Friday Oct. 22, 2010
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Music and video before class today featuring Andrea Bocelli and Elisa ("La Voce del Silencio" and "Dancing")

The Experiment #2 reports have been graded and were returned in class today.  You can revise your report if you want to; the revised reports are due on or before Friday, Nov. 5.  Please return your original report with the revised report.

Experiment #4 materials were distributed today.  You can pick up your materials on Monday if you didn't today. 

Here's Topic #1, the first of 10 that we covered today.


At bottom left in the figure above a 40 F day with 30 MPH winds will feel colder (because of increased transport of energy 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.

Evaporative cooling (upper right) will make you  feel cold if you get out of a swimming pool on an 80 F day with dry air.  You won't feel as cold if the air is humid.  Sling psychrometers make use of this to measure relative humidity and dew point.

Your body tries to stay cool by perspiring.  You would still feel hot on a hot dry day.  The heat index measures how much hotter you'd feel on a hot humid day.  The combination of heat and high humidity is a serious weather hazard because it can cause heatstroke (hyperthermia).   Your sweat won't evaporate as quickly on a humid day and your body might not be able to keep itself cool. 

Topic #2 Cloud condensation nuclei can be found at the top of p. 91 in the photocopied classnotes.
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).
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 come from evaporating drops of ocean water.

Topic #3 was a short (homemade) video 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 allows the salt to flow freely out of the shaker when needed.

The following figure (at the bottom of p. 91) was Topic #4.


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."

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 coming up at the end of class.

Fog is a relatively rare event in Tucson.  To produce fog you first need to increase the relative humidity (RH) to 100%


You can do this either by cooling the air (radiation fog) or adding moisture to and saturating the air (evaporation or steam fog).  Both will increase the ratio in the RH formula above.

Topic #5 explains how radiation fog forms.  Radiation fog is probably the most common type of fog in Tucson.  The ground cools during the night by emitting IR radiation (left figure below).  The ground cools most rapidly and gets coldest when the skies are free of clouds and the air is dry (except for a thin layer next to the ground.
 
Air in contact with the ground cools and radiation fog can form (right figure above).  The fog cloud is cold dense air and "hugs" the ground.

Radiation fog is sometimes called valley fog.


The cold dense foggy air will move downhill and fill low lying areas.   Because the fog reflects sunlight, it is often difficult for the sun to warm the air and dissipate thick clouds of valley fog.

Topic #6 dealt with steam fog or evaporation fog (also sometimes known as mixing fog).  This is commonly observed on cold mornings over the relatively warm water in a swimming pool.



Water evaporating from the pool saturates the cold air above.  Because the fog cloud is warmer than the cold surrounding air, the fog clouds float upward.

It's the same idea when you "see your breath" on a cold day


Warm moist air from your mouth mixes with the colder air outside.  The mixture is saturated and a fog cloud forms.

The following two figures weren't shown in class. 
You might remember the following two reactions from earlier in the semester when we were talking about photosynthesis and combustion

Combustion sometimes adds enough water vapor to the air to saturate the air.  Clouds form in that case.  Here are a couple of examples

There is enough water vapor in automobile exhause to saturate the air and form a cloud (of course the cloud coming from the exhaust pipe is burning oil or something like that).  Exhaust from a house furnace or hot water heat contains water vapor.  When the relative humidity is high you'll frequently see a cloud coming from one of the vents pipes on the house roof.  People will sometimes mistake this for smoke and will call the fire department.
Topic #7 was a demonstration, the cloud in a bottle demonstration.  Cooling air, changing relative humidity, condensation nuclei, and scattering of light are all involved in this demonstration.


We used a strong, thick-walled, 4 liter flask (vaccum flasks like this are 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 faint cloud became visible at this point.  The cloud droplets are too small to be seen with the human eye.  You can see the cloud because the water droplets scatter light.




The demonstration was repeated an additional time with one small change.  A burning match was dropped into the bottle.  The smoke from the match added lots of very small particles, condensation nuclei, to the air in the flask.  The cloud that formed this time was quite a bit "thicker" and much easier to see.

Topic #8 Clouds are one of the best ways of cleaning the atmosphere

Cloud droplets (water droplets) form on particles, the droplets "clump" together to form a raindrop, and the raindrop carries the particles to the ground).  A 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.

Topic #9 consisted of two small plastic petri dishes that were passed around class.  One contained granulated sugar, the other powdered sugar.  You were supposed to look at them and determine which one look whitest.  I'll see if I can't take a photograph of them and insert the picture into the class notes.  This difference in appearance is related to the next and last topic of the day.

Topic #10 Clouds, condensation nuclei, and global warming


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).   Just like in the cloud-in-a-bottle demonstration, the cloud that was created when the air was full of smoke particles was much more visible than the cloud made with cleaner air.  The petri dish filled with powdered sugar was more whiter (more reflective) than the one filled with granulated sugar.

This is has implications for climate change.  Combustion of fossil fuels adds carbon dioxide to the atmosphere.  There is concern that increasing carbon dioxide concentrations 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 too that clouds are good absorbers of IR radiation).