Monday Oct. 22, 2012

We had a visitor in class today from Peru so I thought some music from the Andes might be appropriate.  You heard "Tierra de Vicunas" and "Ocaso" from a group named Sukay.  Turns out they're from Bolivia not Peru and I wasn't able to find them on YouTube.

The challenging In-class Optional Assignment from Friday was collected today (you had the option of turning it in last week or today).  You'll find answers to the questions on that assignment at the end of today's notes.  There is another Optional Assignment due on Wednesday.

It's a few days early, but the Quiz #3 Study Guide is now online.  Note the change in the location of the Tuesday 4-5 pm review.

The Experiment #3 reports are due next Monday.  You should try to return the materials this week so that you can pick up a copy of the Supplementary Information handout.

We quickly reviewed the material on the formation of dew, frozen dew, and frost that we didn't have time for last Friday (you'll find that discussion at the end of the notes from last Friday).  Today we will be looking at what happens when air above the ground is cooled to and below the dew point temperature (i.e. we'll be looking at Pts. 2, 3, 4 and 5 in the figure below).

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.

And here, in a painful piece by piece kind of way, are answers to the questions on the challenging assignment handed out in class last Friday.

Here is, I think, the easiest part of the 1st question.  What will happen to the values of the mixing ratio, r, the relative humidity, RH, and the dew point temperature, Td, if you add moisture to or remove moisture from a parcel of air?

The job of the mixing ratio is to tell you how much water vapor is actually in the air.  One of the jobs of the dew point is to do the same thing.  So both of these variables will increase when you add moisture to the air and decrease when moisture is removed from the air. 

The value of the relative humidity depends on both the mixing ratio, r, and the saturation mixing ratio rs.  The saturation mixing ratio depends on air temperature (warm air can potentially hold more water vapor than colder air).  But in this part of the question temperature is remaining constant, so the value of rs won't change.  RH will change in the same way as r and Td.

In this part of the problem we warm the air but don't add or remove any moisture.  Since moisture isn't being added or removed the mixing ratio and the dew point temperature will remain constant.

The relative humidity depends on r (which stays constant) and rs (which will change because the air is warming).  The value of rs will increase as the air is warmed. Since it is in the denominator of the RH equation, the RH will decrease.

This is just the opposite situation.  We're cooling air but not adding or removing moisture (as long as you don't cool the air below it's dew point temperature).  The values of r and T d will remain the same.  The RH will increase (eventually reaching 100% when you cool the air to the dew point).  RH decreases because cooling the air decreases the saturation mixing ratio. 

Finally we look at what happens when you cool the air below the dew point temperature.  In a previous lecture we saw that this is one way of removing moisture from air.  It's like wringing moisture out of a sponge.

The RH reaches 100% when the air has cooled to the dew point.  As you cool air below Td the air's capacity for water vapor, the saturation mixing ratio, continues to decrease.  The air finds itself with more water vapor than it can hold.  The excess condenses.  The mixing ratio and the dew point temperature will decreases as the air loses water vapor.  The RH will remain at 100%, the highest it can get.

There were two problems on the other side of the page.

Td and r have the same job, telling you how much water vapor is actually in the air.  The city with the highest dew point temperature will also have the highest mixing ratio and the highest amount of water vapor in the air.

Saturation mixing ratio depends on temperature.  The city with the warmest air will have the highest rs and could potentially hold the most water vapor.

Finally the difference between Ta and Td gives you an idea of the relative humidity.  A small difference means high relative humidity and vice versa (no difference between Ta and Td means RH = 100%.).

The figure below gives the answers to the last question.