Why damp days feel colder

The News (New Glasgow) - - WEATHER -

Yester- day, I ex­plained wind chill. I did so to get to a ques­tion that Kevin Red­mond sub­mit­ted late last month. Kevin un­der­stands the con­cept of wind chill but won­ders why the hu­mid­ity is not re­flected in the per­ceived tem­per­a­ture us­ing cur­rent wind-chill met­rics.

It cer­tainly does feel colder on a damp day, Kevin; here’s why.

Our cloth­ing keeps us warm by trap­ping air be­tween our body and our clothes. Our body then warms this thin, layer of air. The air trapped can’t eas­ily cir­cu­late to trans­fer heat and cool our bod­ies. On a cool damp day, how­ever, the layer of air trapped be­tween you and your clothes con­tains more water mol­e­cules. It takes more heat en­ergy to warm water than it does to warm air be­cause water has a higher heat ca­pac­ity. If the layer of air next to your skin is damp, it will take more of the body’s heat en­ergy to warm it. That trans­fer of heat from the body to the air will cool you down.

Sky con­di­tion is an­other fac­tor. A damp day is al­most al­ways over­cast. On a dry sunny day, your body is more likely to be warmed by the sun.

The fac­tors such as cloud cover, where in­di­vid­u­als hap­pen to be and mois­ture trapped in a layer of air be­tween cloth­ing and a body can be mea­sured but on an in­di­vid­ual ba­sis. Too many vari­ables ex­ist to make this a cal­cu­lated value that’s added to a fore­cast to keep peo­ple safe.

So Kevin is cor­rect, it does feel colder on a damp day but that’s not what wind chill is mea­sur­ing. Wind chill is a value that in­di­cates the per­ceived tem­per­a­ture of ex­posed skin cooled by the wind. As wind speeds and tem­per­a­tures fluc­tu­ate, so do the cal­cu­lated val­ues or feel-like tem­per­a­tures.

Did you know… a re­cent sur­vey in­di­cated 82 per cent of Cana­di­ans use wind-chill in­for­ma­tion to de­cide how to dress be­fore go­ing out­side in the win­ter.

Cindy Day is SaltWire Net­work’s Chief Me­te­o­rol­o­gist.

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