Concepts: How does a (cold) front defroster melt ice on the other side of the glass?


#1

When you turn on your defroster fan, within a minute or two, it starts to clear a little area at the bottom of the iced over windshield. Yet, the car engine is not yet warmed up, so it’s blowing cold air. How does blowing cold air on the inside somehow start to break down the ice on the outside of the glass??

I think I have seen some cars with a blower vent on the outside hood, and this would make more sense as there is direct air blowing onto the iced glass. Even though it’s still just cold air. But, most cars don’t have outside vents and just the inside vent. How does this work? I understand how WARM air from the inside would conduct through the glass and melt the outside glass, but not cold air when the car has just started up.


#2

Cold is a relative term. The air only has to be warmer than the glass.


#3

Put a thermometer in that air the next time this happens and we’ll go from there.


#4

Maybe it’s true that molecules in motion are warmer than the ones that aren’t moving. I just made that up because I don’t remember any science from that place we used to go to all day when we were little.


#5

Coolant circulating through the heater core is not restricted by the thermostat.


#6

35F air will feel cold, still melt ice.


#7

Moving air causes evaporation. Just go to Walmart and visit their bathrooms. The dryers blow cold air which still dry your hands! If the water is frozen, moving air can actually cause sublimation which means evaporation with out going through the melting stage.

Those who grew up on the farm may recall your mother putting wet clothes on the line outside in freezing air, which dried just the same, only slower than inside.


#8

"Those who grew up onthe farm may recall your mother putting wet clothes on the line outside in freezing air, which dried just the sam, only slower than inside. "

I was with you until that last part. More than once my Grandmother would bring pants in from the line frozen stiff. Of course that was Minnesota.


#9

Nothing to add, lots of great responses. Let me add that if the AC is also on, then as the frost melts, it evaporates off the window quicker and appears to clear quicker for that reason too. In older cars without AC, there was much more water left on the window, running down the glass and getting everything wet before it could evaporate. Without AC, older cars were less healthy environments with more mold and bacteria IMO. AC with a cabin filter do a better job overall, both winter and summer.

Even under non freezing conditions, AC does an amazing job of keeping windows clear with no heat at all. For frost, all that is required is that the cabin be above freezing and with AC, the windows will clear with little apparent heat.


#10

Pleasedodgevan, “temperature” is actually a quantitative measurement of atomic activity. Different measurement scales exist largely due to the reasoning behind their creations. In “celcius”, water (at “sea level” ambient pressures; 29.92 inches of mercury) freezes at zero and boils at 100. One scale, Kelvin, places zero degrees at that point where all atomic activity ceases. Subatomic particles actually behave differently when approaching “absolute zero”. They begin to line up in “planes” that allow electrons to zip straight through without hitting any resistance, resulting in “superconductors”.

This subatomic stuff is an area of “quantum physics” that I can’t pretend to understand, but to say that molecules in motion are warmer is sort of a roundabout way of recognizing what temperature actually represents.


#11

Sublimation does not apply here, he wants to know about the ice on the OUTSIDE of the windshield, not the inside. As soon as the engine starts, it starts heating up the coolant and the coolant circulates through the heater core, even though the temp gauge is till sleeping on the post and the thermostat is closed tighter than a bank vault. As pointed out, even 35 degrees is high enough to melt ice.


#12

When the thermostat is closed, the coolant is still passing through the heater hoses and radiator, just not the engine block water jackets?


#13

No. When the thermostat is closed the coolant still circulates in the engine and heater.


#14

A closed T-stat simply restricts the coolant from free passage through the radiator.


#15
pleasedodgevan2 December 10 Maybe it's true that molecules in motion are warmer than the ones that aren't moving. I just made that up because I don't remember any science from that place we used to go to all day when we were little.

Yes, but taking into consideration that the mean velocity of air molecules is approximatetly the same as the speed of sound, the low speed blast of air coming out of your heater vent hardly factors in.
Now, if you are coming out of orbit at a velocity of about 5 miles per second, the air you start hitting is going to be very hot from your perspective.