What should I wear today?

The forecast comes from the National Weather Service, hour by hour. We take the daytime hours still ahead of you (tomorrow's, once today's are over) and use their average temperature, the strongest wind and the highest chance of rain or snow.

Next we work out how cold or hot that feels. In cold wind we use the Weather Service's wind chill formula; in heat and humidity, its heat index formula. Otherwise it is simply the air temperature.

Then we adjust for you. If you run cold, we dress you as if it were 7°F (4°C) colder; if you run warm, 7°F (4°C) warmer. Sitting still counts as 3°F (2°C) colder and exercise as 10°F (6°C) warmer. Being out 30 minutes or less counts as 4°F (2°C) warmer; 4 hours or more counts as 4°F (2°C) colder when it is still below 65°F (18°C) after those adjustments. Below 72°F (22°C), when the wind chill formula isn't already in use, a steady wind of 15 mph or more counts as 5°F (3°C) colder, and 25 mph or more as 7°F (4°C) colder.

The result is what we call the dressing temperature. It falls into one of the ranges below, and each range has a starting outfit. Rain, wind, snow, sun, exercise and a big gap between the day's low and high then change it. From a 20% chance of rain the card adds “Compact umbrella”, and from 40% “Umbrella”.

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