You can walk outside in shirt sleeves in −40°C if it is absolutely calm but you may require the thickest of layers at −5°C if the wind is above gale force. Even the wind generated by walking can cause frostbite; a gale at −40°C is very dangerous.

The ‘sensation’ scale – how you will feel at a given wind chill temperature

• Below −25°C: Freezing cold
• −25 to −15°C: Bitterly cold
• −15 to −10°C: Very cold
• −10 to 0°C: Cold
• 0 to 10°C: Very cool
• 10 to 15°C: Cool
• Above 15°C: Mild

The National Weather Service in the United States has developed a different formula for measuring wind chill.