The thermometer reports the air; your skin reports the EXPERIENCE, and two things drive them apart. In summer, moisture: sweat cools you by evaporating, and near-full air won't take the water, so heat piles up. Canada measures that with the humidex — air temperature plus a moisture bonus computed from the dew point. Calibrate your gut with three landmarks: a dew point of 15 °C adds about 4, 20 °C adds about 8, 21 °C adds about 9. Muggy is measurable.

In winter, wind: moving air strips body heat far faster than still air, so the skin loses the fight sooner. The wind chill index (the 2001 formula both Canada and the US adopted) turns air temperature and wind speed into the still-air temperature that would feel the same. The working rule: a strong winter wind roughly doubles the bite — −10 °C in a 30 km/h wind wears like −20. First job on every question: pick the RIGHT index. A calm, mild day needs neither — the thermometer is already telling the truth.