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 them are interpolated and the two darkest are 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 and heat do not peak together, which is the whole reason a block here can be violet, ember, or striped with both. UV 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.

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. A block turns green only when both are true at once: an hour or more before your skin reddens, and a heat index under 90.

The heat number is the National Weather Service heat index, computed from air temperature and humidity. Their scale calls 80 to 90 caution, 90 to 103 extreme caution, and 103 to 124 danger, where cramps and exhaustion get likely with prolonged activity. It is a shade number. Standing in full sun can add up to 15 degrees to it.

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 block 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 and the numbers stay current. The stamp under the location tells you how old they 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 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 category boundaries: Lans P. Rothfusz, The Heat Index Equation, National Weather Service 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.

Colour carries meaning here and never decorates. Violet is ultraviolet load, ember is heat load, green is the window where both are clear, slate is after sundown.