Friday Oct. 19, 2007

All of the 1S1P Assignment #1 reports have now been graded. 
The revised Expt. #1 reports have also been graded.

Optional Assignment #5 (Humidity) is due at the beginning of class next Monday.
1S1P Assignment #2 reports are due next Wednesday.
Experiment #3 reports are due Monday Oct. 29.
Good sunny weather is predicted for the next several days.  Collect your data soon so that you can return the experiment materials next week and pick up the supplementary information sheet. 
The revised Expt. #2 reports are also due Mon., Oct. 29.
Today and Monday we will be discussing many of the phenomena below.  They all involve cooling air to (and/or below) the dew point temperature.  The air becomes saturated (RH=100%) and water vapor begins to condense.


It turns out that it is much easier for water vapor to condense onto something rather than just forming a small droplet of pure water   Near the ground water vapor will condense onto cold objects on the ground (the grass, automobile, and newspaper above).  In air above the ground water vapor condenses onto small particles in the air called condensation nuclei.  We'll learn a little bit about these today. 


In the first example air  starts out with a temperature of 65 F early in the evening.  It cools to 35 F during the night.  When the air reaches 40 F, the dew point, the RH reaches 100%.  As the air temperature drops below the dew point and cools to 35 F water vapor will condense onto the ground or objects on the ground (such as an automobile).  This is dew.

The dew point is the same but the nighttime minimum temperature is below freezing in the second example.  Dew will form again on this night when the air temperature reaches 40 F.  Once the air temperature drops below 32 F though the dew will freeze and form frozen dew.

In the third example both the dew point and nighttime minimum temperatures are below freezing.  When the air temperature drops below the dew point, water vapor turns directly to ice (deposition) and forms frost.  The dew point in this case is sometimes called the frost point.

The air never becomes saturated in the fourth example because the nighttime minimum temperature never cools to the dew point.  You wouldn't see anything on this night.
When air above the ground reaches 100% relative humidity it is much easier for water vapor to condense onto small particles in the air called condensation nuclei than to just form a small droplet of water.  There are hundreds even thousands of these small particles in every cubic centimeter of air.  We can't see them because they are so small.

You can learn why it is so hard to form small droplets of pure water by reading the top of p. 92 in the photocopied class notes.

Water vapor will condense onto certain kinds of condensation nuclei even when the relative humidity is below 100% (again you will find some explanation of this on the bottom of p. 92).  These are called hygroscopic nuclei.

A short video showed how water vapor would, over time, preferentially condense onto small grains of salt rather than small spheres of glass.

The start of the video at left showed the small grains of salt were placed on a platform in a petri dish containing water.  Some small spheres of glass were placed in the same dish.  After about 1 hour small drops of water had formed around each of the grains of salt (shown above at right).  The figure above wasn't shown in class.

In humid parts of the US, water will condense onto the grains of salt in a salt shaker causing them to stick together.  Grains of rice apparently will keep this from happening and allow the salt to flow freely out of the shaker when needed.
Even though condensation nuclei and cloud droplets are too small to be seen, we can tell when they are present because they scatter light.  The demonstration described below was done in class to make clearer what scattering really is.




In the first part of the demonstration, a thin beam of bright red laser light was shined across the front of the classroom.  No one in the class could see this beam of light.  To see the beam you would need to stand over where the beam struck the wall and look back toward the laser.  The laser light is very intense and could damage your eyes, so this wouldn't be a very good thing to do.

Students in the class could see a red spot on the wall because the light hitting the wall was scattered or splattered and sent off in a multitude of directions.  A individual ray of laser light was sent to everyone in the class (and because the intense light is split up into so many rays, the individual rays are weaker and safe to look at).


Next we clapped a couple of chalkboard erasers together.  When particles of chalk dust fell into the laser beam they intercepted some of the laser light and scattered it.  Again everyone in the room got their own personal ray of light coming from each of the particles of chalk.  We use chalk because it is white, it scatters rather than absorbs light.  What would you have seen if black particles of soot had been dropped into the laser beam?


In the 3rd part of the demonstration we made a cloud by pouring some liquid nitrogen into a cup of water.  The numerous little water droplets made very good scatterers.  So much light was scattered that the spot on the wall fluctuated in intensity (the spot dimmed when lots of light was being scattered, and brightened when not as much light was scattered).

The blue color of the sky is caused by the scattering of light by air molecules.  The air molecules preferentially scatter the shorter wavelengths in sunlight.  We should at some point later in the semester have time to discuss the blue color of the sky as well as phenomena such as haloes and rainbows which involve the refraction (bending) of light.

This figure (redrawn after class for improved clarity) shows 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 light 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."

The middle picture shows what happens when you drive from the dry southwestern part of the US into the humid southeastern US.  One of the first things you would notice is the hazier appearance of the air and a decrease in visibility.  Because the relative humidity is high, 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."

Finally when the relative humidity increases to 100% fog forms.  Fog can cause a severe drop in the visibility.  The thickest fog forms in dirty air that contains lots of condensation nuclei.  We will see this effect in the cloud-in-a-bottle demonstration planned for class next Monday.