Sunday, October 14, 2012

Science Sunday: The Basics of Clouds

After taking all this time to write this post about clouds, you'd think that I could identify the types of clouds in this image.  Nope.

"I'm a dreamer.  I live to dream and reach for the stars, and if I miss a star then I grab a handful of clouds." ~Mike Tyson

Like Mike Tyson, today we'll be missing the stars and instead grabbing at clouds.  Sure this seems somewhat random, considering what my subject matter (not to mention my profession) has typically been,  but this post is borne out of two completely unrelated things.  First and foremost is my desire to learn about things outside of my field.  I spend hours each day, every day of the week accumulating knowledge about astronomy in general and stars in particular.  This has resulted in a sort of scientific tunnel vision, where I neither know of or pay attention to anything that doesn't concern stars.  I feel like I've lost touch with a lot of the science that goes on in the rest of the world, and so I'd like to reach out every now and again and learn about what other things the Universe has to offer beyond the various happenings of huge balls of burning hydrogen/helium plasma.

The second motivation for this post comes from my girlfriend's affinity for clouds.  Some days she just hangs out outside and looks up, staring at the shapes and different species of cloud, getting lost in their dance across the sky.  Then she calls me or sends me a text message to tell me about them, to which I invariably respond with my own personal disdain for clouds (I live in Seattle.  I have more cloud than I know what to do with).  Even though there's no love lost between clouds and myself, I'd still like to know what makes them tick.  Hence, we have this post.

Since I actually have no real training in this subject, I'm actually going to try to rely a little less on Wikipedia and a little more on websites with "atmospheric", "physics", "chemistry", or "meteorology" in the name.  Also, if you want to skip much of the scientific inquiry here (in which case I'd ask you why you're even looking at a post entitled "Science Sunday"), you can jump to the very brief summary at the end.

As with any subject that I might write about, but actually know very little about going in, let's start with the basics.  According to the American Meteorological Society, clouds are a visible aggregate of minute water droplets and/or ice particles in the atmosphere above the Earth's surface.  That's pretty straight forward, and falls right in line with the idea of clouds as these large wispy things.  But how does water come to be suspended up in the atmosphere?

Warm air gives these water molecules a chance at freedom.
Well mon frère, when water gets evaporated from the surface of the Earth (whether from land or from oceans), it rises into the air.  Why does water vapor rise?  While writing this, I realized that I actually couldn't answer this question off the top of my head because I simply did not know.  Hadn't really thought about it before.  It was just one of those things that I knew happened because I have always known it to happen.  So, I asked Google "what makes water vapor rise?" and was rewarded with a nice and concise answer.  Water vapor is a gas made of water molecules, each one of which contains one oxygen atom bound to two hydrogen atoms.  While the surrounding air is a mixture of different gases, the bulk of it is composed of diatomic nitrogen (~78%) and diatomic oxygen (~21%).  The molecular mass of water is 18 atomic mass units (16 + 1 + 1), while the molecular mass of the overwhelming majority of air molecules is heavier by far (14+14 = 28 and 16+16 = 32 respectively).  Thus, as water molecules are lighter than the surrounding air, they will rise when vaporized until they have time to mix with the air.  The resulting moist air is lighter than any nearby dryer air at the same temperature, and will then also rise above said dryer air.

As the moist air rises, it expands due to encountering lower air pressure (gases increase in volume when the pressure on them decreases) and cools down (through a negligibly small exchange of heat with the surrounding air) to the dew point of that air, which is the temperature at which the air becomes saturated (cannot hold anymore vapor).  How does this air decrease in temperature and yet exchange no actual heat with the surrounding gas?  This has to do with two of those little wonders of thermodynamics (thermo), Heat and a little something called Work.

This is external heat input changing the temperature of the gas.
This is NOT what's happening with our moist air.
In thermo, a gas is said to be heated (or cooled) when the temperature of that gas increases (or decreases).  Fairly simple there.  The important thing though is to distinguish between a gas having its temperature altered because of a change in the gas itself, and a difference in because of some energy exchange with its surroundings.  Temperature is a measurement of the average kinetic energy of the ensemble of particles that comprise some substance.  Essentially this means that temperature measures the average of the speeds of individual atoms in a gas.  High-temperature gases have particles that are very energetic, and vice versa.  A change in temperature of a gas is therefore a change in the energy of the particles of that gas.  This is an important point, so read it again if you didn't get it the first time.

