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 those two peaks slide out of step is the reason Split view exists. Combo view answers the simpler question, whether this hour is any good, by showing whichever of the two is worse.

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.

Start with the finding that reframes this. In a study of 905 heat illness cases across UK primary care, 74 percent were exertional, and of those, 67 percent followed an ordinary walk rather than running or hard play. The hot car is the famous killer. The walk is the common one.

There is no published table that converts degrees into minutes for a pet dog. Nobody has run that trial. So this number is built from three things that have been measured, and it is a rule of thumb, not a reading.

The first is the ceiling. Military working dog practice puts activity bouts at five to twenty minutes with rest between, which returned 80 percent of dogs to within half a degree of their baseline core temperature, and above 86 degrees fifteen minutes is treated as the limit on a work period. That fixes the hot end of the curve.

The second is humidity. A dog cannot sweat. It sheds heat by panting, which is evaporation, so the drier the air the better it works. Evaporative loss starts to suffer past 35 percent humidity and is effectively cancelled past 80 percent. Humidity is added back here as degrees on that basis, and its weight grows as the air closes on the dog's own body temperature. At 70 degrees a dog is still dumping heat the ordinary way and losing evaporation costs it little. At 100 degrees evaporation is very nearly all it has, and damp air there is worth ten degrees against it.

Between those, the minutes fall in a straight line: an hour at 68 degrees, half an hour around 77, fifteen minutes at 86, ten at 90, and past 95 it stops being a walk.

The third number is the ground, and it is the one people miss. Asphalt in open sun with no wind runs about fifty degrees hotter than the air: 77 degree air measured 125 degree pavement, and 87 degree air measured 143. A paw pad burns on a 124 degree surface in about a minute. The estimate here climbs with the sun and falls slowly after it sets, because asphalt gives its heat back over hours. Settle it with the back of your hand: five seconds on the ground, and if you cannot hold it, neither can they.

All of this is set for a medium adult dog of ordinary build, 20 to 30 kg, with a normal muzzle. The same study found what moves that: flat faced breeds and Chow Chows are far higher risk, up to seven times a Labrador, dogs under two years old are the most affected age group, and every dog over 10 kg carried higher odds than the small ones. Heavy double coats sit in the same direction. Breed and size settings are coming; until then, adjust it in your head.

A dog will keep walking to keep up with you long after it should have stopped. The clock is yours to keep, not theirs.

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.

Pavement against air temperature: James J. Berens, Thermal Contact Burns From Streets and Highways, JAMA, 1970, the source of the 77 degrees air to 125 degrees asphalt figures still quoted today.

Canine heat illness, the 74 percent exertional and 67 percent on walks figures, and the breed and bodyweight odds: Emily J. Hall, Anne J. Carter and Dan G. O'Neill, Dogs Don't Die Just in Hot Cars, Animals, 2020, drawn from VetCompass primary care records.

Work and rest bouts, and the 86 degree ceiling on a working period: US Army veterinary heat injury guidance for working dogs, and published military working dog core temperature recovery work.

Evaporative cooling failing past 35 and 80 percent humidity: working dog heat stress literature.

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