The Science of Everything Podcast - Episode 109: Weather Part I

Episode Date: August 2, 2020

An introduction to key concepts relevant to understanding weather, including relative and absolute humidity, atmospheric stability, cloud formation and classification, types of precipitation, and the formation and classification of air masses. Recommended pre-listening is Episode 90: Climate Systems.   If you enjoyed the podcast please consider supporting the show by making a paypal donation or becoming a patreon supporter. https://www.patreon.com/jamesfodor https://www.paypal.me/ScienceofEverything

You're listening to The Science of Everything podcast, episode 109, weather part one. I'm your host, James Fodor. So in this episode, we're going to start a hopefully two-part series looking at the weather. And as sort of preparatory to that, in this episode we're going to be talking about basic concepts that are important to understanding the weather, including humidity, atmospheric stability of air parcels. We'll talk about cloud formation and different types of clouds and precipitation, and also look air masses. This will lead up into the next episode when we talk about different types of storms and weather patterns including mid-altitude cyclones, tropical cyclones, thunderstorms, and

concluding with a look at tornadoes. Recommended pre-listening for this episode is episode 90 on climate systems and some of the prerequisites in turn of that, which gives some basic background that will be of help in understanding some of the concepts that I talk about here. So let's make a start and start talking about humidity. I think most people have basic idea of what humidity is. It refers to the concentration of water vapor that's present in the air. Evaporated water is one of the components of air. It forms only a tiny fraction of the air, but it is very important for the development of weather. Contrary to what a lot of people think, you can't actually see water vapor in the air, at least under nearly all conditions. What people talk

about as steam is actually a suspension of liquid water that is held in the air by updrafts of hot air, including boiling water vapor. But the water vapor itself, when it's actually in a gasiest state, is not generally visible. However, the air can hold a fairly large amount of water. The amount of water that's held in the air is referred to as the relative humidity. So the relative humidity is expressed as a percentage, indicating the amount of water that's in the air as a percentage of the maximum possible amount of water that could be in the air at that temperature. So relative humidity is contrasted with absolute humidity, which describes the water content of the air expressed in a unit such as grams per cubic meter or grams per kilogram of air or something like that. So relative humidity is perhaps somewhat more useful because you can tell how close the air is to being what's called saturated with water, which means air is saturated with water vapor when maximum possible amount of water vapor is present in the air at that temperature.

and as I've mentioned a number of times, the amount of water vapour that can be present in the air depends on the temperature. The higher the temperature of the air, the more water vapor it's capable of holding. The dew point refers to the temperature at which a specific mass of air must be cooled down to become saturated with water vapor. So, for example, if we start out with air at 30 degrees and there's a certain amount of water vapor content in that air, it might be at, say, 50% humidity. But then as we cool the air down, the amount of water in that air, assuming, you know, no other changes, stays the same. But the temperature goes down, because we're cooling it down, thereby increasing relative humidity, because the maximum amount of water vapor that can be held in the air is decreasing, but the actual amount that's in the air is the percentage of water vapor in the air is a percentage of the maximum possible amount increases. And when that happens, that is referred to as the dew point. So that temperature at which the air becomes saturated with water vapor is the dew.

So high dew points, as in high temperatures, indicate high humidity, meaning that you don't have to decrease the temperature of the air very much in order to reach saturation. The amount of water vapor that can be held in the air increases roughly exponentially with temperature. So that means that low temperatures can only hold a very small amount of water, but higher and higher temperatures by the time you get to 30, 40 degrees can hold, much, much more water. And so you can have these sort of non-linear effects of changing the temperature and how much that changes humidity.

As we'll see going forward, this can play a very important role in a wide range of weather phenomena. Okay, so with the concept of humidity under our belts, then let's talk about the concept of atmospheric stability, which is extremely important for weather. So atmospheric stability is a measure of the tendency of the atmosphere to impede or discourage a vertical motion. So the basic idea here is that what happens when a parcel of air is, you know, or a quantity of air is caused to be lifted by some force.

There's a number of so-called lifting mechanisms that can cause air to rise up. And I'll just go through them briefly before we come back to atmospheric stability because we need to understand sort of how vertical air motion can get started in the first place. So one of them is called orographic lift. This refers to air being pushed up over mountains. So they're sort of moving along across the surface and there's a mountain range in the way the air obviously has to go over the mountain and so it's pushed upwards by the mountain. So that's one cause of lift.

