What I mean by “sweet spot” is this: everyone knows that a locked, windows-up car left in 90F heat will be an oven in no time. Everyone also knows that a locked, windows-up car left in -30F cold would be positively frigid inside (right?). Assume:
[ul]
[li]a sunny day[/li][li]ordinary shadeless suburban mall/supermarket concrete parking lot[/li][li]cars’ engines are not running[/li][li]there is no climate control engaged[/li][/ul]
Just a parked car laid bare to the elements (well, except for those windows and doors ).
Anyway – so what’s the outside air temperature at which the inside of our hypothetical car would feel comfortable. Let’s define “comfortable” as “within a few degrees of 72F”. An outside air temperature that allows a person inside this car to sit for several hours without ill effects save for perhaps thirst, boredom, and nature’s call.
My common-sense guesstimate is around 30-40F. I wonder how much direct sun helps in this situation when you’re dealing with bitter cold … would a car left in the sun in 0F air temperature warm up to around 60F inside? I also wonder how much difference humidity makes here.
I have no idea why a parent would ever consider leaving a child alone in a car for 6 to 8 hours.
This fact sheet was posted in a previous thread by Hello Again which shows that at 72 F a car interior temperature rose to over 110 F after an hour. Since the graph flattens out, you can extrapolate that the interior of a car left for hours in the sun will heat up approximately 50 F above the ambient temperature. So now all you have to do is figure out what temperature a child can withstand for 6 to 8 hours and work backwards to determine the ambient temperature that corresponds to. Even at a moderate temperature I’d expect dehydration to be a problem for that duration, nevermind hyperthermia. For a shorter, more realistic time period (an hour perhaps), I’d guess that an interior temperature above 85 or 90 degrees would be dangerous for a baby, which would correspond to an ambient temperature around 50 F.
Part of this will depend on the child in question (i.e. their rate of respiration and overall volume), and the car itself in terms of total volume. This is very different if the kid is in a Smart car versus a Chevy Suburban. I wonder if carbon dioxide build up might be an issue for such long periods as well in either car. I suspect it would be.
Assuming that any excess CO2 gas is vented, and there is water and/or empty bottles for the child to relieve their bodily function, I think the car would have to start out very cold and eventually warm up to the 72 degrees point near the end of the cycle. I would actually guess something closer to zero as a starting point. Through respiration and perspiration, the child would warm up a modest size car considerably over a 6-8 hour period.
I useed to live in Las Vegas. Winter there is still very sunny, but many tourists are surprised to find daytime highs in Feb in the upper 40s.
A closed-up car left in the sun all day was very comfortable to hop into wearing just light summer clothes. Neither heat nor AC was required as one drove away. The same car in the same spot on a rare overcast day was pretty chilly. A jacket & heater were very nice to have. Once it got to be early spring & daytime highs moved up into the 60s, the car interior would be uncomfortably hot after sitting out all day.
For clarification, let’s define this person as a healthy 18-35 year old of either sex. Absolutely NOT a child.
All we’re interested in measuring is subjective comfort level . We’re not trying to put people in dangerous situations. Therefore, let us further stipulate that the adult in the car consents to the study, and is free to leave the car at any point without penalty. Still further: they don’t have to actually stay in the car to establish comfort … they can get out much earlier and merely affirm they felt like they could have stayed 8 hours if they had to.
Note that the answer is going to be different for a car that you leave completely empty and then come back to later in the day, and one that has a 100 watt heater (i.e., a person) continually running in it.
I live in Minneapolis. (It even says so, in the corner over there.) It’s not uncommon in late fall and late winter, when it’s starting to either get cold or just warm up (say, 30 to 40 degrees F), to leave work on a windy but sunny day (so therefore, kinda cold outside; definitely jacket weather) and get into a nice comfortable car. Yet with the temps in the 70s lately, the car interior is starting to get stuffy and hot when I leave work, even on cloudy days.
My work’s parking lot is very similar to a mall’s - a couple of small trees that don’t really provide much shade, open, that sort of thing.
I’d say that your common-sense estimate is pretty close.
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