Wind Chill / Heat Index / Dew Point

The US National Weather Service formula is 35.74 + 0.6215T − 35.75V^0.16 + 0.4275T·V^0.16, with T in °F and V in mph. It is calibrated for temperatures at or below 50°F with wind of at least 3 mph.

Temperature measures the air itself; the heat index measures how hot it feels to your body. Sweat cools you less in humid air, so the heat index rises with humidity even when the thermometer doesn’t.

The dew point is the temperature air must cool to for water vapor to condense into dew. A higher dew point means more moisture in the air — it is the best single number for how ‘muggy’ it feels.

Below 55°F dew point feels dry and comfortable; 60–65°F starts feeling sticky; above 65°F feels muggy; and above 70°F feels oppressive.

It feels like about 21°F. Wind strips the warm layer of air off your skin, so the same thermometer reading feels noticeably colder as wind rises.