How this recommendation is worked out
The suggestion above starts from the air temperature and shifts it about 4 °C colder whenever the wind box is checked — a rough stand-in for the same wind-chill effect this site's dedicated wind chill calculator works out precisely. That adjusted, "effective" temperature is then matched against seven clothing bands, from short sleeves at the top down to full winter layers at the bottom. Rain adds a waterproofing note on top of whichever band the temperature falls into, since what to wear when it is wet is a different question from how many layers to put on.
Why the thermometer alone is not enough
Two days at the same air temperature can call for different clothing entirely depending on wind and moisture — a still, dry 10 °C afternoon is comfortable in a light jacket, while a windy, wet 10 °C afternoon can feel closer to freezing against exposed skin. That is the same reasoning behind the feels-like temperature and wind chill calculators elsewhere on this site — the number on the thermometer is a starting point, not the full picture.
This tool trades precision for simplicity: it is a single fixed adjustment rather than a full wind-speed input. For an exact figure, use the wind chill calculator with the actual wind speed, then decide clothing from that adjusted temperature instead.
No — the suggestion assumes ordinary walking-around activity. Higher exertion (running, cycling, manual work) generates significant body heat, so dressing for the resting version of a given temperature usually means overdressing once moving; a common rule of thumb is to dress as if it were 10 °C warmer than it actually is when planning to exercise outdoors.