Tuesday Nov. 2, 2010, Election Day
Three Simon and Garfunkel songs before class today ("A Hazy Shade of Winter", "The Sparrow", and "American Tune")

The Experiment #1 revised reports have been graded and were returned in class today.  Experiment #2 revised reports are due this coming Thursday (unless you've been given extra time).

Many of the Expt. #3 reports were turned in today.  Some people picked up the materials late and have been given extra time.  If you haven't returned your materials please do so as soon as you can.

We covered the collision-coalescence process in class last Thursday.



It can produce rain, drizzle, and virga (rain that evaporates before reaching the ground), but that's about it.


Look at what the ice crystal process can do by contrast.  We'll be covering that today.  There's a lot that can happen inside the cloud and more things that can occur outside the cloud.  By the end of class today you should know something about every precipitation particle in the picture.

We looked at the structure of cold clouds last Thursday.  One of the important points is that there is a large part of the cloud where ice crystals and supercooled water droplets are found together.  Today we'll see how ice crystal process works.  There are a couple of "tricky" parts.
The first figure above (see p.101 in the photocopied Class Notes) shows a water droplet in equilibrium with its surroundings..The droplet is evaporating (the 3 blue arrows in the figure).  The rate of evaporation will depend on the temperature of the water droplet.  The droplet is surrounded by air that is saturated with water vapor (the droplet is inside a cloud where the relative humidity is 100%).  This means there is enough water vapor to be able to supply 3 arrows of condensation.  Because the droplet loses and gains water vapor at equal rates it doesn't grow or shrink.


This figure shows what is required for an ice crystal (at the same temperature) to be in equilibrium with its surroundings.  First, the ice crystal won't evaporate as rapidly as the water droplet (only 1 arrow is shown).  Going from ice to water vapor is a bigger jump than going from water to water vapor.  There won't be as many ice molecules with enough energy to make that jump.  A sort of analogous situation is shown in the figure below.  The class instructor could and most of the people in the room could jump from the floor to the seat of a 12 or 15 inch tall chair.  It would be much tougher to jump to the top of the table (maybe 30 inches off the ground) or the podium (maybe 36 inches).  There wouldn't be as many people able to do that.  Guess what I am going to try to do this coming weekend in my backyard.

To be in equilibrium the ice crystal only needs 1 arrow of condensation.  There doesn't need to be as much water vapor in the air surrounding the ice crystal to supply this lower rate of condensation.


Now what happens in the mixed phase region of a cold cloud is that ice crystals find themselves in the very moist surroundings needed for water droplet equilibrium. This is shown below.

The water droplet is in equilibrium (3 arrows of evaporation and 3 arrows of condensation) with the surroundings.  The ice crystal is evaporating more slowly than the water droplet.  Because the ice crystal is in the same surroundings as the water droplet water vapor will be condensing onto the ice crystal at the same rate as onto the water droplet.  The ice crystal isn't in equilibrium, condensation (3 arrows) exceeds evaporation (1 arrow) and the ice crystal will grow.  That's what makes the ice crystal process work.

The equal rates of condensation are shown in the figure below using the earlier analogy.


Most everyone can manage to make the big or the small jump.

Now we will see what can happen once the ice crystal has had a chance to grow a little bit.

Once an ice crystal has grown a little bit it becomes a snow crystal (this figure is on p. 102 in the photocopied classnotes).  Snow crystals can have a variety of shapes (plates, dendrites, columns, needles, etc.; these are called crystal habits) depending on the conditions (temperature and moisture) in the cloud.  Dendrites are the most common because they form where there is the most moisture available for growth.  With more raw material available it makes sense there would be more of this particular snow crystal shape.


Here are some actual photographs of snow crystals (taken with a microscope).  Snow crystals are usually 100 or a few 100s of micrometers in diameter (tenths of a millimeter in diameter). 

