Monday, October 24, 2011

The Truth About Wind Chill

How many times have you heard it on a TV weathercast?   "The temperature is 35F, but with the stiff winds today, the wind chill temperature is 24F, so bundle up!"

 TV folks sometimes refer to the wind chill temperature as the apparent or equivalent temperature, the temperature it would feel like if the wind was calm.

Now if the temperature is 35F and the wind chill temperature is 24F, will water freeze? (Answer: No)

Will your car battery have a harder time starting your car if the wind chill is lower, but the temperature is the same? (No)

What exactly is this wind chill business?

 The original wind chill index was created during the early 1940s by two polar explorers (Siple and Passel), who measured how long it took for plastic bottles of water to freeze at various wind speeds.   If the temperature was below freezing, the bottles would freeze up more quickly when the wind speed increased.  Their original index was not in terms of temperature, but rather in cooling rate.

   In the 1960s, wind chill was expressed in terms of equivalent temperatures--the still air temperature that would produce roughly the same cooling rate compared to the observed temperature and wind speed.   Finally, in 2001 the National Weather Service produced an upgraded wind chill chart that more accurately described the cooling rates at various temperatures and wind speeds (see below):


The whole idea of wind chill is based on the fact that the loss of heat on our skin is related to the difference in temperature between our skin and the cooler outside air (more cooling when the difference was larger) and the wind speed (stronger winds remove heat from our skin faster).   Under calm or near calm conditions, a veneer of "dead air" stays near our skin, and this air, warmed by our skin, act as an insulator.   If the winds pick up, this protective layer is progressively removed.

 The efficiency of batteries only depend on temperature.  If the wind is strong perhaps the battery might cool off faster if it enters an area of cooler temperatures, but the chemical reactions inside the battery only vary with temperature.  Freezing of water depends on the average speed of the water molecules (which is directly related to temperature, the average kinetic energy--energy of motion--of the molecules).  Temperature alone decides on whether water will freeze.  The winds could be blowing 500 mph, but water will not freeze at 33F!

Tonight, eastern Washington is going to be hit by some of the coldest temperatures yet this winter, and the National Weather Service has a freeze warning for the lower elevations of eastern WA.  Here is the forecast temperature for 5 AM.  Most above freezing.


And here are the wind chill temperatures at the same time (lot of them below freezing)


 Wind chill or equivalent temperatures are useful, but there are many other factors that influence cooling rate, such as the amount of solar radiation (sun) and clothing.

 Seattle School Board Information

Jack Whelan, who had run for the Seattle School Board this year, has written a wonderful piece on the current election.  If you would like to read it, do so here.

My recommendations for the Seattle School Board election is here.

And today, another State Auditor's report found that the pressure to underpay for the MLK School came from the highest levels in the School District.

11 comments:

Ivana said...

Speaking of winter, could you please explain why people in Kentucky could see the Aurora Borealis tonight? How often can we see the Aurora in Seattle?

OffBeatMammal said...

unrelated to windchill but some interesting climate change data on the new forecast site WeatherSpark.com ... any thoughts on the validity/usefulness of the data http://weatherspark.com/#!climate;a=USA/WA/Redmond;ctum=0;cth=500;ctmy=15;ctsy=1949;ctey=2010

JewelyaZ said...

Dear Cliff,
It isn't winter yet. It sure has been a cold year though. :-)

Will our kids freeze in the "wind chill" when trick-or-treating?

I got a text message that was good for $160 off a set of 4 studded tires. I think I may just spend the money this year... last year's winter driving scared me.

Ron said...

As I see it, there is one big difference between Wind Chill and "Feels Like". Wind Chill explicitly addresses removal of heat from bare skin. But even when you are bundled up with little or no exposed skin, the wind still does an efficient job of removing heat from your body. Another way to looking at it: would you want to be in calm conditions with an air temp of 20F, or would you want a windy day where the air temp is 35F, but the wind chill is 20F? I'd gladly take the calm day. So for me, Wind Chill underestimates the impact of the wind.

Unknown said...

I think it's fair to say that the MM5 model grossly misrepresented the boundary layer cooling last night in E WA... typical local boundary layer effect. Kudos to the NWS for catching what the model often misses.

RLL said...

The mischief was putting or implying temperatrue with wind chill. If they had dome something like 'tonights low temps will be 22 degrees, with WC18" -the two metrics clearly deliniated.

Jiggy said...

Hi Cliff, I work for the Canadian Wheat Board and had a question regarding your wind chill article. Could you call at 204-770-6066 when you have a moment? Cheers.

Jacques Marcoux
Canadian Wheat Board
Winnipeg, Manitoba

mjamend said...

I had a friend who, up into his 20's, thought the weather man on TV was talking about the "windshield factor". He thought it was the temperature at which your car's windshield would frost up. When I explained the truth to him, I've never seen someone so flabbergasted.

Bob said...

For 20% of your grade: Assuming an air temperature of 40°F and using the formulae for wind chill and frictional heating, at what speed would you have to jump out of an airplane in your birthday suit and still remain comfortable? ;^}

Michael DeMarco said...

I lived in the Arctic. A calm minus 50 degrees and a wind driven minus 50 can mean the difference between life and death. Throw in some body moisture and you have a serious problem. The numbers underestimate the effect.
As to the Aurora: The aurora is affected by the position of the magnetic north and not true north and thus the East is more likely to see them as they "belly down." Seattle is a poor place to observe due to the light pollution. They were seen in the San Juans.

JP said...

Not sure if you have tried this already, but perhaps it can help w/ your dog?

http://lostpetatlas.com/