Monday, October 28

Condensation nuclei and the formation of dew, frost, haze, fog, and clouds
Here's a visual summary of a part of what we'll be covering next.




A variety of things can happen when you cool air to the dew point and the relative humidity increases to 100%.  When moist air next to the ground becomes saturated (RH reaches 100%) water vapor condenses onto (or, in the case of frost, is deposited onto) the ground or objects on the ground.  This forms dew, frozen dew, and frost. 

When air above the ground cools to the dew point, it is much easier for water vapor to condense onto small particles in the air called condensation nuclei.  It would be much more difficult for the water vapor to condense and form small drops of pure water.  Both the condensation nuclei and the small water droplets that form on them are usually too small to be seen with the naked eye.  We can tell they are present because they scatter sunlight and make the sky hazy.  As humidity increases dry haze turns to wet haze and eventually to fog.  We'll try to make a cloud in a bottle and you'll be able to better appreciate the role that condensation nuclei play. 


Condensation nuclei and the role they play in cloud droplet formation

The air next to the ground cools during the night.  Sometimes it cools enough to reach the dew point.  Water vapor condenses onto objects on the ground and you find everything covered with dew (or frost) the next morning.  When this happens in the air up above the ground you might think that water vapor would simply condense and form little droplets.  This is not the case; we will find that small particles in the air called condensation play an essential role in cloud (and fog) formation.

it is much easier for water vapor
to condense onto small particles
called condensation nuclei 		it would be much harder for water vapor
to just condense and form
small droplets of pure water


We won't go into all of the details that follow in class, though they aren't hard to figure out and understand.  You're free to just skip the details, but do remember that particles make it much easier for cloud droplets and clouds to form. 

When the air is saturated with water vapor (the relative humidity is 100%) the rates of evaporation and condensation above a flat surface of water will be equal.

There's no real reason for picking three arrows each of evaporation and condensation, the important point is that they are equal when the RH is 100%.
It's hard for water vapor to condense and form a small droplet of water because small droplets evaporate at a very high rate.  This is known as the curvature effect and is illustrated below.
 



The surface of the smallest droplet above at left has the most curvature and the highest rate of evaporation (6 arrows).  If a small droplet like this were to form, it wouldn't stay around very long.  With it's high rate of evaporation it would quickly evaporate away and disappear. 

The middle droplet is larger and would stick around a little longer because it does not evaporate as quickly.  But it too would eventually disappear.
The drop on the right is large enough that curvature no longer has an effect.  This drop has an evaporation rate (3 arrows) that is the same as would be found over a flat surface of water.  A droplet like this could survive, but the question is how could it get this big without going through the smaller sizes with their high rates of evaporation.   A droplet must somehow reach a critical size before it will be in equilibrium with its surroundings.
Particles in the air, cloud condensation nuclei (CCN), make it much easier for cloud droplets to form.  The figure below explains why.


By condensing onto a particle, the water droplet starts out large enough and with an evaporation rate low enough that it is in equilibrium with the moist surroundings (equal rates of condensation and evaporation). 

There are always lots of CCN (cloud condensation nuclei in the air) so this isn't an impediment to cloud formation. 

Now back to material that we will cover in class.
 The following information is from the bottom of page 91 in the ClassNotes.



Note that condensation onto certain kinds of condensation nuclei and growth of cloud droplets can begin even when the relative humidity is below 100%.   These are called hygroscopic nuclei.  Salt is an example; small particles of salt mostly come from evaporating drops of ocean water.

I might try to show a video tape, not a digital video but video recorded on a magnetic tape.  It will depend first of all on there being a VCR in the classroom.

 Here are some more of the details that we won't cover in class. 

To understand how condensation onto particles can begin even before the RH has reached 100% we first need to learn about the solute effect






solution droplet
 pure water droplet

Water vapor condensing onto the particle in the left figure dissolves the particle.  The resulting solution evaporates at a lower rate (2 arrows of evaporation).  A droplet of pure water of about the same size would evaporate at a higher rate (4 arrows in the figure at right).  Note the rates of condensation are equal in both figures above.  This is determined by the amount of moisture in the air surrounding each droplet.  We assume the same moist (the RH is 100%) air surrounds both droplets and the rates of condensation are equal. 