Now we move to the second thermo concept, Work.  Work is done when energy is exchanged by either changing the volume of a system (in this case some bubble of air), the pressure within the system, or both.  If a bubble of moist air expands into the dryer air around it, it's physically pushing that dryer air outward and away as its volume increases.  This type of action, simple as it is, takes energy.  Thus, it's doing Work in pushing the surrounding air, and therefore the gas itself loses energy as it expands.

Now let's go back to the difference between an internal energy change in a gas and some external energy changing that gas. An internal change in the gas would be some bubble of air exchanging the kinetic energy (temperature) of the gas particles for an increase or decrease in its size (volume).  An external agent changing the gas would be that same bubble of air having its temperature changed because of some injection of energy into the bubble (radiation input or something).

Cooling from changes in pressure and volume
For rising moist air, there's no heat exchange with the surrounding air to decrease its temperature.  Instead what's happening is that the gas is expanding, and in doing so is drawing energy for that expansion out of the kinetic energy of the individual gas particles.  This decreases the average kinetic energy of the ensemble of gas particles, thus decreasing the temperature of the moist bubble.

As the temperature of the air decreases, its capacity to hold more water vapor also decreases.  What I'm really saying here is that the capacity for the moist air to maintain water in its evaporated state decreases.  Why is that?  This was yet another question that I couldn't answer off the top of my head, but now that I've looked it up it makes plenty of sense.  If you want to keep water vapor from becoming water liquid, you have to prevent water molecules from bonding to one another.  And let me tell you, water molecules love to bond.  In order to do this, you have to have enough energy to break any potential bonds between water molecules.  You can think of it like being a hero in an action movie, holding up a collapsing ceiling so that your friends can escape, where the strength that you use to keep the ceiling up is analogous to the energy needed to keep bonds apart.  As long as you have enough strength to hold the ceiling up, you can keep your friends from being killed.  The second your strength falters though, your friends become a nice, red paste on the dungeon floor.  Similarly, when a gas no longer has the energy to keep bonds between molecules from forming, liquid water forms and condenses out of the air.

So yeah, the temperature of the moist air bubble decreases down to the point (dew point temperature) at which it cannot hold any more water vapor.  If the air is cooled further, it sheds the vapor that it can no longer hold, and liquid water condenses out into a cloud.  This liquid condenses onto what are known as cloud condensation nuclei, which are particles (dust, salt, burnt stuff) light enough to be held aloft by air currents.  These particles are exceedingly small in size.  I'm talking like microscopic (0.000001 cm) at the largest.  My question to the internet was "why does water need to condense onto stuff?  Why can't it just glob up into liquid water on its own?"  I wasn't met with a swift and simple answer, so this time I defer to Wikipedia with a promise to look up an actual source and scientific explanation for this.

So now we've got our clouds.  Large, hulking conglomerations of water molecules condensed onto various dust particles in the atmosphere.  If you just wanted to know how clouds form, you could stop here, or skip to the end where I talk about the different cloud types.  If, however, you're innately curious like myself, you might want an answer to just one more question: how in the hell do these things stay afloat?!  Or as one website puts it, Why don't clouds fall?

Not the CLEAREST diagram in the world, sure.
In position 2, the drag force is less than the force of
gravity, so the particle accelerates downward.  In
position 3, they're equal, so there's no acceleration.
The answer here is that clouds actually DO fall (just learned this while writing).  They just fall extremely slowly.  I defer to the above link for the more quantitative explanation, but I'll point out the quasi-quantitative highlights here.  If you were in Physics 101 (or 151 at my undergrad college because...I don't know why), you'd assume that any falling object accelerates at a rate of 9.8 meters per second every second.  This is the acceleration due to the Earth's gravitational pull on that object, and is much of the reason why people used to think that throwing a penny off of the empire state building would kill someone.  However, if you paid attention in that class, you'd notice that one thing was always assumed with any Falling Object Problem: air friction was neglected.  This is because air friction (aka drag) slows things down as they fall.  Air friction increases as the physical size of your falling object increases, the speed of its descent increases, and the viscosity of the air increases.  A falling object will reach terminal velocity (i.e. will not fall faster) when its gravitational force (based solely on its mass) becomes equal to the drag force.  Water droplets are pretty damn light, so they reach their terminal velocity fairly quickly.  The result is that they "fall" with a gentleness that would make feathers jealous.