Another cause of lift is called frontal lift. We'll talk more about this when we talk about fronts. But basic idea is that one air mass will be pushed up over a cooler air mass at a front when two masses of air sort of come into contact with each other. The warmer one goes over the cooler one, thereby causing it to lift. The third type of lifting mechanism is called convective lift. This occurs when the sun heats up the ground or water, and which thereby heats the overlying air, causing it to expand, because warmer air.

expands, as we've talked about in previous episodes, and as air expands, it becomes less dense, and then it rises. So air that's warmed will rise. And finally, convergence lift. This occurs when there is a low-pressure system. Low-pressure systems can occur for many reasons. We talked about them in some of the previous Earth-Science-related episodes. And if there is a low-pressure system, for whatever reason, air will move inwards, and then once it's moved inwards, it has to go somewhere, and so it sort of pushes upwards. So it sort of moves inwards, collides, and then pushes upwards, because there's nowhere else for it to go. So that's called convergence lifting

is another mechanism by which air can be pushed upwards. So for whatever combination of these factors leads a given amount of air to be pushed upwards in the first place, we can then ask the question, under what conditions will it continue to move upwards or under what conditions will it sort of stay where it is? And this is what atmospheric stability relates to. So to determine whether an air parcel is going to be stable

when it's moving upwards, we need to understand a concept called the lapse rate. The lapse rate is the rate at which an atmospheric variable, in this case it's temperature, so the rate at which the temperature in the Earth's atmosphere falls with altitude. As we've discussed previously, temperature decreases with altitude. That is the case pretty much everywhere. There are exceptions called inversion lanes, but basically it's true everywhere. But the question is, how quickly does temperature fall with altitude? And that does change, depending on the local atmospheric conditions. And so the name that's given for this rate of cooling

with altitude is called a lapse rate. Now the other concept that we need to understand is that of adiabatic cooling and also adibatic heating, but adiabatic cooling is the most rather than one here. Again, we've talked about this in previous episodes. I think we've talked about gas laws, but the basic idea here is that rising air expands and cools due to the decrease in air pressure as altitude increases. So with higher altitude, the air pressure is lower because there's less air sitting on top of it to sort of compress it down. By the ideal gas law, PV equals NRT. Again, if you haven't heard that, you should go back and listen to episode 42 on gases and gas laws for a refresher. But the idea here is that if the pressure has decreased and the amount of air

stays the same because, you know, air doesn't just disappear, then what has to happen is some combination of the volume increasing or the temperature decreasing. And both of those things in fact happens as air rises. So because the pressure decreases and the amount of air is constant, you get an expansion of the air, so the volume increases, it takes a more space, but also the temperature falls. Essentially, and this is a sort of a loose way of thinking about it, the energy that's embodied in the thermal motion of the air molecules, some of that goes into doing work on the surrounding gas, to sort of push out the space to make the space for the increased volume of air, and that's extracted from the thermal motion, thereby reducing the thermal

energy, and thereby reducing the temperature of the gas. Again, it's a bit more complicated in that, but that's a loose way of thinking about it. So the point, The point is, when air rises, it expands and cools. That's called adiabatic cooling. Antibatic heating is the reverse phenomenon. When air falls, it warms up for the exact opposite reason. Now, again, this always happens. Adiabatic cooling is a phenomenon of any rising gas in the atmosphere, but the rate at which air cools as it rises, again, depends on local conditions and various factors. And in particular, there are two rates that are most relevant to the discussion here of atmospheric stability. There's the moist adibatic rate and the dry adiabatic rate.

The moist adiabatic rate is the rate at which a saturated ascending parcel of air cools by adibatic cooling. When we say moist, we mean that it's saturated, so it has the maximum amount of water that's possible for air of that temperature. The dry adibatic rate is the same, but for unsaturated air. So it's just any air that doesn't have the maximum amount of water in it. Now, you might think it's odd as to, well, why is they just one for the maximum 100% saturation and one for anything less than 100%? Wouldn't it, like, vary by the water composition? Well, not really.

And the reason for that is because the latent heat of condensation, that is the energy that's released by the air as it cools and releases energy going from a gas to a liquid form, and thereby condensing out of the air, increases the temperature of the air. and thereby means that the dry adiabatic rate is higher than the moist adibatic rate. So to put that another way, as moist air rises, it has an extra factor that warms it up somewhat, not warms it up in an absolute sense, but keeps it warmer than it would otherwise be. And that is the energy that's released from condensing the water in the air from a gas into a liquid. That's called the latent heat of condensation. It helps to reduce the rate at which the air cools. but that only happens when you have actual condensation.

That will only occur when the dew point is reached, and that only occurs for saturated air. So it doesn't matter so much what percentage saturation of the air is. It's just a question of, has it reached the dew point yet, or has it reached 100% saturation? At that point, it will switch from undergoing the dry adubatic rate to the moist adubatic rate. Now, a few times I've used the phrase a parcel of air.

That just basically refers to a bunch of air that has specific pressure and temperature properties and that we track over time. It doesn't have an extremely precise definition, at least from our perspective. So just think of it as like a bunch of air. Another way to think about this if it helps is imagine blowing up a balloon and then releasing it and as it rises up in the air or whatever floats away or whatever happens to it. The air in that balloon could represent a parcel of air that's bounded by the balloon. But it doesn't have to be physically bounded by anything.

It's more of a concept than anything else. Anyway, a given parcel of air experiences either the moist or the dry adibatic rate depending on its moisture content, as I just mentioned before. So to bring these two concepts together of the adubatic rates and the lapse rate to understand that menstruability, we have to think about a parcel of air that's rising upwards or that starts rising for whatever reason. It could be any of the lifting mechanisms that I mentioned before. So the parcel of air starts to rise. The question is, will it continue to rise? That's the question of atmospheric stability. to answer that question, we need to first consider, is the parcel of air experiencing the moist or the dry adibatic rate?

