MRC_Hans
Penultimate Amazing
- Joined
- Aug 28, 2002
- Messages
- 24,961
I really made this for the benefit of Bjarne. I doubt if he will read it, and if he does, he will probably ignore it. However, some others might also find it mildly interesting, so here it is:
The secret life of clouds
There are many different kinds of clouds. Here, we will only look at convection clouds, the family of clouds called Cumulus.
When you see a small white cauliflower-shaped summer cloud drifting across the blue sky, you may not realize that it is only the visible part of a quite complex system, and only the smallest member of a big family of weather systems that ranges across thunderstorms all the way to the mighty tropic hurricanes and typhoons.
However, let us start at the small clouds. All cumulus clouds start by air rising. This can be due to weather fronts, mountains, or simply the sun shining on the ground. But we need to start even before that: We need to start on a warm calm day. Under quiet, stable conditions, the temperature of the air falls with altitude; if we climb a mountain, it will get colder as we get nearer to the top. If the mountain is high enough, the top will be covered with snow, even if there is a tropical climate at its foot.
Since we also know that warm air tends to rise, this situation is rather counterintuitive; one might expect that it would get warmer as we ascended. However, another thing also happens as we ascend: The pressure falls. The pressure of the air is really caused by the weight of the column of air above, so it follows that as we ascend, the column of air above will get shorter, hence the pressure will fall. If we get high enough, the air will be too thin to breathe.
We need to know one more thing: When air expands, it gets colder. A simplistic explanation is that as the amount of heat energy is spread over a bigger volume, the temperature must fall. This normally maintains equilibrium, because if some air starts to rise, it is cooled, and so stops rising.
So, let us go back to the warm, calm day. Now we add sunshine. The sun heats up the ground, and the ground in turn heats up the air right over it. As the air heats up, the heat gradient becomes higher than the normal equilibrium, and the warm air begins to rise.
A convection cell has been born.
As the warm air rises, it pushes aside air around it and above it, as it expands and cools. Since the rising air is lighter than the air it replaces, it excerts a lower pressure at ground level, so air from the sides start moving in to replace it. This air is warmed by the warm ground, and rises. New air comes in, etc. A column of warm air ascends towards the blue summer sky.
In most cases, it stops there. The supply of new air cools the ground, the warm air cools and disperses, and all you may see is a breeze, or a small vortice, a dust devil.
Wait! How did the vortice come into this? Enter Mr. Coriolis: Our summer day may seem calm enough, but actually Earth is rotating. This means that any spot on the surface is rushing eastwards, at low latitudes at a breathtaking speed. A spot on the equator is speeding east at a clip of over 1000km/h. The closer you get to the poles, the smaller the circle, the slower the speed. Normally, the only thing that makes us notice all this is that the sun appears to move across the sky. Everything around us is travelling with us, so we don't feel the speed.
But, if you travel north or south, something does happen. When you travel north on the Northern hemisphere, you are really moving to ground that moves slower. So your higher eastwards momentum will make you veer to the east. Now, if you are walking, running, or driving, even flying an airplane, you won't notice, because the forces keeping you on course are so strong. However, if you are a stream of air, with little friction around you, you will 'feel' it. The air that rushes in to replace our rising convection cell from the South will veer east; the air coming from the North will veer west. Since the air from the other directions will be pushed aside, the whole thing begins to rotate. Due to the law of conservation of angular momentum, the closer the air spirals in, the faster it rotates.
We have a vortice of air spiraling in under the convection cell. Of course, the rotation tends to continue into the upwards motion, so in the center of the cell we have a column of rotating, rising air. Does this give you a mental image of a tornado? Well, a tornado is just the large-scale version of the phenomenon, but our small convection cell can only at best lift some dust, so it is not really visible.
Let us examine the top part of the cell. The rising air bubble expands and as the air expands, it becomes colder. Also, at the edges of the bubble, it mixes with the cooler, surrounding air. If no energy is added, it will run out of momentum and disperse.
Enter water vapor. To understand this, we need to look at bit at the properties of water.
Water has three phases: Solid, AKA ice, liquid, AKA water, and gas, AKA vapor.
Let us begin with ice. If we take one gram of ice, at, say -10 deg C (WARNING: This exercise is metric, including the somewhat obsolete energy unit calorie, because this gives nice round figures) and heat it, we will find that the temperature rises by 1 deg C for each calorie (cal) of heat energy we add. So after adding 10cal, we reach 0 C.
Now something weird happens. We can keep adding heat, but the temperature stays at 0. If we could distribute the heat perfectly in the ice, we would observe that, after adding 80cal, it would suddenly turn into water. As ice is a poor heat conductor, what we will probably see is that the ice gradually melts, but the temperature will stay at 0 till all the ice has melted, which will be the case after adding 80cal. After the ice has melted, the temperature will again rise by 1C for each cal added.
So the phase transition from ice to water consumes 80cal/g.
If we heat further till we reach the boiling point, a similar thing will happen: The temperature will stop at 100C, till all the water has evaporated. Only, this will require a whopping 539 cal! Remember these energies, 80cal plus 539cal.
