Wednesday, February 18, 2015

Let's put a freeze on the wind chill factor

You might have heard: It's going to be cold in Charlotte.

It's what people are talking about in our office and yours. The forecast is for unseasonably cold temperatures, brutally cold temperatures, so cold that weather people will quickly run out of adjectives trying to describe how godawful cold it is.

At some point, perhaps even before those adjectives lose their power to wow us, the weather people will resort to a different, brain-numbing phrase. 

The Wind Chill Factor. 

The wind chill factor serves one purpose: It lets us make bad weather seem spectacularly bad. 

Otherwise, it has no real-world value. It doesn't tell you how cold your skin is getting. Air temperature determines that. For example, if the air temperature is 37 degrees but the wind chill is 24, you are not in danger of getting frostbite. The air temp needs to be below freezing for that. 

Likewise, it doesn't tell us when our pipes will freeze. Wind can make things more frigid, which can accelerate the freezing of water, but air temperature is again the main factor. (Researchers at the University of Illinois have determined the pipe-freezing threshold is about 20 degrees. So make sure those outdoor faucets are covered tonight, Charlotte.)

Why even have a wind chill factor? The measurement was developed in 1945 by Antarctic explorers Paul Siple and Charles Passel, who put together an index that they felt would capture how cold we feel at various temperatures with wind blowing. 

There is an actual formula for wind chill. It's been tweaked some since 1945, but basically it assumes that it's nighttime, your exposed face is about five feet off the ground, and you're walking about 3 mph directly into the wind. If you're standing next to a building, or in the sun, or at 2 in the afternoon, you're feeling something different. 

And even then, wind chill is essentially only a calculation of that moment - not a general calculation of temperature - because the wind doesn't blow at a constant, steady rate. Yes, it's colder on your skin when the wind blows, but that number just can't be measured precisely.

Why, then, is it cited so often? As always, blame the media. The original wind chill index went largely ignored until the media got hold of it, specifically at the legendary Ice Bowl 1967 NFL championship game between Dallas and Green Bay in Green Bay. Temperatures that day were -13 degrees, but that didn't sound nearly as legendary as a wind chill of -48. 

So this week, when your weather person says the wind chill is -20, it's a dubious figure at best. That doesn't mean you shouldn't cover your skin when you go outside, especially if it's windy. And you definitely shouldn't do anything like this.

But wind chill is nothing more than meteorological boasting, and as with most boasting, that means 1) there's some exaggeration going on, and 2) nobody is that impressed, anyway. 

Peter St. Onge


tarhoosier said...

Thank you for this. Wind chill is temperature with a press agent. Only for exposed heated items. Who goes out in sub zero temps with no clothing? Has minor significance in determining loss of heat for idling engine of car (heat producing object) or when to wear gloves, scarf and hat. Other than that it gives weather agents a WOW event to crow about. Value for making personal decisions? Close to zero (unadjusted).

blockhead said...

I hate television weather hype as much or more than the next guy. However, you're totally wrong about wind chill. I run, bicycle and walk daily, regardless of weather and temperature. There is an enormous difference between running on a 10-degree morning with no wind, and one in which there's even a 5 or 10 mph wind - not to mention the 30 mph and up winds that normally accompany a cold front. I can be toasty at 10 calm degrees, but nearing frost bite and hypothermia at the same temperature with wind. Huge, huge difference. My guess is, this screed was written by someone who sits on the coach - like many of my friends - and expounds on how they just love winter and cold weather.

Mark Helms said...

While we're at it can we ban the ridiculous new fad of naming winter storms?

blockhead said...

I won't belabor the point but a moment longer. I agree with Mark, btw, about the ridiculous thing of naming storms. However, discounting wind chill is silly and dangerous, if you understand thermodynamics. Heat "pools" around a stationary body, which is why an air-cooled engine (or a liquid-filled radiator) needs to be moving to keep from overheating. It's also why moving air (wind) dissipates body heat so much faster. Why do you think we have "windbreaker" jackets, etc.? Incidentally, the ideal arrangement for keeping warm on zero mornings remains a natural wool inner garment or garments, and an exterior high-density, windproof shell, one for insulation, the other to prevent heat dissipation. I'm sure I'll see you all out at 5 tomorrow for your morning runs. (I'll beat you to it - only a nut...)

Fred Johnson said...

I beg to differ. There is a thermodynamic precedent for wind chill; it is called wet-bulb temperature. Wet bulb temperature takes into account the evaporative cooling effect (if you are standing out in 60 degree air and your face is wet, it will feel much cooler than if your face is dry.)
The mechanical engineer that designed the HVAC system at your home or office used wet-bulb temps in determining capacity of the equipment. So it's not just marketing. Love, Jim Cantore

James Edgar said...
This comment has been removed by the author.
James Edgar said...

blockhead - I don't think the message here is that there is no value in wind chill, but that the number cited is not necessarily accurate. I, for one, applaud someone calling out these meterologists for putting semi-accurate information out in public in order to justify their existence. These people are far too self-important. I will never forget sitting on my couch during a summer thunderstorm listening to Brad Panic-ovich openly question the FAA for allowing flights to take off and land at the airport, as if his meterology degree made him more qualified than the FAA to decide when it's safe to land a plane. Steve Useless-son and Shreiking Thomas are just as bad. They need to rein in the hype and panic. A lot.

blockhead said...

I totally agree with Mr. Edgar. I question how many of these "meteorologists" really are. There's also a danger that by constantly crying wolf, when there is legitimate danger, viewers will have become inured to warnings. My point is simply that wind chill is incredibly dangerous. Though it is unlikely that anyone like me who's nuts enough to be outdoors in all kinds of weather would be ill-informed enough to be influenced by Peter's misinformation, there's a remote chance there is. Dressing for 10 degrees in calm air could literally be deadly when it 10 degrees with a 40 mph wind.

Larry said...

What about those who transport fruit in the bed of pickup trucks. Do they not need to know this in every weather report?

Mike Hermann said...

In summer we add the humidity to the temperature to get the heat index. But we don't then factor in the wind to subtract some from the temperature like we do in the winter? Seems we only like to know how miserable we are.

The Observer Editorial Board said...


I don't think anyone would recommend dressing inappropriately for 10 degree weather with 40 mph winds. I would guess that meteorologists saying "It's 10 degrees out, and the winds can reach 40 mph" would do just fine as a warning instead of the hype and inaccuracy of wind chill.