That just depends on its relative humidity. The other question we have to answer is, what is the ambient lapse rate of the surrounding air? Now, that, as I mentioned, depends on local, environmental and climactic conditions. But this just refers to the rate at which the temperature changes with altitude or decreases with altitude in the surrounding non-moving air. So the air surrounding the parcel isn't moving. It's just kind of sitting there doing its thing. The parcel of air is rising up, it, the rising parcel of air, is going to cool down because of adiabatic cooling. The surrounding air is not cooling down because it's not rising up, but it's colder at higher altitudes because that's a property of the atmosphere under most conditions.

And so the rate at which the stationary air cools with higher altitudes is the ambient lapse rate, and that can vary with environmental factors. Whereas the rate at which the rising parcel of air cools with altitude as it moves to higher altitudes is, I'm going to, either the moist or dry adepatic rate, depending on whether it's saturated. So now let's think about how these two things relate to each other, the ambient lapse rate and the adiabatic lapse rate. Now, the key thing to ask is whether the ambient lapse rate is higher than or lower than the adiabatic lapse rate. If the adiabatic lapse rate is lower than the ambient lapse rate, this means that the air that's being displaced upwards cools more slowly than the surrounding air, the ambient air. So the adiabatic lapse rate is lower than the ambient abs rate. Lapse rate, this means that the rising air cools down more slowly.

Because the lapse rate, it shifts how fast does it cool? If this is the case, this means that the rising air, the air parcel, becomes warmer relative to the surrounding atmosphere. Not absolutely warmer, it's still cooling down, but warmer compared to the surrounding atmosphere as it rises up. Now, we know that warmer air is less dense and therefore tends to rise further. So in this situation, the air mass keeps rising, and therefore the air is said to be unstable. So again, if the adiabatic lapse rate is lower than the ambient lapse rate, the air cools down more slowly, which means it gets relatively warmer compared to the surrounding air, and therefore it keeps rising because it's less dense. That's unstable air. So the idea here is if you sort of give that air an upward push through one of the lifting mechanisms we mentioned, it will keep going up.

Conversely, if the adiabatic lapse rate is higher than the ambient lapse rate, that means that the air mass that's displaced upwards cools down more quickly than the ambient air. If the air becomes cooler than the surrounding atmosphere, that means it's more dense and will tend to fall, or at least resist upward motion. Such air is said to be stable. So if the air gets relatively warmer than the surrounding atmosphere, it will keep going up and therefore is unstable. if the air gets relatively cooler than the surrounding ambient air, it will become more dense and therefore it will resist upward motion and it is stable. There's a third possibility which occurs when the ambient lapse rate is between the moist and the dry antibiotic rates for the parcel of air.

Because in this condition, initially, the parcel of air will be unsaturated because it's at a lower altitude, it's warmer, and therefore its relative humidity is going to be lower. it therefore cools down at the dry adibatic rate, which is relatively faster, and that means that under this condition, when the air is at a lower altitude and when it's unsaturated, it's stable, because at the dry adabatic rate, it cools down faster than the surrounding air, therefore, meaning that it gets more dense and that it won't rise further. However, if the environmental lapse rate is just such that it's between the dry and the moist adibatic rates, what happens is once the air parcel reaches a critical elevation, it will reach saturation, so 100% humidity. And when that happens, it will stop rising at the dry heat abatic rate because it's not dry anymore. And it will start rising at the moist antibiotic rate, which is slower.

That is, it cools down more slowly because of the additional energy of the water condensing out, causing it to heat up a bit more. Therefore, under this condition, the air is actually becomes warmer relative to its surroundings, and therefore becomes less dense and therefore rises. So the basic point here is that

if the environmental lapse rate is between the dry and the adiabatic lapse rates, you can have a condition where, which as I mentioned is called conditionally unstable, whereby the air parcel is stable as long as it doesn't rise too much, but if it's forced to rise too much for whatever reason, it will switch from being dry to moist because the relative humidity goes up and therefore it will become unstable. And so this is a particularly interesting combination, which can give rise, obviously, to interesting weather phenomenon, whereby you can have a stable situation, but then if something shifts it, it can become unstable, and therefore it will keep rising. Okay, that is a brief summary of atmospheric stability, and this is, although it might seem about arcane, you know, why do I care if an air pass will keeps rising or not, the reason this is important is because rising air and weather air keeps rising is critical for pretty much all of the weather phenomena that we're about to look at, cloud formation, precipitation, air masses, fronts, cyclones, and everything else. It all depends on rising air. And so if air is not, if air passes are stable, it won't keep rising and therefore you don't

get these weather phenomenon, or at least they're much rarer and more difficult to form under stable conditions. Most of the interesting weather phenomena that we're going to look like occur under unstable atmospheric conditions. As an introduction to that, let's start by talking about clouds, which are obviously a very familiar weather phenomenon. So a cloud is an aerosol consisting of a visible mass of liquid droplets or sometimes frozen crystals and other particles suspended in the atmosphere. So aerosol being liquids or sometimes solid suspended in a gas.

