Warning: Another long post with technical content, but hopefully in explanations that most are able to follow and understand 🙂
Please everyone, do not listen to bda because he clearly does not know much about the subject and is merely guessing. And guessing incorrectly in fact.
It appears to me that bda still believes “wind chill” is something that happens on an intercooler, or your windscreen, and appears to believe that an intercooler, by itself, can cool the internal charge air to a temperature that is lower than the ambient air temperature of the air flowing over the external surfaces of the intercooler.
Any quick search of google will point you to “high school” level physics resources that confirm an intercooler can do no such thing. It is a physical impossibility.
If I come back to the contrived scenario of water freezing on a windscreen when a car begins to move in air temps of +1°C, I can again confirm that this phenomenon has absolutely NOTHING to do with “wind chill”. “Wind chill” is not a phenomenon that applies to inanimate objects.
The phenomenon of liquid water freezing into ice on a windscreen travelling through air temps of +1°C can be accountable through many different factors:
1: is your car’s air temperature sensor reliable enough for you to know that the air is not actually 0°C or below? NO is the correct answer here.
2: have you travelled to a different altitude in “Normal Conditions”? Normally, air cools as you ascend in the atmosphere and the difference in a theoretical “Standard Atmosphere”*** (in still air) between 0.5°C and 0.0°C is less than 250 feet. Not much of an elevation change. The actual Environmental Lapse Rate varies on a day-to-day and an hour-by-hour basis, so the elevation change in the real world to experience a 1°C change in air temperature can vary wildly and a “Dry Adiabatic Lapse Rate” can easily be in the region of 3 or 4°C cooling every 1,000 feet of ascent. That’s 1°C per 25o feet or 0.5°C every 125 feet. That’s an insignificant height difference in the real world, where, again, both the environmental conditions at the surface and the environmental lapse rate can be significantly different over relatively short distances.
2: have you travelled DOWN-HILL into cooler valley temperatures when “katabatic” conditions exist. Katabatic conditions are when the air cools, for example, overnight. Cool air is more dense and tends to flow down hills and valleys. The temperature in a valley, for instance, can be significantly colder in the morning the air temperature higher up the valley slope, because the colder, denser, heavier air has collected in the bottom of a valley. This is katabatic conditions and can lead to valley fog or valley frost on lower slopes, in reverse to normal conditions.
1 and 2 with amendments: With a little bit of air movement, a slight breeze or a stiffer wind, there are significant effects where the terrain surface affects the prevailing conditions. These are called “orographic effects” and range from a breeze blowing colder air out of a narrow valley into a wider flood plain down-stream, or rotor effects where air at hill-top level is dragged toward the surface in the leeward side of the hilly area. These orographic effects have a significant invisible effect on prevailing conditions, such that you can’t predict what the conditions might be unless you have a firm grounding in meteorology. (Which, as an aviator and an instructor, I do.)
3: have you driven behind a hedge in the morning, when the sun is particularly low on the horizon, perhaps on an northeast-to-southwest road segment early in the morning, where the hedge to the southeast conceals and protects a wet road surface from the sun, where otherwise the surrounding roads are dry? This can set up local cold-spots in relatively small and very localised areas. While there is significant lag in car air temperature sensors, the liquid water on the windscreen reacts much, much more rapidly, and much more readily than the number displayed on your dashboard. Meanwhile, you drive back out of the localised pocket of colder air and your air temperature display hasn’t registered the localised dip. It may not be a hedge, but instead a row of housing in a residential area. We’ve all seen that just after sunrise that frost on some residential roads thaws easily in sunlight because of the parallel angle of the sun, while other roads perpendicular to the direction of sun are still treacherously white. Air temperature in these areas is extremely localised.
4: Has there been cold soak of your windscreen from other parts of the surrounding car body-work? It is very conceivable that in using water to clear any frost on the windscreen that the surrounding metal body panels have not been defrosted or heated to temps above their overnight low. Metal car surfaces do reach much lower temperatures than the air temperature overnight. Metal cools readily. It is a good conductor and a good radiator. Once you understand how the air is cooled, this makes more sense. Some people think that the air is cooled “directly” and that it can lose heat by radiating it away. This is incorrect. Air is an excellent insulator. It does not conduct readily nor radiate any heat of its own. The air does not lose heat without something solid to absorb the heat energy away. That surface is the ground. Or your car. The solid surfaces of the ground and car metal bodywork radiate heat away, particularly on a clear night. It is the air in contact with these surfaces that gets cooled by these cold surfaces. This is where the only cooling takes place. At the microscopic interface between air and the surface that it is in contact with. A little bit of air motion really helps to cool a larger air mass. On an absolutely flat calm clear night, you tend to get dew and frost forming on the surfaces. Car metal bodywork is an excellent example of where water vapour moisture content of the air is “sublimated” directly onto the metal as solid water frost, without going through the liquid stage. It is common to find frost on your car in the morning even when the air temperature is above freezing. It is also common to see frost on your car and no other garden surfaces, such as your grass lawn or pavement, simply because metal conducts and radiates more readily than those other surfaces. Fog is also a condition that is directly due to the air being cooled only by the surface on which it contacts. However, fog requires a gentle breeze for it to form. Water condenses as dew on a still night. When there is around 5mph wind, a gentle mixing of the air occurs and the air that is cooled directly by the surface is lifted away from the surface by turbulence in the moving air. This lifts the liquid water “cloud” up off the ground, causing (radiation) fog to form…
… but to move back to the original point – about cold metal surfaces that can easily be much colder than the surrounding air temperature – and these cold metal surfaces of the car body can and do easily conduct any heat energy away from the windscreen, causing liquid water on a windscreen to re-freeze when the windscreen itself cools back to sub-zero. This is quite prevalent, and you will notice the edges of the screen freeze first. Which is quite normal, because water will only be present around the edges because it doesn’t sit on the glass, particularly if you have any sense as a driver and use your windscreen wipers to clear it away from the main screen, LOL.