You'll find some much better photographs and a pile of addtional information about snow crystals at www.snowcrystals.com

A variety of things can happen once a snow crystal forms.  First it can break into pieces, then each of the pieces can grow into a new snow crystal.  Because snow crystals are otherwise in rather short supply, ice crystal multiplication is a way of increasing the amount of precipitation that ultimately falls from the cloud.

 

Several snow crystals can collide and stick together to form a snowflake.  Snow crystals are small, a few tenths of a millimeter across.  Snowflakes can be much larger and are made up of many snow crystals stuck together.  The sticking together or clumping together of snow crystals is called aggregation (I frequently forget this term.  If I can't remember it I don't expect you to remember it either)


Snow crystals can collide with supercooled water droplets.  The water droplets may stick and freeze to the snow crystal.  This process is called riming or accretion (note this isn't called collision coalescence even though it is the same idea).  If a snow crystal collides with enough water droplets it can be completely covered with ice.  The resulting particle is called graupel.  Graupel is sometimes mistaken for hail and is called soft hail or snow pellets.  Rime ice has a frosty milky white appearance.  A graupel particle resembles a miniature snow ball.  Graupel particles often serve as the nucleus for a hailstone.  Riming and graupel are terms you should remember.
This figure gives you an idea of how hail forms.


In the figure above a hailstone starts with a graupel particle (Pt. 1, colored green to represent rime ice).  The graupel falls or gets carried into a part of the cloud where it collides with a large number of supercooled water droplets which stick to the graupel but don't immediately freeze.  The graupel gets coated with a layer of water (blue) at Pt. 2.  The particle then moves into a colder part of the cloud and the water layer freeze producing a layer of clear ice (the clear ice, colored violet, has a distinctly different appearance from the milky white rime ice), Pt. 3.  In Tucson this is often the only example of hail that you will see: a graupel particle core with a single layer of clear ice.

Hail that falls to the ground in Tucson usually just has a graupel core and a single layer of clear ice.  In the severe thunderstorms in the Central Plains, the hailstone can pick up additional layers of rime ice and clear ice and hailstones can be composed of many alternating layers of rime and clear ice.  An  unusually large hailstone (around 3 inches in diameter) has been cut in half to show (below) the different layers of ice.  The picture below is close to actual size.

Hail is produced in strong thunderstorms with tilted updrafts.  You would never see hail (or graupel) falling from a nimbostratus cloud.  


The growing hailstone can fall back into the updraft (rather than falling out of the cloud) and be carried back up toward the top of the cloud.  In this way the hailstone can complete several cycles through the interior of the cloud (the motions are probably more complex than shown here, the hailstone probably moves horizontally inside the cloud).


One last figure showing some of the things that can happen once a precipitation particle falls from a cloud
We'll assume the precipitation falling from the cloud is either a graupel particle or a snow flake.  Moving from left to right, the falling particles can move into warmer air and melt.  The resulting drops of water fall the rest of the way to the ground and would be called RAIN.  This is how summer rain in Tucson forms.  Note sometimes the grauple will reach the ground before melting.

Sometimes the falling raindrops will evaporate before reaching the ground.  This is called VIRGA and is pretty common early in the summer thunderstorm season in Arizona when the air is still pretty dry.  Lightning that comes from thunderstorms that aren't producing much precipitation is called "dry lightning" and often starts brush fires.

Rain will sometimes freeze before reaching the ground.  The resulting particle of clear ice is called SLEET.  FREEZING RAIN by contrast freezes only after it reaches the ground.  Everything on the ground can get coated with a thick layer of ice.  It is nearly impossible to drive during one of these "ice storms."  Sometimes the coating of ice is heavy enough that branches on trees are broken and power lines are brought down.  It sometimes takes several days for power to be restored.

Last Friday I gave the students in the MWF section of the class a chance to submit what they thought might be appropriate questions for this week's quiz.  Click here to see a sampling of what they came up with.


During the last part of the class we returned to a topic that we skipped earlier when we were covering humidity variables.



We'll try to answer and understand the 2 questions above.