As I mentioned before with humidity, clouds don't consist of water vapor. I mean, there is water vapor in them. But what we talk about as the cloud, I guess, is, well, it's the atmosphere plus the stuff in it that we see. So what we're seeing are the visible liquid droplets and frozen crystals and other stuff, including sometimes pollutants. That's what reflects the sunlight, and that's what we actually see as the cloud. So, I mean, a cloud in that sense is mostly air, but it becomes visible because of the things in the air. Clouds can form pretty much whenever air becomes saturated, meaning, of course, that the air reaches 100% humidity, water then begins to condense out of the air and forms liquid droplets. Now, those liquid droplets don't immediately fall as rain because the liquid droplets that form initially, in most cases, are extremely small, much smaller than raindrops.

So, typical raindrop is about two millimeters in diameter, obviously it varies a bit, whereas a typical cloud droplet is about 0.02 millimeter, so about 1-100th of the size of a rain droplet. So when you're talking about liquid water in the atmosphere, again, this is not water vapor, which is what can you, to humidity. Here I'm talking about liquid water droplets. They can remain aloft in the atmosphere if there's a balance between the force of gravity, which obviously pulls them down, and forces that push them up. So what might those forces be? Usually it's air currents or uplift of air that push it usually warm air, but not necessarily that's pushing it upwards. So clouds often form when there's some force that some lifting mechanism that causes a massive air to move upwards, or a graphic lift, frontal lift, convective lift and so forth, that I mentioned before. If that air is unstable,

then it will continue rising. As the air reaches dewpoint, so as it reaches 100% humidity, the moisture in the air begins to condense out, forming liquid droplets. Those droplets will often stay suspended in the air, at least for a time, because of the force of the air pushing upwards on those droplets, because the air is continuing to rise, or like new air is rising up underneath it. So they're pushing it upwards. So basically, updrafts of air help to keep the water droplets suspended in the atmosphere. And you don't have to have a very high, like a very large force for that to be possible because they're not very large. And so basically there's, like many other things, there's a balance.

If the water droplet becomes too large, then the upward force becomes too small relative to the downward force of the force of gravity acting on it. Basically because the upward force depends on the surface area of the droplet because that's the area over which the upward force can act, the surface of the droplet, whereas the downward force of gravity just depends on the volume of the droplet, which of course affects its mass, and gravity is dependent on mass. So basically, the water droplets can stay suspended in the atmosphere so long as they don't get too big. When they get too large, they fall as rain or some other form of precipitation, which we'll talk about in a moment. Now, I mentioned that if the air becomes saturated, then water condenses out of it and forms water droplets, which typically form clouds. However, that's not quite true because for clouds to form, water doesn't just spontaneously condense out of the atmosphere, even at 100% humidity. For that to occur, you need what are called cloud condensation nuclei, or sometimes just condensation nuclei, if it's not in the context of a cloud. But these are small particles, typically only a few hundred nanometers across, so much smaller even than the cloud droplets, which I mentioned are smaller in turn than rain droplets.

So these are very tiny particles. And basically they're just like specks of smoke or pollution or soot, bits of sea salt, dust, clay, really any stuff that's been caught up in the air. Sulfates from volcanic activity is another example, bits of organic matter that have found their way into the atmosphere. So very, very tiny particles we're talking about. Any of these types of particles can act as a condensation nuclei. And basically this is just a surface that the tiny droplets of water begin to condense on and sort of join up together. Without these cloud condensation nuclei, it's extremely difficult for water to condense out of the atmosphere. And I think I read somewhere that you have to get to something like 400% relative humidity

in order for water to spontaneously condense out of a context in which there are absolutely. no condensation nuclei. Of course, that's extremely unlikely to happen because there's always stuff in the air, even if in very tiny amounts. And you don't need many of these things to get the water to condense. But anyway, cloud condensation nuclei are small particles that allow the water vapor to condense once you reach 100% humidity. They condense out of the atmosphere, and as long as they remain suspended in the atmosphere by updrafts of air, they form clouds. There are many different ways of classifying clouds. Anyone who looks up at the sky can tell that there are different types of clouds and people since time and memorial really have

classified them in different ways. Modern methods of cloud classification are typically based on the altitude of clouds as well as their method of formation, vertical stratification, and physical properties or appearance. I'm going to talk a bit about the different types of clouds. First I'll talk about the physical forms of clouds, like mostly based on their appearance, and then I'll talk about different categories of clouds based on their altitude, high level, mid-level, and load level clouds. It's obviously quite difficult to talk about cloud types without being able to show you what they look like. I'll post a few diagrams on the Facebook so you can get a visual idea. But because many of you will have seen these clouds at various times, hopefully you'll have some

idea of what I'm talking about. You can sort of visualize it a bit. And again, there are many types of classification systems or degrees of detail. I'm only going into a modest degree of detail because this is just an introductory podcast and there's no need to get over the complicated. So, First of all, let's talk about some of the physical forms of clouds. One property of clouds is called stratiform. Now, stratiform clouds appear in a stable air mass conditions. So already we're bringing that concept of atmospheric stability in. So when I talk about stable or unstable conditions or atmospheric conditions,

again, that's referring to whether air that begins to be pushed upwards by some lifting mechanism, whether it continues to move upwards, or whether it sort of gets stuck and stays where it is. And so that's very important for cloud formation, as we'll see. So stratiform clouds appear in stable conditions and have a flow. sort of sheet-like appearance and they can form at any altitude. So on an overcast day, if the sky just looks like there's a carpet over it, that's called a stratiform clouds or it's likely be stratiform clouds. Another descriptive term is syruform, this is spelled with a C.