Most people have witnessed this heat-soak of cold metal body panels have an effect when they see the inside of the windscreen mist up. The edges mist first. And now you know why this might be 🙂
5: Finally, and this is perhaps what you were driving at with the incorrect “wind chill” theory. Once we’ve discounted the above likely scenarios of elevation change and localised areas of cooler air, you need to be in a very specific set of circumstances for liquid water to freeze in air temps close to, but not below freezing. First of all, you need to be sure that the whole car structure has a temperature of above zero. (which normally isn’t the case, and heat transfer by conduction always occurs in the car structure which means that it takes a lot longer than you might expect for the surrounding air to heat the entire car structure up to above zero). Next, you need air temps very close to freezing, but also to be very, very dry, with next to no water vapour content. This is the important part. Dry. No moisture vapour present in the ambient air. We all know, by experience that your clothes will dry much more quickly on a breezy day when compared to a still air day while the other conditions of air temperature and relative humidity remain the same. So it is here with the “wind chill”, which isn’t a real phenomenon. Rather, what is happening is that when to car is at rest, the microscopic depth of the air-to-windscreen interface doesn’t evaporate much of the liquid water away before it becomes saturated and cannot hold any more water vapour content. On a relatively still day (and let’s face it, that’s when frost normally occurs as I’ve already explained above) the air in contact with the windscreen cannot be replaced by the dry air that surrounds it, because no air movement is taking place. However, when the car begins to move, suddenly, the air that flows over the windscreen replaces the air already in contact and can evaporate a little bit more liquid water and carry it as water vapour. As this evaporation takes place, latent heat needs to be absorbed for the change of state of the water to go from liquid to vapour. That latent heat is supplied by the mass of water and also by the glass of the windscreen. As the latent heat is absorbed, and the vapour is carried away by the air flowing over the windscreen, that is then being replaced by drier air, which can then absorb some more latent heat, and so on… this cools the water and the windscreen that is in contact with the water. That cooling effect can bring the surface temperature of the water and screen to below zero, and if there is enough water transfer taking place into vapour, the remaining water can freeze due directly, and ONLY to the process of cooling due to latent heat of evaporation that is absorbing heat energy away.
THAT, at point 5: that is not “wind chill”. It does require movement of the air, this is true, but that isn’t “wind chill”. It requires movement of air because without movement, not enough water can be evaporated quickly enough to absorb enough latent heat of evaporation. It is the process of latent heat absorption that cools the system to below ambient temperatures. Not by virtue of simply a moving air mass, because on it’s own, the entire system cannot be cooled to less than ambient temperatures just by a “wind chill”. Using the term “wind chill” is to misunderstand the effect entirely.
Moreover, bda, your absolute guesswork of:
“I’d probably run with it when the air is below say 15c and take it off when the temps are above 16c November to April have it on, April to October, have it off! best of both worlds then. innit”
… this is pure fabrication with no understanding of the thermodynamic principles behind it. Guesswork. And not even an “educated” guess.
So, on the subject of removing the blanking plate. Don’t allow those that know nothing about it persuade you that +15°C is in any way a real cut-off point. It isn’t. Temperature is certainly not the only physical property that determines whether a blanking plate is useful or not. It could easily be argued that relative humidity is of much more relevance.
Suffice to say that I’m absolutely happy with my own car having it missing, after it has simply fallen off, in *all* conditions of the year here in the UK.
Congratulations to any reader who has read this entire post, and even more kudos to you if you understood the principles behind my explanations.
Yours Aye
Mark H
*** The International Standard Atmoshpere (or ISA for short):
ISA conditions are defined as:
Sea level temperature +15°C; sea level pressure 1013.25 millibars (mb) or the new aviation term HectoPascals (HPa); lapse rate 1.98°C per thousand feet.
In a theoretical ISA atmosphere, an altitude increase of 1,000ft means that the air ambient temperature is around 2°C lower than where you began.
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