Tuesday Mar. 27, 2012
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One full song "Icare" and part of another "Ibis" perhaps from the Cirque du Soleil presentation of Alegria which I went to see a year or two ago. 

The Experiment #3 reports were collected today.  It usually takes at least 1 week to grade them so you should expect to get them back sometime next week.

We're getting to the point in this semester where you should already have completed an experiment report or currently be working on an experiment.  Here's a list of people that don't seem to have done that yet.  If you're on this list you should get in touch with me right away.

The Controls of Temperature assignment was also collected today.  Answers to the questions on that assignment will appear online soon.

I handed out what I consider to be a fairly challenging Optional Assignment.  Students were free to turn it in at the end of class but could also hang onto it, work out the questions over the weekend, and turn it in at the start of class on Thursday.  You can download the assignment here if your interested in doing the same.

After class last week I went looking and found a good real world example of the rain shadow effect in Oregon.   







The figure above at left shows the topography of the state (here's the source of that map).  Winds generally blow from west to east across the state. 

Coming off the Pacific Ocean the winds first encounter a coastal range of mountains.  On the precipitation map above at right (source) you see a lot of greens and blue on the western sides of the coastal range.  These colors indicate yearly rainfall totals that range from about 50 to more than 180 inches of rain per year.  This is where temperature rain forests are found. 

That's the Willamette River, I think, in between the coastal range and the Cascades.  This valley is somewhat drier than the coast because air moving off the Pacific has lost some of its moisture moving over the coastal range. 

What moisture does remain in the air is removed as the winds move up and over the taller Cascades.  Yearly rainfall is generally less than 20 inches per year on the eastern side, the rainshadow side, of the Cascades.  That's not too much more than falls in Tucson which averages about 12 inches of rain a year.

Wikipedia considers the Tibetan Plateau one of the best examples of a rain shadow. (here's the source of the picture).  This example wasn't shown in class.


The Himalayan mountains stretch across the lower left 1/3 of the picture.  The land below and to the left of the mountains appears somewhat green in the picture.  This is because moist air moving from lower left toward the upper right leaves most of its moisture on this side of the mountain range.  The upper right 2/3rds of the picture, the Tibetan plateau, is in the rain shadow and appears very dry and brown in the photograph.

Most of the year the air that arrives in Arizona comes from the west, from the Pacific Ocean (this changes in the summer).  It usually isn't very moist by the time it reaches Arizona because it has travelled up and over the Sierra Nevada mountains in California and the Sierra Madre mountains further south in Mexico.  The air loses much of its moisture on the western slopes of those mountains. 

Next in our mix of topics was measuring humidity.  One of the ways of measuring humidity is to use a sling (swing might be more descriptive) psychrometer.




A sling psychrometer consists of two thermometers mounted side by side.  One is an ordinary thermometer, the other is covered with a wet piece of cloth.  To make a humidity measurement you swing the psychrometer around for a minute or two and then read the temperatures from the two thermometers.  The difference between the dry and wet bulb temperatures can be used to determine relative humidity and dew point (you look up RH and Td in a table, it's not something you can easily calculate). 

You know I like to beat some concepts to death.  But also it's a pretty good example of where you can take some pretty basic concepts that you understand and use them to really understand something else.


The figure shows what will happen as you start to swing the wet bulb thermometer.  Water will begin to evaporate from the wet piece of cloth.  The amount or rate of evaporation will depend on the water temperature (the 80 F value was just made up in this example).  Warm water evaporates at a higher rate than cool water.

The evaporation is shown as blue arrows because this will cool the thermometer.  The same thing would happen if you were to step out of a swimming pool on a warm dry day, you would feel cold.  Swamp coolers would work well (too well sometimes) on a day like this.

The figure at upper left also shows one arrow of condensation.  The amount or rate of condensation depends on how much water vapor is in the air surrounding the thermometer.  In this case (low relative humidity) there isn't much water vapor.  The condensation arrow is orange because the condensation will release latent heat and warm the thermometer.
Because there is more evaporation (4 arrows) than condensation (1 arrow) the wet bulb thermometer will drop. 


The wet thermometer will cool but it won't cool indefinitely.  We imagine that the wet bulb thermometer has cooled to 60 F.  Because the wet piece of cloth is cooler, there is less or slower evaporation.  The wet bulb thermometer has cooled to a temperature where the evaporation and condensation are in balance.  The thermometer won't cool any further.

You would measure a large difference (20 F) between the dry and wet bulb thermometers on a day like this when the air is relatively dry.

Here's the situation on a moister day.  There's enough moisture in the air to provide 3 arrows of condensation.  You wouldn't feel as cold if you stepped out of a pool on a warm humid day like this.  Swamp coolers wouldn't provide much cooling on a day like this.


The wet thermometer only cools a little bit before the rates of evaporation and condensation are equal.

