health Organization already publish, the one printed on sunscreen and used in every weather app: green low, yellow moderate, orange high, red very high, violet extreme. The heat column runs on the National Weather Service heat index bands: nothing under 80, yellow caution, orange extreme caution, red danger, magenta extreme danger. Neither ramp is invented here, so a colour means the same thing on this page as it does everywhere else.

The one liberty taken is that the sun column is keyed to your skin rather than to the raw index. The published scale is quietly calibrated to fair skin, so at type II the colours land exactly where the official index would put them. Choose a darker type and the same colours arrive later in the day, which is the point of a scale that is about harm rather than about sunlight.

The black mark down the left edge of a day is your window: an hour or more before your skin reddens, and a heat index under 90. That is looser than both columns being green, deliberately. Waiting for a perfect hour in August means never going outside.

The UV index is not a temperature and not a percentage. It measures how fast sunlight is reddening skin, scaled so one point equals 25 milliwatts of sunburn causing energy per square metre. That scaling has a useful side effect: the index is roughly the number of standard sunburn doses you collect in an hour of full exposure. Index 8 for an hour is about 8 doses.

A standard dose (one SED) is 100 joules per square metre. How many you can take before your skin visibly reddens is your minimum erythema dose, and it is the only part of this that depends on you.

Two of these are published anchors, type I at 2.5 doses and type IV at 5. The rows between are interpolated and the two darkest extrapolated along the same slope, so treat every number as a bracket, not a stopwatch. Two people inside one category can differ by a factor of two, and medication, pregnancy, and recent sun all move the line.

The clock assumes bare skin facing open sky. Shade, clothing, a hat, and a low sun all stretch it. Water, pale sand, and snow shorten it by bouncing extra light back up at you.

Ultraviolet tracks the sun's angle and peaks within about an hour of solar noon. Heat lags, because the ground spends the afternoon giving back what it absorbed all morning, so the hottest hour usually lands between three and six. Watching the two columns slide out of step is the reason they sit side by side instead of being merged into one score.

That gap is the opening. Early morning is often clear on both counts. Late evening is usually clear on UV and still failing on heat.

The heat number is the National Weather Service heat index, computed here from air temperature and humidity. It is a shade number. Standing in full sun can add up to 15 degrees to it, which is why athletic trainers and the military use wet bulb globe temperature instead, a measurement that folds in sun angle and wind. If you are planning hard exercise rather than a walk, that is the number to go find.

SPF is a multiplier on time, not a percentage of protection. SPF 30 means it took thirty times the dose to redden the tested skin. The catch is that the test uses 2 milligrams of product per square centimetre, and measured real world application runs closer to a quarter or half of that, which lands most people nearer a third of the number on the bottle.

Two practical consequences. A high number does not buy an unlimited day, it buys a longer one, and it stops buying anything the moment it sweats or towels off. And reapplying beats the number on the bottle: a fresh coat of 30 outperforms a four hour old coat of 70.

Everything past the current hour is a forecast. UV forecasts are unusually good, because the sun's angle is arithmetic and the only real unknown is cloud, but a bank of cloud arriving an hour early moves a cell a shade. The heat side inherits normal temperature forecast error, roughly two to three degrees on day one and widening across the five.

The page refetches every ten minutes and whenever you come back to the tab, so the five days roll forward on their own. The stamp under the wordmark tells you how old the numbers are.

This is a planning tool, not medical advice. If you take a photosensitising medication, have lupus or another photosensitive condition, or are following a dermatologist's instructions, their numbers beat these.

Forecast and place search: Open-Meteo. UV comes from the CAMS European atmosphere model, temperature and humidity from national weather models.

UV index colours and category names: EPA UV Index Scale, which follows the WHO international standard.

UV index definition and the 25 milliwatt scaling: WHO, WMO, UNEP and ICNIRP, Global Solar UV Index: A Practical Guide, 2002.

Index to doses per hour, and the type I and type IV dose anchors: Richard McKenzie, How is UV Index related to UV Dose?

Heat index equation and band names: Lans P. Rothfusz, The Heat Index Equation, NWS technical attachment SR 90-23, 1990, and the NWS heat index chart.

Skin categories: Thomas B. Fitzpatrick, The validity and practicality of sun-reactive skin types I through VI, Archives of Dermatology, 1988.