Fri., Mar. 23 notes

Friday Mar. 23, 2007

The optional assignment and the Experiment #3 reports are due next Monday.

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 portion 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.

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 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.

Clouds are one of the best ways of cleaning the atmosphere (cloud 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. Gaseous pollutants can dissolve in the water droplets and be carried to the ground by rainfall also.

Now armed with some knowledge of condensation nuclei, humidity, and scattering of light we are ready for another demonstration. We will try to make a cloud in a bottle.

We used a strong thick-walled 4 liter flask (flasks like this are designed not to implode when all of the air is pumped out of them, they aren't designed 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. 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 cooling increases the relative humidity of the moist air in the flask to 100% (probably more than 100%) 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 a second time (perhaps a third time) with one small change. A burning match was dropped into the bottle. The smoke from the match consisted of lots of very small particles that act as condensation nuclei. The cloud that formed this time was somewhat "thicker" and easier to see.

The concentration of particles in the air can have on the appearance of a cloud.

A cloud composed of a large number of small droplets is more reflective than a cloud composed of a smaller number of larger droplets. This something that interests the people studying climate change. Combustion of fossil fuels adds carbon dioxide to the atmosphere but also adds condensation nuclei. The increasing greenhouse gas concentrations are expected to warm the earth. More particles might make clouds more reflective though which could cool the earth.

We started off on a small detour in the second part of the class.
There are a variety of optical phenomena in the atmosphere (rainbows, haloes, sundogs, mirages, the green flash) that occur because of refraction.

Refraction is the bending of light that occurs when light travels from one transparent material into another.

Light entering a denser material at an angle bends toward the normal. The normal is a line drawn perpendicularly to the boundary between the two materials. This is shown at the top of the figure (point 1) as a beam of red laser light passes from air into a piece of plastic.

Light passing from a high density to a low density material bends away from the normal. You can see this is the lower part of the figure (point 2) above as the light beam travels from the plastic back into the air.

The following figures show some of the atmospheric optical phenomena that are produced by refraction.

In this figure light from a distant star is bent as it passes from space (density = 0) into the atmosphere (higher density). The person on the ground thinks the star is higher in the sky than is really the case.

A halo is a ring of light that surrounds the sun or moon. Sundogs are patches of light seen to the left and right of the setting (or rising) sun. Both are produced by bending of light that occurs as light travels from low density air into (and out of) higher density ice.

Light follows a more complicated path to produce a rainbow. The light is first bend (toward the normal at Point 1) as it passes from the air into the water drop. The ray of light is reflected off the back inside surface of the raindrop at Point 2 and is refracted again (away from the normal at Point 3) as it exits the raindrop.

Production of an inferior mirage is also a little complicated. Sunlight strikes the tree is rays of light are scattered in all directions. One ray (1) goes directly to the person. The other is initially directed downward toward the ground. The hot low density air next to the ground bends this ray of light and eventually sends it toward the person (2). The person sees two images of the tree. The real image when looking along light ray 1 and a virtual image when looking down in the direction of light ray 2.