Syroform clouds are a type of clouds that look like semi-detached or partly merged filament. So they're kind of like long and string. They form high in the atmosphere. One thing I should mention is that when I talk about the atmosphere here, I'm always talking about the troposphere, which is the lowest level of the atmosphere, up to about 10 kilometers up, and basically all clouds and all of the clients that we will talk about occur in the troposphere. A few types of clouds can sometimes poke up into the stratosphere, and there are very specialized

forms of clouds that occur at higher altitudes, but pretty much all clouds, and for that matter, most weather phenomena occur in the troposphere. So I talk about the different levels of the atmosphere in episode 89 on the atmosphere, so have a check back at that if you want a refresher. but hitherto and previously when I talked about the atmosphere, I'm talking about the troposphere. So these seriform clouds, which look like long, stringy filaments, they occur at high altitudes. Then there are cumuliform clouds. Now, these appear as like heaps or tufts or kind of like cotton balls. These are sort of the, I guess, the canonical stereotypical cloud.

If you were to draw a cloud, you'd probably draw a cumuliform cloud or, you know, a stereotype form of it. Now, these are generally more localized than the stratosform clouds, just like, up at the sky. Cumuliform clouds occur under more localized conditions of lift, which is why they're more localized rather than covering the whole sky. A fourth major type or also property of clouds is stratocumuliform. Now these are clouds of structure that have a combination of cumuliform and stratoform, as their name indicates stratumuliform. So they're just kind of a cross between stratiform and cumuliform clouds, because obviously there's a continuum. Generally they result from more limited instability, so in other words, more stable conditions. Often when there's,

an inversion layer, so that's a temperature inversion that prevents upward motion of the air, and that can keep a sort of a more blanket cloud formation. And the last form of cloud, or sort of property characteristic of clouds that I'm going to mention is cumulonimbab form. It's a bit of a tongue twister there, but these are very large clouds that have towering vertical extents. So I mentioned before that vertical stratification is one property of plants. This means how much they have vertical structure. Most clouds are more or less flat or flat-ish relative to the atmosphere, but cumulonimbus clouds or clouds that have cumulonimbabre form properties or characteristics have very substantial vertical extent and cover a wide range of

altitudes throughout the atmosphere. They occur in highly unstable air, as you might expect, given that they have a large vertical extent. So basically what we've covered here are sort of five types or physical characteristics of clouds. So there's stratus form, which is like flat like a sheet. they occur in more stable conditions Syroform like stringy they occur at very high altitudes cumula form that's sort of puffy or tufts they occur under

sort of more localized unstable conditions stratocumular form combination of stratiform and cumuliform so combination of sort of sheet like and puffy and cumulonimbabre form which are vertically extended these are also the types of clouds that give rise to thunderstorms and a lot of rain as we'll talk about later and these clouds

typically form under very unstable conditions. Okay, so that's the basic sort of physical characteristics of clouds. Now I'm just going to talk a bit about the different types of clouds that form depending on the altitude. So let's start with high-level clouds. These occur at altitudes from three to eight thousand meters in polar regions and then it changed a little bit depending on the temperate reaches and the tropics. I'll probably just give the numbers for polar regions just for simplicity because otherwise there's too many numbers, but bear in mind that the atmosphere is different thicknesses at different altitudes. So the altitudes are generally much higher at the tropics,

the polar regions because the atmosphere essentially bulges around the equator because of the Earth's rotation. So these high altitude clouds are many kilometers up and are divided into three main categories that I'll talk about. First of all, there are the plain cirrus clouds. So these are fibrous wisps, a very delicate syroform ice crystal clouds. They show up very clearly against the sky and often occur near the jet stream, which I've talked about in the past. I won't go over that again here, but jet stream is high altitude, very fast-moving air. these clouds don't produce any precipitation. In fact, I don't think any of the serious clouds produce significant amounts of precipitation.

So the next type of cloud I'll talk about is cirro cumulus. So these are very pure white puffy clouds. So they look like basically a bunch of those puffy clouds, but very small far away, which is kind of because that's what they are. So if you see sort of very white-looking clouds that are kind of puffy, but with small dots or like, I guess, kind of like a network of blue, between them, that's kind of, you're probably looking at a cirricumulus cloud. They're composed of ice crystals and other super cool water droplets.

