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06.17.08
Clouds from Both Sides Now
by
Okay, we've looked at this picture over a couple of past articles. We've seen how the earth is in balance at the top, with 342 watts coming and going per square meter. And we see that the surface of the earth is also in balance, with 496 watts coming and going. What are those 496 watts about? And how can there be a sustained difference of 154 watts?
Well, let's look at the atmosphere. It is also in balance. The picture is a bit confusing because of the two colors of the atmosphere. What is it trying to say?
The idea is that there is an energy source "in the middle" of the atmosphere. Actually it is meant to represent energy coming from all through the atmosphere. What is that energy? It is the heat radiation coming from the atmosphere itself. The atmosphere is material and has a temperature, and so it radiates infrared, as we described last time.
WHy does the atmosphere get its temperature? Well, taht turns out to be very complicated and interesting, and that isn't covered here, but it has one. Interestingly, most of the heat energy in the atmosphere comes from below.
The biggest piece is infrared energy radiated from the surface, 390 watts. S
ome if it is conducted from the surface, the way your hand gets warmer if it touches a hot coffee mug on a cold day. There is no infrared involved here. It is just a flow of heat energy directly. The meteorological jargon is "sensible heat". This is marked with an "S" and it isn;t a big player, just 16 watts.
Then there is the "L" term of 90 watts. This part is about water phase changes. As water evaporates near the surface, the surface cools. Then as it precipitates in the upper atmosphere, the upper atmosphere warms. The energy meanwhile is carried in the water vapor, and is correctly called "the latent heat of vaporization" or sometimes the "latent heat of fusion" since ice also forms at high altitudes and even evaporates at low ones.
All of this is how energy gets into the atmosphere from below. Energy also comes in from above, about 68 watts of direct heating from the sun. The only way the atmosphere balances this accumulation of energy is by infrared radiation, both downward and upward. The upward radiation is important because it closes the circle, and it would be nice if we could measure it directly, and a satellite to do this job has been built, but it has been mothballed, apparently for budgetary reasons though many people think this is a false economy. The downward radiation, though, is crucial. That is the part that closes the balance of the surface as well as of the atmosphere.
So where did the extra energy come from? The extra energy isn't anything mysterious. It is just heat energy bouncing back and forth between the atmopshere and the earth. It takes very little time for it to be replenished if it gets low or dissipated if it gets high. You can think of it as a slightly wobbly constant, the same way the amount of water in the clouds wobbles around a bit but on the average doesn't change very much from month to month.
Suppose someone leaves a hose on on a slanted property for a long time. The amount that spills off will eventually match the amount that comes out of the hose. How much water stays on the property depends on how bumpy the property is. If it's paved, there won't be very much, while if someone takes a jackhammer to the pavement and makes a dirty gravel, more water will stay on the property, but before too long the same amount as ever will spill off.
OK, so what determines how sticky the atmosphere is to outgoing radiation? You will not be surprised when I tell you it is called the "greenhouse effect". We'll discuss that in more detail next time.
Well, let's look at the atmosphere. It is also in balance. The picture is a bit confusing because of the two colors of the atmosphere. What is it trying to say?
The idea is that there is an energy source "in the middle" of the atmosphere. Actually it is meant to represent energy coming from all through the atmosphere. What is that energy? It is the heat radiation coming from the atmosphere itself. The atmosphere is material and has a temperature, and so it radiates infrared, as we described last time.
WHy does the atmosphere get its temperature? Well, taht turns out to be very complicated and interesting, and that isn't covered here, but it has one. Interestingly, most of the heat energy in the atmosphere comes from below.
The biggest piece is infrared energy radiated from the surface, 390 watts. S
ome if it is conducted from the surface, the way your hand gets warmer if it touches a hot coffee mug on a cold day. There is no infrared involved here. It is just a flow of heat energy directly. The meteorological jargon is "sensible heat". This is marked with an "S" and it isn;t a big player, just 16 watts.
Then there is the "L" term of 90 watts. This part is about water phase changes. As water evaporates near the surface, the surface cools. Then as it precipitates in the upper atmosphere, the upper atmosphere warms. The energy meanwhile is carried in the water vapor, and is correctly called "the latent heat of vaporization" or sometimes the "latent heat of fusion" since ice also forms at high altitudes and even evaporates at low ones.
All of this is how energy gets into the atmosphere from below. Energy also comes in from above, about 68 watts of direct heating from the sun. The only way the atmosphere balances this accumulation of energy is by infrared radiation, both downward and upward. The upward radiation is important because it closes the circle, and it would be nice if we could measure it directly, and a satellite to do this job has been built, but it has been mothballed, apparently for budgetary reasons though many people think this is a false economy. The downward radiation, though, is crucial. That is the part that closes the balance of the surface as well as of the atmosphere.
So where did the extra energy come from? The extra energy isn't anything mysterious. It is just heat energy bouncing back and forth between the atmopshere and the earth. It takes very little time for it to be replenished if it gets low or dissipated if it gets high. You can think of it as a slightly wobbly constant, the same way the amount of water in the clouds wobbles around a bit but on the average doesn't change very much from month to month.
Suppose someone leaves a hose on on a slanted property for a long time. The amount that spills off will eventually match the amount that comes out of the hose. How much water stays on the property depends on how bumpy the property is. If it's paved, there won't be very much, while if someone takes a jackhammer to the pavement and makes a dirty gravel, more water will stay on the property, but before too long the same amount as ever will spill off.
OK, so what determines how sticky the atmosphere is to outgoing radiation? You will not be surprised when I tell you it is called the "greenhouse effect". We'll discuss that in more detail next time.
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