To transfer water from ice to vapor, we need to invest 619cal/g, enough to make a similar substance red-hot, if it wasn't for the phase transitions.
Back to our convection cell: All air contains some water vapor. The hotter the air, the more water vapor it can contain. If the rising air contains sufficient water vapor, as it rises and cools, it will reach the condensation point. Water vapor is totally transparent and normally invisible. Once moist air cools, however, the vapor condenses to a mist of fine droplets, which is the material clouds are made of. So at a certain altitude, we can suddenly see the rising air bubble, because the air in it becomes filled with mist.
This is the reason most clouds have a distinct base; at that altitude, condensation starts, and the rising column suddenly becomes visible.
However, something else also happens. Remember it took 539cal to evaporate 1g water? Well, when it condenses, all that energy is released in the form of heat! This does not mean that the air heats up again, but it means that it can now rise and expand quite a lot while not getting much colder. Instead, it feeds on the stored heat in the water vapor.
What does this mean? It means that once a cloud forms, the cell can start to fuel itself. It is no longer depending on hot ground, or some other source of rising air. Instead the condensation-powered convection simply sucks fresh moist air in, gets new condensation, and the process goes on. In this mode, the cloud can drift for hundreds of miles with the prevailing wind, while it lives off the land of moist air below it.
What if there is A LOT of moisture in the air? Well, the more moisture, the more fuel to the convection cell, so it may grow, sucking in more air, growing even more. Our small cumulus cloud starts to grow tall and because of its own shadow, it begins to look darker, and more ominous.
It has one more source of energy: As the mist rises high enough, the air expands so much that its temperature falls below the freezing point, and now the water droplets start to freeze. This releases another 80cal/g, and the top of the cloud is now 4-6 kilometers up.
At this point, some of the ice crystals begin to clot and fall back through the cloud, melting to rain, and we have a rain shower. When the ice melts to rain, it steals back 80cal/g, but with 539cal/g of condensation energy, the cloud has plenty of fuel.
One of the best sources of water vapor is a large, warm body of water. So when a thunderstorm that started on a sunny field in Western Africa, late in summer drifts out over a warm Atlantic ocean, it lands in a feast of a virtually unlimited energy supply. If the surface temperature of the ocean is above 33C, a run-away growth starts, and we have a tropical hurricane.
Once it starts, it drifts across the ocean, waxing and vaning dependent of the supply of water vapor, and ending up on the American coast where it can cause disasters. Once it drifts inland, it starts to die, for two reasons: The increased friction over land slows down the inrush of fresh air, and the land air is inevitably drier. Both contribute to starve the hurricane of energy, and it runs out of momentum and dissolves.
Hans
The secret life of clouds
There are many different kinds of clouds. Here, we will only look at convection clouds, the family of clouds called Cumulus.
When you see a small white cauliflower-shaped summer cloud drifting across the blue sky, you may not realize that it is only the visible part of a quite complex system, and only the smallest member of a big family of weather systems that ranges across thunderstorms all the way to the mighty tropic hurricanes and typhoons.
However, let us start at the small clouds. All cumulus clouds start by air rising. This can be due to weather fronts, mountains, or simply the sun shining on the ground. But we need to start even before that: We need to start on a warm calm day. Under quiet, stable conditions, the temperature of the air falls with altitude; if we climb a mountain, it will get colder as we get nearer to the top. If the mountain is high enough, the top will be covered with snow, even if there is a tropical climate at its foot.
Since we also know that warm air tends to rise, this situation is rather counterintuitive; one might expect that it would get warmer as we ascended. However, another thing also happens as we ascend: The pressure falls. The pressure of the air is really caused by the weight of the column of air above, so it follows that as we ascend, the column of air above will get shorter, hence the pressure will fall. If we get high enough, the air will be too thin to breathe.
We need to know one more thing: When air expands, it gets colder. A simplistic explanation is that as the amount of heat energy is spread over a bigger volume, the temperature must fall. This normally maintains equilibrium, because if some air starts to rise, it is cooled, and so stops rising.
So, let us go back to the warm, calm day. Now we add sunshine. The sun heats up the ground, and the ground in turn heats up the air right over it. As the air heats up, the heat gradient becomes higher than the normal equilibrium, and the warm air begins to rise.
A convection cell has been born.
As the warm air rises, it pushes aside air around it and above it, as it expands and cools. Since the rising air is lighter than the air it replaces, it excerts a lower pressure at ground level, so air from the sides start moving in to replace it. This air is warmed by the warm ground, and rises. New air comes in, etc. A column of warm air ascends towards the blue summer sky.
In most cases, it stops there. The supply of new air cools the ground, the warm air cools and disperses, and all you may see is a breeze, or a small vortice, a dust devil.
Wait! How did the vortice come into this? Enter Mr. Coriolis: Our summer day may seem calm enough, but actually Earth is rotating. This means that any spot on the surface is rushing eastwards, at low latitudes at a breathtaking speed. A spot on the equator is speeding east at a clip of over 1000km/h. The closer you get to the poles, the smaller the circle, the slower the speed. Normally, the only thing that makes us notice all this is that the sun appears to move across the sky. Everything around us is travelling with us, so we don't feel the speed.