And then finally, we've got the cirrus stratus clouds, which is like the stratus form of the cirrus. So this is a thin ice crystal that appears, again, at very high altitudes. It tends to give rise to halos because of refraction of the sun's rays so they can cause the sun to stand out very clearly. They're extremely wispy, and so they don't have clear threads in them like the cirrus clouds do, nor they, a little. little puffy like the cirric numeralous clouds. They can give rise to very interesting optical effects

due to the diffraction of the sun there. Now let's move from the high altitude to the mid-altitude clouds. So as you would have noticed, the sort of prefix that's used is sero or cirrus for high-altitude clouds. The accompanying or related term for mid-level clouds is alto. Alto clouds form from around 2,000 meters up to around 4,000. thousand meters near the poles and then a bit higher shifted up at their more mid latitudes. So let's think about as being between maybe two to three kilometers and then three kilometers above is the high altitude clouds. There's two main types that I'll talk about here. There's the alto cumulus and the alto stratus.

So you can probably get an idea of what these are. The alto stratus are like the carpet like ones. Altocumulus are like the puppy ones. The main difference in terms of the physical appearance is just that the alto stratus clouds are much less diffuse than the cirrus. stratus clouds, they don't give rise to that halo optical effect thing. Alto-cumulus clouds appear kind of bigger and more separated, partly just because they're much closer and so that they don't appear as kind of particulate as the cirricumulus clouds do, but otherwise

they're sort of broadly similar. Altostratus clouds can sometimes produce precipitation, especially if they're a bit darker, but again, most middle altitude clouds don't produce that much in the way of precipitation. precipitation, precipitation mostly comes from vertically structured clouds. Moving now to the low altitude clouds. Now these ones, unlike the mid and the high altitude ones, don't really have a special prefix for them. The three main types are stratus cumulus and strata cumulus. So again, hopefully you're seeing the pattern now.

Stratus refers to like the carpet-like ones. Cumulus is the puffy ones. Stratocumulus is kind of like a combination of the two. At low altitudes, the stratus ones, the stratus cloud look pretty similar to alto-stratus. they're just kind of a bit, they're thicker, often darker, though not always, and often resembles fog that's just being elevated, because essentially that's what it is. Only typically quite weak precipitation occurs from stratus clouds, whereas cumulus clouds are the really big, puffy, stereotypical clouds that you probably think of. They indicate fair weather, usually occurring in relatively stable conditions, but with localized instability, as we'll see a bit later. Stable atmosphere conditions usually means that it's going to be sort of warmer and sunnier, or at least sunnier, and we'll talk a bit more about why that is later.

And at this low level, both clas are made up of aeropolis rather than ice crystals in most conditions here. So the basic way I think about it is that you've got your low level clouds, which are the stratus. They're the carpet-like ones and the cumulus, which are the puffy ones, and then there's stratumulus, which is like a combination of the two. And then you've got the variance of those at middle and high altitude. So you've got your alto-stratus, your alto-cumulus, your cero-cumulus, your cero-cumulus. And then there's another type of clans at high-altitude is called ceres, which is a very, wispy, streaky ones that you don't really get at lower altitudes. So again, that's one way of classifying clouds.

You can go out and have a look and see what clouds you can see in the sky now, or next time you go outside or next time there are clouds around. But we haven't yet talked about the vertically stratified clouds, which are in some sense the most interesting clouds. However, I'm going to put that on hold until we get to talking about storms, thunderstorms and also mid-altitude cyclones and other related phenomena, because that's basically where these clouds, including the Cumuline nimbus and the Nimbostratus clouds really come to shine because these clouds occur in highly unstable atmospheric conditions that give rise typically to most of the precipitation and other interesting phenomena as well.

So we'll come back to those types of clouds in due course. For the moment, I just want to mention a few other things. So fog is basically just a cloud that makes contact with the ground. If we vertically categorize the clouds that we've talked about, fog is any cloud that touches the ground. so from altitude zero up to wherever. Low altitude clouds are anything below two kilometers. Mid-level clouds at the poles is anything from about two to three kilometers or a bit higher at mid-altitudes.

High altitude clouds is anything from, say, three kilometers to up to a maximum of 10 kilometers, but more around the tropical areas. So most clouds that we probably think about and interact with are occurring at the mid and the low levels, and so those are up to probably only a few kilometers high. So most jet aircraft travel, I think, just around the top of the troposphere. So they're going to be above most of the clouds, not all clouds. But that's one of the reasons why you'll often see yourself flying above the clouds, because most of the clouds that we sort of interact with typically,

apart from cumulonymus, but the non-vertically shatterified ones are mostly well below aircraft cruising altitudes. Coming back to fogs, just briefly, there's many reasons why fog can form, but some of the most common types of reasons why fog forms are radiation fog. This is caused by cooling of the land after sunset. So obviously the incoming solar radiation stops at sunset and thereby emission of radiation by the now cooling land cools down the land, which cools down the above air, which then causes the air temperature to fall, reaching the dew point, thereby causing water vapor to precipitate, forming fog.

Another type of common cause of fog is called advection fog. This occurs when moist air passes over a cool surface by basically wind. It's pushed thereby by differences in pressure or whatever other factor, and is thereby cooled. So the difference here is that it's not stationary air that's cooling because it's getting dark. It's air that was moist, that's then cooled by moving over a new surface. So this is very common at sea, where you have moist air encountering cool waters because it's moved over from land or from another part of the ocean that was warmer. It's cooled down by the water, and thereby reaches its dewpoint and precipitates. There's many other causes of fog as well, but those are two particularly common ones.

