Wind Chill

DRussell · Jan 12, 2022

Wind chill has always been a bit confusing to me. Maybe the smart people on here can explain it better.

I'm driving to work this morning and the OAT was 11F. There's a bit of ice on my windshield and I ran the defrost all the way to work, about 20 miles. Anytime I'm stopped at a stop sign or red light the ice would immediately begin to melt and slide down the windshield. As soon as I started driving, especially over 25-30 it would freeze right back up despite the warm air from the defrost on the windshield.

The difference has to be the wind from driving. Isn't that essentially wind chill? I thought wind chill wouldn't have affected that as I've always understood it to not affect inanimate objects like metal, glass, etc. I thought wind chill simply made it feel colder than it actually is to flesh, but wouldn't affect whether water would actually freeze sooner or not.

cjunrau · Jan 12, 2022

Windchill is in reality the wind blowing the heat away.
If you take a 1500watt heater in a shelter and hold your hand 2 ft away it is toasty warm. Now go into the wind and that heater does nothing as the heat is being blown away. On your window same thing. Your heater can heat the glass until to much cold air is blowing the heat away faster than you can heat it.
A block heater on a vehicle is the same. Tractor sitting in the middle of a field plugged in on a windy day at -20 will never warm up. Put that same tractor in a sheltered area where the wind cant blow and plug in and the heat stays in it till its warm enough to start at even -30.
Humans skin is the same way. If the wind cant blow the cold away from you , you can stay warm. Wind blows and takes the heat away.

kcm.MN · Jan 12, 2022

Consider the difference between a tractor with a clean radiator and another tractor that has a radiator full of dirt and chaff. Both tractors are identical except for how well the radiator works.

Air flowing over such surfaces can make a very huge difference.

Another way to look at it is that your windshield has much more area to heat when moving than it does sitting still. When sitting, the same air is in contact with the glass. When moving, the air is constantly moving, therefore the heat from your defroster is spread out over a vastly larger area of air.

It's much colder here (NW MN) than where you live. I can tell you, I actually prefer -20F with calm winds to 35F with a wind chill of 10F. Why? The wind tends to find a way into my jacket, hat and gloves, so it's colder.

Personally, I think the method they use to calculate wind chill needs a makeover!

Steve in VA · Jan 12, 2022

Not really. Wind chill is an estimate of how it feels. For an inanimate object, the air temp is the air temp independent of wind. However, for biological purposes, evaporation can effectively sub-cool the skin. In your example, the ambient temp was below freezing so the water on the windshield would freeze. When you stop, the defroster warms the glass above freezing. Once moving, the air that has been warmed and it refreezes.

Bill(Wis) · Jan 12, 2022

The wind wipes away heat bringing the surface of your windshield down to the ambient outside air temperature. Or close to it, you did have some heat on the windshield but the wind was taking it away. The wind will never reduce the temperature of an object to below the temperature of the air surrounding it.

jon f mn · Jan 12, 2022

What most mean by wind ot affecting inanimate things is that no matter how much wind there is the temp can never get below to real temp. So if the real temp is zero no matter how hard the wind blows a tractor can't get below that zero mark. The wind will make it cool faster, but not colder. Where many get confused is when they put their tractor in an unheated shed and it starts better than outside. The reason for that is inside the shed, even with no heat, it is warmer than outside. And protected from wind it can take days for a tractor to cool to actual outside Temps so a regular used tractor will start better inside.

Steve@Advance · Jan 12, 2022

Wind chill is a combination of things. Mainly moisture on your skin evaporating and carrying away heat, and the moving air over your skin, also carrying away heat. Your body can only produce so much heat, so it quickly feels the difference.

Inanimate objects also experience this to an extent. A heated object will loose it's heat faster with cold air moving across it, and even faster still if there is evaporation also taking place, as in the object is wet.

As far as the windshield freezing when moving, 2 things, the evaporating water was lowering the temperature more efficiently when moving, but probably the one thing having the greatest effect would be the warm air from the engine coming through the gap at the back of the hood.

When stopped, the air flow slows down, the air has more time to warm as it passes through the radiator and over the warm engine, carrying the heat to the windshield. Once moving the cold air dilutes the warm to the point it is scattered and diminished, so it has little effect on the ice.

Pete in Holland MI · Jan 12, 2022

All about the speed of heat (or cool) transfer (loss or gain).

If it is 30 degrees STILL air temperature, the wind can blow and achieve 15 degrees wind chill, but the air temperature is still 30 degrees. YOU, on the other hand will get colder, faster, when the wind is blowing because you are losing temperature like it is 15 degrees STILL air temperature.

You benefit from wind chill in the summer too. When it's 90 outside, and you drive with your arm outside of the window, you are still blowing the 90 degree air on your arm, but it is cooling your arm like it is a cooler STILL air temperature.

john in la · Jan 12, 2022

Now to throw a sledge hammer into the conversation.

Wind can not reduce the temperature of an inanimate object.
Just like the wind removes heat from your body making you feel colder than it really is (wind chill) the wind was removing what little heat you had in your windshield.

But a clear night sky can lower the temperature of an inanimate object.
It is why a metal roof condensates or ice forms on your windshield when the air temperature is only 35 degrees.

DRussell · Jan 12, 2022

The condensation forms because the surface of the windshield or metal of the car is colder than other nearby surfaces.

I've always understood that the metal or glass surfaces freeze before other nearby surfaces because the material they are made from more readily lose heat than other surfaces, such as grass/ground/concrete etc.

showcrop · Jan 12, 2022

I guess that I was around 17 when wind chill was invented. I was working on the ski patrol at a pretty decent sized ski area, and they gave us all a stack of wallet sized cards to pass out to people as we cautioned them about this new windchill thing. All the time as I was growing up we would work and play out in the freezing cold and wind and come in and complain about how the white patch stung as it turned pink again, three days later the skin would peel and then we would be back out there freezing it again.

jimg.allentown · Jan 13, 2022

The answer to your question is two factors at work.

First is evaporation. When any liquid (water in this case) changes state from a liquid to a gas, it absorbs energy (heat) which reduces it's temperature. In your case, when you are moving, some of the liquid water is evaporating and therefore reducing the temperature on the outside of your windshield.

Second is the temperature differential. In scientific terms, there is no such thing as cold. Only relative levels of heat. So, taking into account that the outside air is (as you stated) 11 degrees, it is well below the freezing point of the water on your windshield. In still air, the heat moves from your defroster inside to the glass in the windshield. That warms the ice to above the freezing point of the water (ice). Once you begin moving, you have 11 degree air removing the heat from the outside of the windshield so the malted ice refreezes.

Wind chill is based on evaporation from a moist surface. It primarily affects flesh. It does not and cannot affect a dry hard material like iron or steel or aluminum.

Wind Chill

DRussell

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cjunrau

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kcm.MN

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Steve in VA

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Bill(Wis)

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jon f mn

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Steve@Advance

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Pete in Holland MI

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john in la

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DRussell

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showcrop

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jimg.allentown

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