Learning by candlelight
To see a world in a grain of sand… – William Blake
While enjoying a recent effect of Global Warming, a week-long blackout brought on by a freak ice-storm which devastated the central Massachusetts region, I had ample opportunity to contemplate how a candle’s flame behaves.
It’s often said that here on the earth’s surface, air convection is the ruling heat-loss mechanism. And how. We’re like fish living at the bottom of an ocean, yet are seldom aware of how our effort to generate heat is constantly thwarted by the very medium we’re breathing. It’s not that air is a good conductor, it’s that once it does conduct it won’t stand still. Due to gravity, heated air becomes lighter in weight and rises away, while cooler air is displaced downward and steals more heat from the source. This process shapes a candle’s flame and even influences its color.
Hold a candle at any angle and the flame always points upward, away from the earth’s center. The flame responds to gravity. It would otherwise look like a ball, not a teardrop, but the currents it generates push colder air into it, thus squeezing it into something more cylindrical. This air infiltrates the flame itself, so, although currents keep bringing in fresh oxygen to use, the cooling effect is profound. The net result is a vigorous flame that’s too cool to burn efficiently. The black soot a candle emits is unburned carbon, a symptom of incomplete combustion. Due to air convection, then, a candle flame is never as hot as it could be although it’s brighter than it would be.
All because air moves so nimbly in a gravitational field.
The oddness of this being so familiar to us, the appearance of a candle in zero gravity is somewhat startling.
The flame is spherical because no convection occurs.
Blue because of complete combustion. Dimmer because of a slower rate of oxygen replenishment in static air.
As I waited night after night for the electricity to return, candlelight kept teaching me about moving air’s talent for removing heat, hampering any effort to keep warmth “down here” by constantly sending it up and away. Good thing for a heat-containing roof, then; it lessens the harm considerably. The earth itself lacks any such roof, however. And imagining that certain radiation-absorbing gases provide one is only to confuse radiation with convection.
A physical lid over a heat source decreases the zone of circulating air, thus reducing the cooling rate. But an open “lid” of gas that’s capable of absorbing radiant energy will convect around like any other gas, stealing heat and doing nothing else except radiating the very energy it has received by radiation, having zero power to confine it.
Rather than limiting the area in which heat-loss occurs, then, a radiant absorber constitutes no barrier to radiation at all — it’s merely a second radiator that relays heat away. And, just as there’s no such thing as “back-convection” — where a flame makes itself hotter by the air currents it creates — or “back-conduction” — where a colder object raises the temperature of what it’s in contact with — there’s no such thing as “back-radiation.”
Redirecting radiant energy back to the source cannot increase its temperature.
In all its forms, heat spontaneously moves from a more intense zone to a lesser. What makes convection particularly dynamic and meddlesome is that a cool mass also keeps moving to the heat source — a double whammy.
A lot can still be learned by candlelight.
As additional food for thought, cast your mind over this:
Concentrated CO2 exposed to infrared will get somewhat warmer than everyday air. But this only proves that everyday air (99.96% of which is nitrogen, oxygen and argon) is more transparent to IR and less apt to be heated that way. Air molecules, CO2 included, initially acquire heat by contact with warmer surfaces. Via mutual collisions and convective transport, this heat gets spread around within an airmass.
To some slight degree, CO2 also has the option of acquiring heat by radiative transfer. But, rather ironically, it cannot radiatively transfer this heat to the nitrogen, oxygen and argon molecules which surround it because, as said, they are largely infrared-transparent. As a result, an excited CO2 molecule is obliged to share its heat just like the rest of them do, by bumping into other molecules. In short, there’s nothing special about CO2 in a real-world context. Outnumbered 2500 to 1, CO2’s energy is lost in a busy buzz of collisions, its radiative properties wasted.
Moreover, any heated gas radiates infrared — and in this case 99.96% of the gas consists of molecules other than CO2.
Yet no one seriously imagines that back-radiation from 99.96% of the air has a role in raising the earth’s surface temperature.
Only when CO2 comes up do we lose touch with reality.
Here’s a succinct point: Immersed in the vacuum of space, the earth has but one means of losing heat: radiation. And what does carbon dioxide do? It radiates.