So that concludes a section on clouds. Hopefully, clouds make a bit more sense now. So let's go on and talk a bit about precipitation, which is, of course, the thing that clouds are probably most well known for. So precipitation is any product of condensation of atmospheric water vapor that falls under gravity. So any type of water that is falling from the clouds due to gravity is precipitation. So precipitation includes rain, snow, sleet, ice pellets, hail, drizzle, and many other words that are used in different cultures to describe somewhat different forms of precipitation. And basically these just refer to the physical state of the precipitation as it's falling.

So it's all water, but water has many forms. It can be liquid, which is classic rain, but it can also occur in various solid forms and sort of semi-solid liquid solid combinations, which account for many of these different words. So as I mentioned before, precipitation occurs when clouds accumulate, sufficient condensed moisture droplets that they begin to condense into sizes that are too large to remain aloft and therefore they fall. Or it could also be the case that the updrafts that keep the existing water droplets are loft stop for whatever reason and therefore the whatever water droplets are still present

in the cloud will condense and fall down. Of course, just because water starts falling from the sky doesn't necessarily mean it will fall to the ground because obviously if it comes from a higher altitude as it falls, the temperature of the atmosphere increases, thereby reducing the relative humidity, other things being equal, and thereby potentially allowing the water to evaporate once again. And so it won't necessarily fall to the ground unless it's able to fall far enough. Air turbulence tends to promote precipitation because air turbulence causes more of these condensed water droplets to collide with each other, which produces bigger droplets, which then increases the rate at which sufficiently

large droplets form that are able to fall to the ground and produce rain. other forms of precipitation. So that's often why unstable conditions like thunderstorms or lightning, which the product of highly unstable atmospheric conditions and air turbulence and so forth, are associated with a lot of precipitation. So wind and rain kind of go together. Rain drops vary in size from a fraction of a millimeter to about nine millimeters in diameter. I think I said before two millimeters is a kind of an average, but they're significantly larger, as I mentioned, than the moisture droplets that form clouds. there's a common image of a raindrop which, you know, has that classic teardrop shape.

I think a lot of people know that that's not what a raindrop actually looks like. That shape is the product of water falling from some sort of aperture and the surface tension with the water that sort of pulls it backwards into that sort of teardrop shape. But because that's not what's happening when water falls from the cloud, there's no, there's nothing that it's connected to, nothing that pulls it up into that shape. Water droplets either have roughly a spherical shape if they're quite small or, if they're larger, they become more oblate, which basically means they're like a squash circle, kind of like a donut, not those donuts that are a hole in the middle, you know, the ones that are

just like an oblate shape. If raindrops get too large, they tend to break up in the middle into smaller droplets, so there's a limit to how big they can get. Whether precipitation falls as rain, snow, or ice pellets or something else depends on many factors, but particularly important is how warm the air is. So if the air is very cold from cloud all the way down, then precipitation is more likely to fall as snow, that is actual ice crystals, that have had a chance to form larger like snowflakes or at least partial snowflake structures, which then fall all the way to the ground. On the other hand, on the other extreme, if the air is fairly warm,

or maybe not extremely warm, but at least much warmer above freezing point all the way down, then the precipitation will just fall as rain. But there's other combinations in between. So, for example, you might have a very cold layer around the cloud, which is where the precipitation form. So initially it might form snow, but then it might pass through a relatively warmer layer below, which can cause the snow to melt into basically rain.

But then if it passes through a colder layer, still, so this would be an inversion layer when the air gets a bit warmer and then colder again if you go up. So if it passes through an inversion layer, it could cause the snow to melt, but then refreeze more quickly and form ice pellets. So that's one way you could have hail, which are little pellets of ice rather than crows.

crystals, what you have when you have actual snow. There's other possibilities as well, such as the warm layer could be not sort of in the middle, but right near the ground. So you could have a situation in which the snowflakes sort of just start to melt or just melt as they hit the ground, but then if the surface, they could sort of melt just before they hit the ground, but then when they hit the ground, the ground could be slightly cooler and then causing them to refreeze. So that's freezing rain. And as many combinations as you can imagine, and it's actually quite complicated as to exactly what form water takes in different combinations of temperature and humidity and so forth. So I'm not going to go into all the details of that, but just be aware that basically it's all

the sort of the same thing when it comes to the cloud's point of view. It's moisture falling out of it in the form of raindrops. But what happens to it between that, the falling out of the cloud and hitting the ground, kind of depends on what form the precipitation takes. Okay, so let's conclude this first part by talking about air masses. And this will form the basis for what we get into next time when we start talking about fronts and met outstudent tropical cyclones and then getting into thunderstorms. I've mentioned the concept of an air mass once or twice, but I haven't defined it, and this is where I'm going to do that. So in meteorology, an air mass is a volume of air that's defined by its temperature and water vapor content. So basically

