Rates a run by how it will feel, not just the temperature. GoDay scores every hour from 6am to 9pm, dark hours included, because plenty of people run before sunrise, and picks the best hour of each day.
A thunderstorm holds the score at 2. A feels-like above 35°C (95°F) holds it at 3, and rain over 4 mm an hour at 4. Poor air quality (US AQI over 150) holds it at 4, and over 200 at 2.
Daylight counts too: once it's dark the score is held at 7.
Cooler than most people expect: GoDay rates a feels-like of 5 to 15°C (41 to 59°F) as ideal. Past 23°C (73°F) it starts to count against the run, and past 28°C (82°F) it's hot.
The dew point measures the moisture in the air. Above about 18°C (64°F) sweat stops cooling you well and a run feels muggy, even when the temperature looks fine.
Yes. GoDay uses the US Air Quality Index: up to 50 is good and up to 100 moderate. Above 150 the score is held down, because hard breathing pulls more pollution into your lungs.