How Does a Fan Cool You Down?

How does a fan cool you down if it can't cool the room? It doesn't lower the air temperature at all: the electricity it uses ends up as heat. It cools your skin instead. In an 82 °F (about 28 °C) room, your 91 °F (about 33 °C) skin warms a thin layer of still air next to it. A breeze of about 4.5 mph (2 m/s) sweeps that layer away, and one estimate puts your heat loss at about 200 watts instead of 20: ten times as fast. This video also covers the second effect, evaporating sweat: each gram takes about 2,400 joules of heat with it, so 100 g (about 3.5 oz) of sweat in an hour carries away about 67 watts, about two-thirds of the 100 watts your body makes at rest. It also shows why a fan in an empty room only uses electricity, and why a fan helps less in very humid air. Try it yourself: ▶ https://www.metric-conversions.org/temperature/fahrenheit-to-celsius.htm Chapters: 0:00 The puzzle 0:25 The warm layer next to your skin 1:33 Evaporation 2:04 A worked example 2:36 Limits and takeaway Sources: • Fans: The First Line of Cooling (Minnesota Commerce Department, Home Energy Guide): https://mn.gov/commerce-stat/pdfs/fans-the-first-line-of-cooling.pdf • Staying cool, saving energy (GRDA, crediting energy.gov): https://grda.com/staying-cool-saving-energy • What Happens to Your Used Electricity? (Green Building Advisor): https://www.greenbuildingadvisor.com/article/what-happens-to-your-used-electricity • Heat Balance in the Human Body (UBC): https://c21.phas.ubc.ca/article/heat-balance-in-the-human-body-2 • Fahrenheit to Celsius (metric-conversions.org): https://www.metric-conversions.org/temperature/fahrenheit-to-celsius.htm • Understanding Wind Chill (NWS): https://www.weather.gov/safety/cold-wind-chill-chart • Phase Changes (HyperPhysics): http://hyperphysics.gsu.edu/hbase/thermo/phase2.html • NIST Handbook 44, Appendix C: https://www.nist.gov/document/nist-hb-44-2024-appendix-c-general-tables-units-measurement • Heat Index (NWS Amarillo): https://www.weather.gov/ama/heatindex Is there anything else you'd like us to cover? Let us know in the comments below.