Paging the Hive's Defroster Whisperers

I live in Ontario (the province), so I spend a lot of the winter driving around with my defroster on. Way back when we had our first Mazda MPV, I was out with the kids one cold winter night, made a right turn, and the defroster blew cold air for a few seconds. I had a moment of existential dread, as one does ("Oh, God, we'll all freeze to death and they won't find our bodies until spring!"), but then the air warmed up again. Eventually I accepted this as an MPV quirk, or Feature, especially when our second MPV did the same thing. Then the 2014 Kia Rondo did it, and now I'm damned if it doesn't happen in the 2020 Odyssey too. Just right turns, never left.

I'm now at the point where I treat the brief blast of cold as a refreshing interlude, but it never happened in the Tercel, the Voyager, or anything else I can recall. So my question is, basically, What the hell is going on?

My 87 4Runner would do that for more than a few seconds because there was an air bubble in coolant line leading to the heater core.

That's typically air in the coolant.

Seems like the air should be coolant temp.

Don't most defroster run the AC as well as heat because air conditioning is not just cooling but dehumidifing which is helpful for moist windshields. 

bearmtnmartin (Forum Supporter) said:

Seems like the air should be coolant temp.

It is, but air carries less than 1/4 the heat energy as water.  It's also far less dense, so it is far slower at releasing heat to the heater core.

Same reason you can stick your hand in a 350 degree oven and not get burned, but you get sizzled if you touch the oven rack.  They're both 350 degrees, but the steel has a lot more heat energy in it.