Here's a summary


A large difference between the dry and wet bulb temperatures means the relative humidity is low.
A small difference means the RH is higher.
No difference (the bottom figure) means the relative humidity is 100%.  Any evaporation from the wet thermometer is balanced by an equal amount of condensation from the surrounding air.



Evaporative cooling will make you feel cold if you get out of a swimming pool on a warm dry day.  You won't feel as cold if the air is humid and the relative humidity is high.  This reminds me of something we covered earlier in the semester.




We learned that a 40 F day with 30 MPH winds will feel colder (because of increased transport of energy away from your body by convection) than a 40 F day with no wind.  The wind chill temperature tells you how much colder it will feel ( a thermometer would measure the same temperature on both the calm and the windy day).  If your body isn't able to keep up with the heat loss, you can get hypothermia and die.

Now something similar but new.  Your body tries to stay cool by perspiring.  You would feel hot on a dry 105 F day.  You'll feel even hotter on a 105 F day with high humidity; your sweat won't evaporate as quickly.  The heat index measures how much hotter you'd feel. The combination of heat and high humidity is a serious, potentially deadly, weather hazard because it can cause heatstroke (hyperthermia).  


A variety of things can happen when you cool air to the dew point and the relative humidity increases to 100%.  Point 1 shows that when moist air next to the ground is cooled to and below the dew point, water vapor condenses onto (or is deposited onto) the ground or objects on the ground.  This forms dew, frozen dew, and frost. 

Air above the ground can also be cooled to the dew point.  When that happens (Point 2 above) it is much easier for water vapor to condense onto something rather than just forming a small droplet of pure water.    In air above the ground water vapor condenses onto small particles in the air called condensation nuclei.  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 (Point 3) because they either scatter (haze or fog) or reflect (clouds) sunlight. 

It's going to be a busy day, we'll be learning about all of this before the end of class.

The following confusing figures are found on p. 90 in the photocopied ClassNotes.



It might be a little hard to figure out what is being illustrated here.  Point 1 is sometime in the early evening when the temperature of the air at ground level is 65.  During the course of the coming night the air will cool to 35 F.  When the air temperature reaches 40 F, the dew point, the relative humidity reaches 100% and water vapor begins to condense onto the ground.  You would find your newspaper and your car covered with dew (water) the next morning.

The next night is similar except that the nighttime minimum temperature drops below freezing.  Dew forms (condensation) and first covers everything on the ground with water.  Then the water freezes and turns to ice.  This isn't frost, rather frozen dew.  Frozen dew is often thicker and harder to scrape off your car windshield than frost. 

Now the dew point and the nighttime minimum temperature are both below freezing.  When the RH reaches 100% water vapor turns directly to ice (deposition).  This is frost.

What happens on this night?  Because the nighttime minimum temperature never reaches the dew point and the RH never reaches 100%, nothing would happen.  I've seen some textbooks refer to this as black frost but I don't like to use that term.  You have probably heard of black ice.  Black ice does sometimes form on road surfaces and is a very dangerous driving hazard.  Because it's hard to see you can hit it with your car and lose control. 

When the relative humidity in air above the ground (and away from objects on the ground) reaches 100%, water vapor will condense onto small particles called condensation nuclei.  It would be much harder for the water vapor to just condense and form small droplets of pure water (you can learn why that is so by reading the top of p. 92 in the photocopied class notes).  There are always lots of CCN (cloud condensation nuclei in the air) so this isn't an impediment to cloud formation.
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.  Salt is an example; small particles of salt mostly come from evaporating drops of ocean water.

A short homemade video (my first actually) that showed how water vapor would, over time, preferentially condense onto small grains of salt rather than small spheres of glass.  The figure below wasn't shown in class.


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 but not the glass grains (shown above at right).

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 absorb moisture which keeps this from happening and also break up lumps of salt once they start to form.  Grains of rice might also be used because they won't fall out of the holes in the salt shaker together with the salt.  You'll find this discussed in an interesting Wikipedia article about salt.

The following figure is at the bottom of p. 91 in the ClassNotes.




This figure 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 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 a few tens of miles.

The middle picture 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.  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."  Visibility now might now only be a few miles.

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.  That is part of the reason the Great London Smog of 1952 was so impressive.  Visibility was at times just a few feet!  We could see this effect in the cloud-in-a-bottle demonstration that was performed next.

Cooling air, changing relative humidity, condensation nuclei, and scattering of light are all involved in this demonstration.



We used my backup flask in class.  Normally I use use a strong, thick-walled, 4 liter vacuum flask (designed to not implode when all of the air is pumped out of them, they aren't designed to not explode when 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 100% ( probably more than 100% momentarily ) and water vapor condenses onto cloud condensation nuclei in the air.  A very faint cloud became visible at this point. 




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.  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).

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 IR radiation and also emit IR radiation.

Clouds are one of the best ways of cleaning the atmosphere


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 later this week.