Only because your car is on a treadmill.
This is similar to a question I’ve asked in a couple of forums, and never got an answer.
Is the heat exchange greater if I leave the refrigerator door open while making a sandwich, or if I keep wafting the circulation by opening and closing the door several times to take things out and put them back? In other words, was my mother right or wrong when she yelled “close the fridge, you’re wasting electricity”.
If it’s 40F and 100% RH, then when it hits the 32F ice cream it’s going to cool further and leave behind condensate.
If it’s 40F and 100% RH and mixes with ambient air (at 90F and a dewpoint >32F), then the resulting blend will have a dewpoint above 32F, which will likewise cool further and leave behind condensate when it hits the 32F ice cream.
The only way we’re drying out the ice cream is if the ambient air has a dewpoint below 32F (welcome to Moab, UT), and/or the AC evaporator is operating at less than 32F (and so dehumidifying its output to a dewpoint below 32F). In the latter case, something is wrong with the AC.
I think the buoyancy-driven convection associated with leaving the door open for a solid 5 minutes is going to warm things more than just quickly opening/closing the door a few times. Either way I think the cost of the energy involved is pennies.
For best efficiency, think about what you want on your sandwich, then grab all those items in as few visits to the fridge as possible. Put the items back (in another single visit) as soon as your sandwich is made so that you minimize how much they warm up.
I’m trying to explain the observation “the ice cream isn’t melting”. I think condensation on the ice cream wouldn’t lead to this specific observation. If we could confine this flow from the A/C and stream it across the ice cream, then you would be correct; in all cases, water would condense on the ice cream. Unfortunately we are mixing two different air flows and much of what Bernoulli’s Principle predicts will fall apart. With RH’s low enough, the evaporation will proceed at a much much quicker rate than condensation. It’s deadly to stand a few feet behind a jet engine at full throttle, not so much 100 yards away. The air flows mix and come closer to equilibrium by then.
That’s one of the knocks on A/C, it dries the air out so much. If you bring up Moab, UT; then single digit RHs shouldn’t surprise you (it’s not the triple digit temperature, it’s the double digit humidity that’ll getcha).
All I’m saying is that if your ice cream stops melting when you hold it to the A/C vent … it’s because the melt is evaporating so quickly … NOT because the 40ºF air is drawing energy out of the 32ºF solid keeping it frozen.
Observation: The OP asked about an ice cream bar, not about exposed ice cream in a cone or dish.
Question: Air conditioning by definition dehumidifies. Why is it being assumed that the output air is at 100% RH?
Just to be fair to my wife and her friends, I don’t think anyone thought the AC vent would keep the ice cream frozen indefinitely. I think they just believed that it would melt more slowly that way, giving them more time to get it back to the apartment.
ETA: Gary is right; also, it was in a wrapper, and my wife’s friend was holding it by the end of the stick.
It seemed to me the most likely condition to produce a counter-example to my claims; and see, I was right on that score. I do have to add to my claim the condition of low initial RHs.
Because for an air conditioner to dehumidify it has to lower the temperature of the air passing through it below the dew point (100% RH) so the water vapor condenses and drains away. Cold saturated air pumped into a warm space dehumifies as it mixes with the ambient air. Or think about it like this, cool a small tank of air (something sealed tight, like a jug put in the refrigerator) until the water vapor condenses, then drain away the water. When you heat it back up the air inside will be drier than before. An air conditioner does the same thing just not to all the air at once.
In practice the outlet air is not exactly 100% because if it is then you’d see fog coming out the vents. It’s still well into the 90% + range though, but since not all the air effectively touches the evaporator coil there’s still some mixing going on to keep the outlet from being completely saturated.
Buy your membership? Huh?
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Science teachers: this is an excellent question for students to explore in a science fair project. In fact, the thread can even be used as a component of the literature search. xo,
Dr. C.
Ice cream bar(s), in a wrapper! Blanket is the way to go! Insulate, insulate insulate! Hell, they use to have special “ice cream bags” at the supermarket when I was a kid!
My claim is that if the ambient dewpoint is higher than 32F, then any mixture of ambient air and HVAC-cooled air will produce condensation on the ice cream.
HVAC evaporators have an operating temperature above freezing. If this were not true, they would frost up and block the airflow, just like the evaporator in a cheap dorm-room fridge. Given that this is the case, an HVAC system cannot get the dewpoint of its output air down to 32F. If you take its output air and move it across the ice cream, the air temp will drop to 32F - and the ice cream will develop condensation on it (this again assumes ambient dewpoint above 32F).
For a 90F ambient temperature, the RH needs to be below about 12% (i.e. the ambient dewpoint needs to be about 32F) in order to have neither condensation nor evaporation when the air hits the ice cream. This is exceptionally dry for the great outdoors. It’s average for a place like Moab in June, but not for most of the US.
In the rare case of a 90F day with ambient RH below 12%, the air very close to the ice cream will be very close to 32F, so its RH will be very high, and evaporation will be exceedingly slow. Evaporation is an unlikely explanation for an observation of “the ice cream isn’t melting;” if the ice cream isn’t melting, it’s more likely because it was very recently taken from a 0F freezer and its surface hasn’t yet warmed up to 32F.
That would be an amazing rate of evaporation. The only way to achieve an evaporation rate like that would be a massive amount of heat, which isn’t going to come from 40F HVAC air. Point an acetylene torch at an ice cube, and most of it is going to melt and drip away rather than evaporate.
Cool! Be sure to report back your findings. 
Very little will evaporate from the ice cream.
There is an equation, but one would need the ice cream surface area, amount of air flow (mass flow rate), heat transfer coefficient and the heat capacity of the ice cream. (CpH of air is easily looked up).
Temperature of the AC air at the ice cream also needs to be determined; it will warm from mixing immediately upon leaving the duct. Air temp after the ice cream is also needed.
Experimentation would probably take less time. And there would be more ice cream around!