The next figure compares solution droplets that form when the RH is 100% (left figure) and when the RH is less than 100%.


the droplet is able to grow
 the droplet is in equilibrium with its surroundings
even when the RH is less than 100%

The solution droplet will grow in the RH=100% environment at left.  You can tell the RH is less than 100% in the figure at right because there are now only 2 arrows of evaporation.  But because the solution droplet only has 2 arrows of evaporation it can form and be in equilibrium in this environment.


We should remember that much of what we see in the sky is caused by scattering of light.  There is a pretty good demonstration of light scattering during a Startijenn concert shown below.
 

 The figure below is at the bottom of page 91 in the ClassNotes and illustrates how cloud condensation nuclei and increasing relative humidity can affect the appearance of the sky and the visibility.

The air in the left most figure is relatively dry.  Even though the condensation nuclei particles are too small to be seen with the human eye you can tell they are there because they scatter sunlight.  When you look at the sky you see the deep blue color caused by scattering of sunlight by air molecules mixed together with some white sunlight scattered by the condensation nuclei.  This changes the color of the sky from a deep blue to a bluish white color.  The more particles there are the whiter the sky becomes.  This is called "dry haze."  Visibility under these conditions might be anywhere from a few miles up to a few tens of miles.




(source of the image above)

A photograph of fairly severe air pollution in Paris that illustrates an extreme case of dry haze (this is more common and more severe in China and India).   In Paris cars with even numbered license plates weren't allowed into the city on certain days of the week, odd numbers were banned on other days.  Public transportation was free for a short time to try to reduce automobile use. 


The middle picture below shows what happens when you drive from the dry southwestern part of the US into the humid southeastern US or the Gulf Coast.  One of the first things you would notice is the hazier appearance of the air and a decrease in visibility.  It isn't that there are more particles.  The relative humidity is higher, water vapor begins to condense onto some of the condensation nuclei particles (the hygroscopic nuclei) in the air and forms small water droplets.  The water droplets scatter more sunlight than just small particles alone.  The increase in the amount of scattered light is what gives the air its hazier appearance. This is called "wet haze."  Visibility now might now only be a few miles.




Thin fog (perhaps even wet haze)
with pretty good visibility
(source of the image)
 Thick fog
(visibility was less than 500 feet)
(source of the image)

Pictures of fog like we sometimes get in Tucson (maybe once a year).  The picture at left is looking east from my house and was taken early in the morning at the start of the spring semester in 2015.  The picture at right is the view to the west.  Visibility was perhaps 1/4 mile.



Finally when the relative humidity increases to 100% fog forms and water vapor condenses onto all the condensation nuclei.  Fog can cause a severe drop in the visibility.  The thickest fog forms in dirty air that contains lots of condensation nuclei.  That is part of the reason the Great London Smog of 1952 was so impressive.  Visibility was at times just a few feet!

Making a cloud in a bottle
Cooling air (caused by sudden expansion) & increasing relative humidity, condensation nuclei, and scattering of light are all involved in this demonstration.





We used a strong, thick-walled, 4 liter vacuum flask (designed to not implode when all of the air is pumped out of them, they really aren't designed to be pressurized).  There was a little water in the bottom of the flask to moisten the air in the flask.  Next we pressurized the air in the flask with a bicycle pump.  At some point the pressure blows the cork out of the top of the flask.  The air in the flask expands outward and cools.  This sudden cooling increases the relative humidity of the moist air in the flask to more than 100% momentarily and water vapor condenses onto cloud condensation nuclei in the air. 

I like it best when a faint, hard to see, cloud becomes visible.  That's because there is something we can add to the demonstration that will make the cloud much "thicker" and easier to see.



The demonstration was repeated an additional time with one small change.  A burning match was dropped into the bottle.  The smoke from the matches added lots of very small particles, condensation nuclei, to the air in the flask (you could see the swirls of smoke, the small particles scattered light).  The same amount of water vapor was available for cloud formation but the cloud that formed this time was quite a bit "thicker" and much easier to see.  To be honest the burning match probably also added a little water vapor (water vapor together with carbon dioxide is one of the by products of combustion).

