http://www.straightdope.com/columns/read/153/is-it-warmer-at-the-top-of-tall-buildings
Is it possible Connie V. was referring to indoor temperatures? In an enclosed space, warmer air is less dense and will rise, after all.
Powers &8^]
That’s exactly what I was thinking. In my little 3 floor townhouse, there is a huge difference between basement and top floor temperature. I haven’t measured, but I’d expect it’s at least 10 degrees difference.
On the other hand, there is no noticable difference in temperature where I work among the 20 above ground floors (the basement is still freezing though). But there, we have a ventilation system that is always on, so that regulates it. There have been times though where the ventilation has had issues and it was noticably stuffy in a matter of minutes (even when it wasn’t hot outside). I wonder just how bad it would get if you had a huge skyscraper and turned the ventilation off for a full day.
I share similar thoughts. It seems that Cecil basically sidestepped the question by rattling off an answer about higher altitudes being colder. It seems the answer would be more complex, and instead more about how big, tall buildings are heated and cooled, how air circulates inside big buildings, and whether any outside atmospheric effects are enough to compensate for indoor circulation.
Agreed. Even for the days when Unca Cece used to toss off a few of these each column, without an in depth answer guaranteed, this one is very poor in comprehension of the question being asked.
Bad Cecil. Bad. Bad.
As a person who has worked on roofs professionally in the past I can say without question that there are times when a roof is much hotter than the ground. On a summer day you are completely exposed to the sun, and if the surface of the roof is dark (which it often is) it will absorb a lot of heat. This has nothing to do with altitude though, and has everything to do with the lack of shade.
When I read the column, it seemed to me that Connie’s friend was talking about the indoor temperature being warmer, but that Connie took it as being the outdoor temperature.
Yes, Connie’s question appears to be about going onto the roof of a building for sunbathing while the ground is buried in snow. Cecil addressed that question. Now it seems immediately obvious that Connie mistook what her friend said about upper floors being warmer, when the friend meant interior conditions.
As for roofs being hotter, they can by the same token that an exposed asphalt parking lot is hotter than a covered garage. More sun exposure combined with heat-retaining surface and you get heat zone. But they aren’t necessarily warmer, and in wintry conditions (especially with overcast skies), they won’t be warm enough to make a difference.
As for interior temps, each floor (and typically zones or rooms on each floor) has a different thermostat so that the air can be adjusted for that floor independently. Thus even if the lower floors take less cooling than the upper floors, the overall temps should be controllable to the occupants’ tastes.
I live on the top floor of a 9-story building, and find that one of the two stairwells (which ends at my floor) is basically unusable in winter, because it acts like a chimney and the top becomes much too hot for coming in wearing a winter coat. The other one, though, goes up to roof access, and remains tolerable at my floor.
I agree.
The issue usually is air circulation. An open townhouse (or even my bungalow with basement) will be much warmer up top if the stairway or other ventilation is open. A 20-storey office building or 9-storey apartment because of fire laws has closed doors between floors, so each floor is a relativey(!) independent thermal ecosystem. Basements will always be cool, barring heating or AC, because there is not a large load of sunlight or conductive heat being forced into them. (And why waste your money heating a basement that is not often used?) I found the top floor of a house, even with a well-insulated attic space, will pick up the heat from the hot sun and conduct it into the top floor rooms. Also, most heating systems and AC are preferentially aimed more at rooms other than the basement.
As for stairwells - mostly, they are open. I’ve seen a few with a firewall divider between one group of floors and another. Usually these have heaters in winter all the way up - and all that heat rises, and most have no outside wall so very little heat loss. With all that convection, the bottom heaters are always going to sense they are a little chilly and keep pumping out more heat. What about your second stairwell? is it piping hot at the top, or is it nicely cooled by a poorly insulated roof access?
The temperature falls with altitude, about 5 degrees F per 1000 feet IIRC. The issue is probably not so much altitude as it is that roofs are dark and absorb the sun’s heat; and if the area is a little sheltered, you don’t lose that ambient warmth easily. If you are lucky an the roof is tilted toward the sun, bonus extra heat. (That would be south-facing, and often cool winds in winter are from the north). Of course, sunbathing in winter - it’s amazing how quickly you WILL chill if there’s a hint of wind.
You’re right, I neglected stairwells and other common zones across multiple floors.
Insofar as the temps closer to the surface of the Earth being warmest, why then do low-lying areas get the first frosts?
Warm air rises and cold air sinks; so why then is the surface temp warmer than higher up (up to a point, of course)?
Once the ground cools below the ambient air temp the cool air starts to settle; is that it? There just seems to be a contradiction otherwise. Normally mountain tops are colder than the valleys below; but the valleys are where the first frosts happen… What a strange planet!
Valleys are subject to cold temperatures, as you suggest, due to cold air sinking and following depressions in topography, which can in some instances lead to frost where the ground has cooled to below the air temperature. On the other hand, mountain tops are cold, as Cecil point out, due to the dominant radiative ground-heating of air in the troposphere, whereby air nearer the ground warms up to a greater extent (all other things being equal), and less ‘ground’ means cooler T. The warm air that you are puzzled about expands as it rises, and and it does so cools (‘adiabatic’ expansion, if you prefer), which also contributes to generally cooler temperatures at higher altitudes (up to a point). There are also numerous other micro-climatic considerations which complicate the temperature such as angle of incidence to sunlight, albedo, cloud cover, etc.
Stranger however are the inverted temperature gradients which persist in the stratosphere and thermosphere (the latter can reach T >1,500°C). This is due to infrared absorption by ozone and other energetic solar radiation. Mount Everest sometimes pokes into the Tropopause (the troposphere/stratosphere boundary), where temperatures cease declining with with altitude.
Anyway I think the point of the original question was concerning internal heating within a building rising, which it does, though presumably compensated for to some extent by the ventilation systems in effect in the Sears Tower. If anyone lives nearby, hopefully they can take a thermometer on their next ascent in the name of science.
Thanks Captain, you are awesome!