So let's summarize the science here.  Water evaporates from the Earth's surface and rises.  Then, it mixes with dry air, expands and cools.  Because of the cooling, water condenses onto dust particles, and aggregates into clouds.  The clouds are then held aloft by the drag in the air and air currents pushing against their descent.  The following graphic is just perfect for this.

Now we get to the different general cloud types (and some notable subtypes).  I had no idea that they were organized into families, genuses, and species like animals are.  Sweet Jesus!  As I'm tired of writing, I'm going to just list their basic qualities (hot and fresh from the Wiki) and show pretty pictures.  After all, I am an astronomer at heart, and astronomers are suckers for pretty pictures.

  • Cirrus - fibrous wisps of delicate white ice crystal cloud that show up clearly against the blue sky.  
  • Cirrocumulus - pure white cloud layer.  Composed of ice crystals or supercooled water droplets appearing as small, unshaded round masses, or flakes in groups, or lines with ripples like sand on a beach [ed - that's a lovely description].  They occasionally form alongside cirrus or cirrostratus clouds at the very leading edge of an active weather system.
  • Cirrostratus - a thin ice crystal veil that typically gives rise to halos caused by refraction of the Sun's rays.  
  • Altocumulus - A cloud layer usually in the form of irregular patches or round masses in groups, lines, or waves.  High altocumulus may resemble cirrocumulus, but is usually thicker and composed of water droplets so that the bases show at least some light-grey shading.  
  • Altostratus - An opaque or translucent veil of grey/blue-grey cloud that often forms along warm fronts and around low-pressure areas where it may thicken into nimbostratus.  Altostratus is usually composed of water droplets but may be mixed with ice crystals at higher altitudes. [ed - I think these are what plague Seattle].
  • Stratocumulus - A cloud layer usually in the form of irregular patches or round masses similar to altocumulus, but having larger elements with deeper-gray shading.  This cloud often forms under a precipitating deck of altostratus or high-based nimbostratus.
  • Stratus - A uniform layer of cloud resembling fog but not resting on the ground. [ed - Nevermind, it might just be stratus in general].
  • Cumulus - Clouds with clear-cut flat bases and domed tops.  The smaller cumulus types are generally associated with fair weather.
  • Nimbostratus - A very thick diffuse dark-grey layer that looks feebly illuminated from the inside.  It normally forms from altostratus and achieves vertical extent when the base subsides into the low-altitude range during precipitation that can reach moderate to heavy intensity.
  • Cumulus Congestus - aka Towering Cumulus; cumulus clouds of great vertical size, usually with dark-gray bases, and capable of producing severe turbulence and showers of moderate to heavy intensity.
  • Cumulonimbus - Heavy towering masses of cloud with dark-grey to nearly black bases that are associated with thunderstorms and showers.  Thunderstorms can produce a range of severe weather that includes hail, tornadoes, a variety of other localized strong wind events, several types of lightning, and local very heavy downpours of rain that can cause flash floods.  Require moisture, an unstable air mass, and heat in order to form.
  • Nimbus - Dense with water and appear darker than other clouds.  They can be characterized by their great height.  They are formed at low altitudes and are typically spread uniformly across the sky.  A nimbus cloud produces precipitation that reaches the ground as rain, hail, snow, or sleet.

And that's all!  This post has me thinking about a lot of other non-astro topics that I want to dig my fingers into.  Gotta love it.  References are at the bottom, after the disclaimer.  As always, mind the disclaimer.  Until next time!

Disclaimer disclaimer, while this is far better-referenced than my typical intellectual swill, it's still MY intellectual swill.  Additionally, I know very little about atmospheric science, and only retain a smattering of thermodynamics that applies to it.  As such, some of my conclusions and musings may be questionable.  This is not to be taken as a definitive source of information.  If anything, maybe just a primer.  But seriously though, learn this for yourself.  Don't rely on me. Feel free to ask questions, and make use of the references!  They're actually there for a reason!  And if I can learn about cloud formation from a few hours of trial and error research, you DEFINITELY can from an organized reference list and oodles of wikipedia.  Science for all!


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