I have found a couple of online versions of the demonstration.  The first is performed by Bill Nye "The Science Guy" and is pretty similar to the one done in class.  The second differs only in the way that is used to caused the sudden expansion and cooling of the air (I didn't care much for the music (probably your opinion of the music I play before class) and I would recommend turning down the sound while watching the video).

Mother Nature's version of the Cloud in a Bottle demonstration





 A brush fire in this picture is heating up air and causing it to rise.  Combustion also adds some moisture and lots of smoke particles to the air.  You can see that initially the rising air doesn't form a cloud (the RH is still less than 100%).  A little higher and once the rising air has cooled enough (to the dew point) a cloud does form.  And notice the cloud's appearance - puffy and not a layer cloud.  Cumulo or cumulus is the word used to describe a cloud with this appearance.  These kinds of fire caused clouds are called pyrocumulus clouds.  The example above is from a Wikipedia article fire-caused clouds.  The fire in this case was the "Station Fire" burning near Los Angeles in August 2009.  We sometimes see clouds like this in the summer when lightning starts a fire burning in one of the nearby forests.  The pyrocumulus cloud caused by the fire is sometimes the only cloud in the sky.

Clouds and climate change
This effect has some implications for climate change.
 

A cloud that forms in dirty air is composed of a large number of small droplets (right figure above).  This cloud is more reflective than a cloud that forms in clean air, that is composed of a smaller number of larger droplets (left figure).  

Combustion of fossil fuels adds carbon dioxide to the atmosphere.  There is concern that increasing carbon dioxide concentrations (and other greenhouse gases) will enhance the greenhouse effect and cause global warming.  Combustion also adds condensation nuclei to the atmosphere (just like the burning match added smoke to the air in the flask).  More condensation nuclei might make it easier for clouds to form, might make the clouds more reflective, and might cause cooling.  There is still quite a bit of uncertainty about how clouds might change and how this might affect climate.  Remember that clouds are good absorbers of far IR radiation and also emit IR radiation.  Clouds often raise nighttime low temperatures.

Clouds are one of the best ways of cleaning the atmosphere.  This is something we mentioned earlier in the semester and you're now in a position to understand it better.




A cloud is composed of small water droplets (diameters of 10 or 20 micrometers) that form on particles ( diameters of perhaps 0.1 or 0.2 micrometers). The droplets "clump" together to form a raindrop (diameters of 1000 or 2000 micrometers which is 1 or 2 millimeters), and the raindrop carries the particles to the ground.  A typical raindrop can contain 1 million cloud droplets so a single raindrop can remove a lot of particles from the air.  You may have noticed how clear the air seems the day after a rainstorm; distant mountains are crystal clear and the sky has a deep blue color.  Gaseous pollutants can dissolve in the water droplets and be carried to the ground by rainfall also.  We'll be looking at the formation of precipitation in more detail next.


Formation of precipitation in clouds

Why is it so hard for clouds to make precipitation?

This figure shows typical sizes of cloud condensation nuclei (CCN), cloud droplets, and raindrops (a human hair is about 50 μm thick for comparison).  As we saw in the cloud in a bottle demonstration it is relatively easy to make cloud droplets.  You cool moist air to the dew point and raise the RH to 100%.  Water vapor condenses pretty much instantaneously onto cloud condensation nuclei to form cloud droplets.  It would take much longer (a day or more) for condensation to turn a cloud droplet into a raindrop.  You know from personal experience that once a cloud forms you don't have to wait that long for precipitation to begin to fall.

 Part of the problem is that it takes quite a bit more water to make a 2000 μm diameter raindrop than it does to make 20 μm diameter cloud droplets .  A raindrop is about 100 times bigger across than a cloud droplet.  How many droplets are needed to make a raindrop?  Before answering that question we will look at a cube (rather than a sphere).





How many sugar cubes would you need to make a box that is 4 sugar cubes on a side?


It would take 16 sugar cubes to make each layer and there are 4 layers.  So you'd need 64 sugar cubes.  The key point is that we are dealing with volumes,  in the case of a cube, volume is length x width x height.

The raindrop is 100 times wider, 100 times bigger from front to back, and 100 times taller than the cloud droplet.  The raindrop has a volume that is 100 x 100 x 100 = 1,000,000 (one million) times larger than the volume of the cloud droplets.  It takes about a million cloud droplets to make one average size raindrop.

