Weather is the input people are most sceptical about, and the one that changes the answer most.
Heat is a volume knob
Warmth speeds up evaporation. The same three sprays that sit politely at twelve degrees will fill a room at twenty-eight, and burn through in half the time. This cuts both ways: a heavy oriental that is beautiful in November is a headache in July, and a light citrus that vanishes in the cold can be exactly right in heat, because the heat is doing the projecting for it.
So the engine does not just ask whether a fragrance suits summer. It asks what the next six hours will do to this fragrance, given how it is built.
Humidity carries
Damp air holds scent molecules and moves them around. High humidity makes almost everything read stronger and rounder, which flatters some compositions and ruins others. Aquatics and fresh citrus tend to thrive. Dense sweet gourmands can become cloying.
Still air versus wind
Wind is the enemy of sillage. On an exposed, breezy evening a quiet skin scent may as well not be there, and something with real projection earns its place. In a still room the opposite is true, and the same projection becomes an imposition on everyone near you.
Why the forecast and not the thermometer
Because you are dressing for the evening you are about to have. If it is fourteen degrees now and dropping to six by the time you leave dinner, the fragrance that suits fourteen is the wrong answer. The app looks at the curve rather than the point, and tells you what will happen to the bottle it picked as the air cools.
That is the whole trick, and it is why the reading includes a line about the next six hours rather than just a score.
