Thanks for the explanations. I think the radiation/convection/conduction/evaporation explanation helps a lot, especially the sun example of radiant heat.
The fireplace is another great example of radiant heat (and a degree of convection) but I have to disagree that the air temperature is the same on the side of me facing the fire as the side facing away from it. There’s an inverse square relationship there and with my body blocking the heat the air between me and the fire is going to be hotter than the air on the side of me away from the fire.
I’m not yet convinced that you feel colder in a cement-walled cellar because your body radiates more heat there. Shouldn’t the intervening air act as an insulator? Unless there are air currents, in which case you’ve got convection going on. How would your body sense that the cellar wall is a heat sink if there is an intervening layer of air?
David, radiation is a form of light, so the air will not insulate it. It’s unlike convection/conduction/evaporation as it does not require any physical contact with you and the surface that you are losing heat to. I would read the wikipedia article for more details. It is an observable phenomenon, so it’s not something you have to just take on faith alone.
When you are by a campfire, the air all around you will not be all that warm. Air is actually a very poor conductor, so you would expect that it would take some time. You can measure that air temperature as well. Furthermore, from the time the fire is started you would expect that to feel the fire’s effect, that it would take time as the air needs to warm up, assuming that a campfire operates primarily by conduction. But instead, the fire will make you instantaneously warm when it is ignited. This is also testable because you will feel the heat from a fire even on a windy day.
You’re right that there are other effects in the fireplace example, convection by colder air coming in from behind is probably the greatest of them. But, the radiant heat from the fire is by far the greatest effect.
To minimize those other effects, put your hand up and move it back and forth sideways so it isn’t blocking the fire from heating the air on either side and is causing enough turbulence to mix the air in any case. You’ll find that the side facing the fire will feel (and become) much warmer.
Air is a poor conductor, but it isn’t a very good insulator against radiant heat. If it were, we wouldn’t have such big shifts between daytime and nighttime temperatures, nor would we feel the almost instant warmth when the sun emerges from behind a cloud.
If you stand in a concrete basement, your body is radiating heat and the walls are radiating heat. The air is largely transparent to this radiation. If you and the walls are at the same temperature, the walls don’t matter because you’ll transfer as much energy to the walls as they transfer to you. If the walls are colder, they transfer less to you than you transfer to them (and the transfer rate drops quickly with temperature).
What Daniel and I are suggesting is that the walls of your basement are colder than the walls of your cellar (which is better insulated), so that you feel colder because you’re transferring more heat energy through radiation to the walls.
What Daniel and I are suggesting is that the walls of your basement are colder than the walls of your cellar (which is better insulated), so that you feel colder because you’re transferring more heat energy through radiation to the walls.
No doubt that radiation has and impact on perceived temperature in an environment. Those examples which were mentioned make a lot of sense and demonstrate the impact although they might be a bit extreme and not necessarily applicable when comparing a basement wall in a residential dwelling versus an insulated cellar wall.
A finished basement will (hopefully) have some level of insulation. Current building code in Ontario is R10 below grade and R20 above grade. A finished wine cellar might have a slightly better rating but its doubtful that it will be anywhere significantly over R20. It would have to be a unfinished basement with virtually no heating (I hope that is not the case) to end up with cooler wall temperatures compared to an active wine cellar with an air temperature of around 50F.
I therefore believe that the humidity level has to be playing a more important role on how the OP perceives the temperature.
Brian,
You are right that the relative humidity in cold air can reach high levels (and it does that a lot) but it is only a relative humidity based on the temperature. It contains very little moisture and when the same air is heated up to indoor room temperature you will end up with extremely low relative humidity levels as low as less then 20%.
Thanks all for elaborating on radiant heat and the lack of air’s ability to insulate against it. I went to sleep last night knowing I was missing some fundamental physics principle.
Your posts reminded me of a debate I had with a co-worker many many years ago as to why a metal desk feels colder than a wood desk in a constant temperature room. He insisted that the surface of the metal desk was colder. I said both had to be the same temperature so it must be due to more rapid heat transfer from your hand to the metal. Neither of us was able to convince the other (a bit of a PhD vs MD thing I think), so he got a thermometer to prove his point. He graciously offered to buy me lunch that day. Conduction rather than radiation but otherwise similar.
I am just a simple civil engineer, I took thermo in college but have not had use of it since then so I am not going to debate the science of why it feels colder in one place or the other and if materials make a difference. I just wanted to know if other people experienced the same thing. I have used a couple of thermometers in the cellar and basement since posting and the temps are correct, so it is a ‘feels like’ issue not a thermostat issue.
For the record, the basement walls are CMU with brick veneer on the outside where it goes above trade. The CMU is filled with vermiculite for insulation. The cellar walls are 2x4 studs with batt insulation, I think i used R19, the ceiling is 2x6 joists with batt, and the floor is 1x2 strips with polystyrene foam board over concerted. Exterior grade glass pane door. The vapor barrier is good enough that you can feel it when you close the door.
I did not mean to imply the humidity doesn’t matter, it does. I think the humidity difference would feel like a 2.5 F temperature, but the cellar is 1 F colder, so that means it would feel 1.5 F warmer if it were only the humidity that mattered.