But, if you travel north or south, something does happen. When you travel north on the Northern hemisphere, you are really moving to ground that moves slower. So your higher eastwards momentum will make you veer to the east. Now, if you are walking, running, or driving, even flying an airplane, you won't notice, because the forces keeping you on course are so strong. However, if you are a stream of air, with little friction around you, you will 'feel' it. The air that rushes in to replace our rising convection cell from the South will veer east; the air coming from the North will veer west. Since the air from the other directions will be pushed aside, the whole thing begins to rotate. Due to the law of conservation of angular momentum, the closer the air spirals in, the faster it rotates.
We have a vortice of air spiraling in under the convection cell. Of course, the rotation tends to continue into the upwards motion, so in the center of the cell we have a column of rotating, rising air. Does this give you a mental image of a tornado? Well, a tornado is just the large-scale version of the phenomenon, but our small convection cell can only at best lift some dust, so it is not really visible.
Let us examine the top part of the cell. The rising air bubble expands and as the air expands, it becomes colder. Also, at the edges of the bubble, it mixes with the cooler, surrounding air. If no energy is added, it will run out of momentum and disperse.
Enter water vapor. To understand this, we need to look at bit at the properties of water.
Water has three phases: Solid, AKA ice, liquid, AKA water, and gas, AKA vapor.
Let us begin with ice. If we take one gram of ice, at, say -10 deg C (WARNING: This exercise is metric, including the somewhat obsolete energy unit calorie, because this gives nice round figures) and heat it, we will find that the temperature rises by 1 deg C for each calorie (cal) of heat energy we add. So after adding 10cal, we reach 0 C.
Now something weird happens. We can keep adding heat, but the temperature stays at 0. If we could distribute the heat perfectly in the ice, we would observe that, after adding 80cal, it would suddenly turn into water. As ice is a poor heat conductor, what we will probably see is that the ice gradually melts, but the temperature will stay at 0 till all the ice has melted, which will be the case after adding 80cal. After the ice has melted, the temperature will again rise by 1C for each cal added.
So the phase transition from ice to water consumes 80cal/g.
If we heat further till we reach the boiling point, a similar thing will happen: The temperature will stop at 100C, till all the water has evaporated. Only, this will require a whopping 539 cal! Remember these energies, 80cal plus 539cal.
To transfer water from ice to vapor, we need to invest 619cal/g, enough to make a similar substance red-hot, if it wasn't for the phase transitions.
Back to our convection cell: All air contains some water vapor. The hotter the air, the more water vapor it can contain. If the rising air contains sufficient water vapor, as it rises and cools, it will reach the condensation point. Water vapor is totally transparent and normally invisible. Once moist air cools, however, the vapor condenses to a mist of fine droplets, which is the material clouds are made of. So at a certain altitude, we can suddenly see the rising air bubble, because the air in it becomes filled with mist.
This is the reason most clouds have a distinct base; at that altitude, condensation starts, and the rising column suddenly becomes visible.
However, something else also happens. Remember it took 539cal to evaporate 1g water? Well, when it condenses, all that energy is released in the form of heat! This does not mean that the air heats up again, but it means that it can now rise and expand quite a lot while not getting much colder. Instead, it feeds on the stored heat in the water vapor.
What does this mean? It means that once a cloud forms, the cell can start to fuel itself. It is no longer depending on hot ground, or some other source of rising air. Instead the condensation-powered convection simply sucks fresh moist air in, gets new condensation, and the process goes on. In this mode, the cloud can drift for hundreds of miles with the prevailing wind, while it lives off the land of moist air below it.
What if there is A LOT of moisture in the air? Well, the more moisture, the more fuel to the convection cell, so it may grow, sucking in more air, growing even more. Our small cumulus cloud starts to grow tall and because of its own shadow, it begins to look darker, and more ominous.
It has one more source of energy: As the mist rises high enough, the air expands so much that its temperature falls below the freezing point, and now the water droplets start to freeze. This releases another 80cal/g, and the top of the cloud is now 4-6 kilometers up.
At this point, some of the ice crystals begin to clot and fall back through the cloud, melting to rain, and we have a rain shower. When the ice melts to rain, it steals back 80cal/g, but with 539cal/g of condensation energy, the cloud has plenty of fuel.
One of the best sources of water vapor is a large, warm body of water. So when a thunderstorm that started on a sunny field in Western Africa, late in summer drifts out over a warm Atlantic ocean, it lands in a feast of a virtually unlimited energy supply. If the surface temperature of the ocean is above 33C, a run-away growth starts, and we have a tropical hurricane.
Once it starts, it drifts across the ocean, waxing and vaning dependent of the supply of water vapor, and ending up on the American coast where it can cause disasters. Once it drifts inland, it starts to die, for two reasons: The increased friction over land slows down the inrush of fresh air, and the land air is inevitably drier. Both contribute to starve the hurricane of energy, and it runs out of momentum and dissolves.
Hans
