Garmin Under Jacket Baro-Altimeter

In theory it's not absolute pressure that's used for floor calculation. It's the CHANGE in pressure that triggers floor count.

In theory it's not absolute pressure that's used for floor calculation. It's the CHANGE in pressure that triggers floor count.

Yeah, but will it be able to detect that change when I wear the watch under the jacket?

Sure it will. Your jacket is not pressurized, is it?

Being inside a car slow down the updating of the altitude reading, even if the car is not pressurized.

A car severely restricts air flow and has a rigid body, so indeed external pressure change can be reflected with a delay inside a car, but a piece of normal clothing, even with limited breathability, will have negligible impact on pressure underneath.

The temperature might affect the output though.

car ventilation need output for air, and it's always open, remove rear bumper and you will see it immediately... pressure is always same in and out. Can you imagine that you can't open door because higher pressure outside car after return back from mountains???
I think we have barometric sensor in watches today but they have some limits still. But for stairs+floors is relevant data from accelerometer too, isn't it?

The temperature might affect the output though.

Yeah ok, cause it's much hotter under the jacket then outside the jacket.