We first must understand that the rate at which water evaporates depends on temperature (see p. 84 in the photocopied ClassNotes).  Hot water evaporates more rapidly than cold water.  Wet laundry hung outside on a hot day will dry much more quickly than it would on a cold day.

Before talking about water, have a look at the grade distribution below.  The average appears is 77%.  Students with grades equal to or greater than 90.0% are exempt from the final. 


If I added 5 pts to everyones grade,
Would the curve shift to the RIGHT  or the  LEFT?
Would the average grade  INCREASE,  DECREASE  or remain the  SAME?
Would the number of people that don't have to take the final  INCREASE,  DECREASE  or remain the  SAME?

 



It seemed like most everyone understood that the curve would shift to the RIGHT, the average grade would INCREASE, and the number of people getting out of the final exam would INCREASE.

The next question is very similar.  Instead of grades, the figure below shows the distribution of the kinetic energies of water molecules in a glass of water.  There's an average and some of the water molecules (the ones at the far right end of the curve) have enough kinetic energy to be able to evaporate (analogous to students that are exempt from the final exam).  You'll find this figure on p. 84 in the photocopied ClassNotes.


You can ask the same kinds of questions: If the water were heated, would the curve shift to the  RIGHT  or the  LEFT.  Would the average kinetic energy of the water molecules  INCREASE, DECREASE  or  remain the  SAME?.  Would the number of water molecules, with enough kinetic energy to be able to evaporate  INCREASE,  DECREASE,  or remain the  SAME?  The shifted curve is shown below  




The value of the average kinetic energy would increase and more molecules would lie to the right of the threshold and be able to evaporate.  Thus we conclude that hot water evaporates more rapidly than cold water.  This is shown pictorially below (the number of arrows is a measure of the rate of evaporation).



Next we'll try to understand why the rate at which water vapor condenses depends on the amount of water vapor actually in the air.

Before looking at water vapor we'll consider an analogous situation, shown below.



When the front door is first opened people will start streaming into the Walmart.  The number of customers in the store (initially zero) ill start to increase.  At some point some fraction of the people inside will start to leave.  Eventually the number inside will grow to the point that the number of people leaving balances the number entering.  The question is how many people would have to be inside the Walmart in order for the two rates to be equal?

In the rate of people entering the store were higher (20 people per minute), the number inside would increase.  If the rate were to decrease then the number of people inside would get smaller.

The "Walmart problem" is very similar to saturation of air with water vapor which is shown on p. 85 in the photocopied ClassNotes.


The evaporating water in Picture 1 is analogous to people entering a Walmart store just as the store opens in the morning.  There is initially no water vapor in the air in the covered glass but it will begin to buildup (Fig. 2).  Some fraction of the water vapor molecules will condense (even though they might have just evaporated), this is shown in Fig. 3.  The rates of evaporation and condensation aren't yet equal in Fig. 3 so the water vapor concentration will increase a little bit more until eventually the rate of condensation balances evaporation (Fig. 4).  The air is saturated at that point.  The water vapor concentration won't increase further.  Saturated air has a relative humidity (RH) of 100%. 

Cups filled with cold and warm water are shown at the bottom of the figure.  Because of different rates of evaporation (slow in cold, rapid in warm water) the water vapor concentrations at saturation will be different.

This would be like going back and redoing the Walmart problem assuming that people were entering the store at different rates:  10 people/minute, 20 people/minute, and 30 people/minute.  You'd end up with 100, 200, and 300 people in the store once the entering and exiting rates were equal.

One last thing to notice in the bottom part of the figure above.  The relative humidities in the cold and warm cups are the same (100%) even though the actual amounts of water vapor in the air are very different.  This is proof again that relative humidity really doesn't tell you how much water vapor is actually in the air.  It only tells you whether the air is full of water vapor or not.

It being Election Day, we held a vote in NATS 101.  The class voted to postpone starting some new material on Climate Change until later in the semester.