Precipitation-producing processes
Fortunately there are two processes capable of quickly turning small cloud droplets into much larger precipitation particles in a cloud.




The collision coalescence process works in clouds that are composed of water droplets only.  This is often called the "warm rain" process.  Clouds like this are found in the tropics (and very occasionally in the summer in Tucson).  We'll see that this is a pretty easy process to understand. 



This process will only produce rain, drizzle, and something called virga (rain that evaporates before reaching the ground).  Because the clouds are warm and warm air can potentially contain more water vapor than cooler air, the collision-coalescence process can produce very large amounts of rain.

The ice crystal process produces precipitation everywhere else.  This is the process that normally makes rain in Tucson, even on the hottest day in the summer (summer thunderstorm clouds are tall and grow into cold, below freezing, parts of the atmosphere).  Hail and graupel often fall from these summer storms; proof that the precipitation started out as an ice particle).  Thunderstorms also produce lightning and later in the semester we will find that ice is needed to make the electrical charge that leads to lightning. 



There is one part of ice-crystal process that is a little harder to understand, but look at the variety of different kinds of precipitation particles (rain, snow, hail, sleet, graupel, etc) that can result.

The Collision-Coalescence process

The collision coalescence process works in clouds that are composed of water droplets only.  Here's how it works.  The picture (found on page 101b in the ClassNotes) below shows what you might see if you looked inside a warm cloud with just water droplets:


The collision coalescence process works in a cloud filled with cloud droplets of different sizes, that's critical.  The larger droplets fall faster than the small droplets.  A larger-than-average cloud droplet will overtake and collide with smaller slower moving ones.


The bigger droplets fall faster than the slower ones.  They collide and stick together (coalesce).  The big drops gets even bigger, fall faster, and collide more often with the smaller droplets.  This is an accelerating growth process - think of a growing ball of snow as it rolls down a snow-covered hill and picks up snow, grows, and starts to roll faster and faster;  or think of an avalanche that gets bigger and moves faster as it travels downslope.

image source

source of this image





Very quickly a larger than average cloud droplet can grow to raindrop size.


The figure shows the two precipitation producing clouds: nimbostratus (Ns) and cumulonimbus (Cb).  Ns clouds are thinner and have weaker updrafts than Cb clouds.  The largest raindrops fall from Cb clouds because the droplets spend more time in the cloud growing. In a Cb cloud raindrops can grow while being carried upward by the updraft (in this case the smaller droplets are catching and colliding with the larger droplets, but the end result is the same) and also when falling in the downdraft.

Raindrops grow up to about 1/4 inch in diameter.  When drops get larger than that, wind resistance flattens out the drop as it falls toward the ground.  The drop quicly breaks apart into several smaller droplets.  Solid precipitation particles such as hail can get much larger (an inch or two or three in diameter).

And actually my sketch at lower left above isn't quite accurate as this video of the breakup of a 5 mm diameter drop of water shows.

The ice crystal process works in most locations most of the time.  Before we can look at how the ice crystal process actually works we need to learn a little bit about clouds that contain ice crystals - cold clouds.

Cold clouds
The figure below shows the interior of a cold cloud (see page 102a in the ClassNotes)

The bottom of the thunderstorm, Point 1, is warm enough (warmer than freezing) to just contain water droplets.  The top of the thunderstorm, Point 2, is colder than -40 F (which, coincidentally, is equal to -40 C) and just contains ice crystals.  The interesting part of the thunderstorm and the nimbostratus cloud is the middle part, Point 3, that contains both supercooled water droplets (water that has been cooled to below freezing but hasn't frozen) and ice crystals.  This is called the mixed phase region.  This is where the ice crystal process will be able to produce precipitation.  This is also where the electrical charge that results in lightning is created.

Ice crystal nuclei

The supercooled water droplets in cold clouds aren't able to freeze even though they have been cooled below freezing.  This is because it is much easier for small droplets of water to freeze onto an ice crystal nucleus (just like it is easier for water vapor to condense onto condensation nuclei rather than condensing and forming a small droplet of pure water).  Not just any material will work as an ice nucleus however.  The material must have a crystalline structure that is like that of ice.  There just aren't very many materials with this property and as a result ice crystal nuclei are rather scarce.  In much of the mixed phase region there are more supercooled water droplets than ice crystals.