temperature and humidity. The difference between an air mass and an air parcel, which I defined previously, is basically just one of size. I mean, it's a bit more than that because an air mass is more of an meteorological term, whereas an air parcel is more of a fluid dynamics term. But we didn't really get into all of those technicalities here. For our purposes, an air mass is just a very large massive air that covers like hundreds of kilometers or maybe thousands of kilometers, and that has reasonably consistent temperature and humidity properties. This is distinguished to a parcel of air, which is a much, much smaller, a volume of air, maybe a few meters or a few hundred meters across. And they refer to in different contexts. So parcel of air, we talked about in the context

of atmospheric stability. Air masses we're talking about in the context of fronts, cyclones, and storms, as we'll get into. So an air mass is a big, big body of air. Now, air masses are classified according to latitude and according to their source as being either continental or maritime. Basically, air masses can form in very cold environments or sort of modern environments or warmer environments, and they can also form either in maritime or wet environments over the ocean or in continent, or relatively drier environments. And so that's how they get their names. For an air mass to form, it has to have uniform properties over a wide horizontal area and must be stationary over what's called the source area, which is where it forms, over its source area for a long enough

time to be homogeneous over such a large region. Also, it has to travel as a fairly homogeneous unit and retain these properties. If it sort of mixes with the existing surrounding air or breaks up into pieces, it's not an air mass anymore. So because of these requirements for horizontally large uniform properties over a wide area, the typical source regions for airmasses are oceans, large deserts, and large forests because, well, they are fairly flat, wide open areas where you can have an air mass form. It's harder to form in areas that are broken up by many mountain ranges, for example, or have a diverse range of vegetation, because you're likely to have different properties over those different regions. Again, this is not hard and fast rule.

Air masses can form under many different conditions, but these are typically, you're looking at oceans, deserts and large forests. So as I mentioned, they're classified by latitude and by where they form. I'll just go through the five main ones that we're going to look at. First of all, maritime polar, which is abbreviated MP. Maritime polar, as the name indicates, a form in polar regions. So closer to the polar. That polar, perhaps somewhat confusingly, doesn't mean at the pole. It just means closer to the pole.

So typically we're talking more a temperate area. I think like Europe, North America, closer to Canada than Mexico, northern China kind of area. That's what's meant by polar. So it is a bit confusing in that context. Maritime, obviously, meaning over the ocean, polar, as I said, refer to the latitude. Now there's also a continental version of polar, which is seepy for continental polar. And it's basically the same latitude, but over land. Now, conversely, there's also a tropical version.

So there's maritime tropical, MT, and continental. continental tropical sea sea. And so tropical, we're talking generally between the tropics. So that's 30 degrees latitude on either side of the equator. So this is everything from sort of, if you draw a line through the middle of the US, through to a line that includes most of South America, but not the bottom. This line passes through basically halfway through the Mediterranean, passes through kind of between North and South Korea roughly. That's at the 30 degrees. North, 30 degrees south passes basically along South Africa and cuts off the bottom of Australia. the tropics area, anything that any air masses that form within that range, called tropical,

so they don't have to be like right at the equator to count as tropical, and depending on whether they're over the ocean, they're maritime or continental. Because so much that the tropics are over ocean, maritime, tropic air masses are quite common. The final type of air mass is continental Arctic air, which is abbreviate CA, and needless to say, it is very cold. There's no distinction here between continental and maritime because in the Arctic it's all frozen. So therefore it has the property of continental air which is being dry. Not surprisingly, maritime air has the property of being relatively moist because of all the evaporated water that occurs over the oceans. Tropical, whether it's maritime or continental, is always much warmer than polar

air which is cooler. And then of course you have Arctic air which is very cold. So none of that's very surprising. There's also an equivalent form of the continental Antarctic mass as well, but most of the sources you'll look at it and focus on the northern hemisphere, and so I've just sort of followed that by focusing on the northern hemisphere. But it's sort of similar in the southern hemisphere, you just sort of have to flip everything. Okay, so we've got maritime polar, maritime tropical, continental polar, continental tropical, and continental Arctic. Those are our combinations. And basically what happens when we get interesting weather phenomena, which we'll talk about in the next episode, is that different.

masses of air collide with each other. And interesting stuff happens when that happens, because you've got atmospheric instability, you've got warmer air going over cooler air, and you've got the mixing together. You've got fronts, which are the basically lines between air masses, and very cool stuff happens at those. When you've got unstable air, you're going to have thunderstorms, vertical clouds occurring because of the moisture condensing out of the air. When you also consider the fact of the Earth's rotation, the koiolts effect, that gives rise to rotations of storms and also cloud structures, which gives rise to mid-laditude and tropical cyclones. So the point is a lot of interesting stuff happens when you start to think about air masses

and start to think about those air masses moving and then colliding together. And basically it's the movement of air masses around that gives rise to most of the weather phenomenon that we think about. And it is that that will form the topic of the next episode when I talk about fronts and different types of cyclones, thunderstorms, and then conclude by looking at tornadoes. So stay tuned for that. hopefully you found this episode interesting. If you liked what you heard,

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That's FODS12 at gmail.com. Thanks once again for listening and I'll talk to you next time.