Tue., Mar. 30 notes

Here's a reminder of where we were working last Thursday. We finished class by looking at the formation of and differences between dew, frozen dew, and fog.

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.

A short video 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.

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 later in the class.

With cold and possibly wet weather being forecast, you might have a chance to see some fog 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.

Probably the most common type of fog in Tucson is radiation fog. 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). Because the fog cloud is colder than the air right above, this is a stable situation. The fog clouds "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.

Steam fog or evaporation fog (also sometimes known as mixing fog) 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.

When you "see your breath" on a cold day (the figure below wasn't shown in class)

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

Next it was time for a demonstration that puts together many of the concepts we have been covering. Cooling air and 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.

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; 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.

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.

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).

We had a little time to start the next topic: identifying and naming clouds.
The ten main cloud types are listed below (you'll find this list on p. 95 in the photocopied class notes).

You should try to learn these 10 cloud names. Not just because they might be on a quiz (they will) but because you will be able to impress your friends with your knowledge. There is a smart and a not-so-smart way of learning these names. The not-so-smart way is to just memorize them. You will inevitably get them mixed up. A better way is to recognize that all the cloud names are made up of key words. The 5 key words, we will find, mostly tell you something about the cloud altitude and appearance.

Drawing a figure like this on a blank sheet of paper is a good way to review cloud identification and classification.

Each of the clouds above has a box reserved for it in the figure.

Clouds are classified according to the altitude at which they form and the appearance of the cloud. There are two key words for altitude and two key words for appearance.

Clouds are grouped into one of three altitude categories: high, middle level, and low.

Cirrus or cirro identifies a high altitude cloud. There are three types of clouds found in the high altitude category..

Alto in a cloud name means the cloud is found at middle altitude. The arrow connecting altostratus and nimbostratus indicates that they are very similar. When an altostratus cloud begins to produce rain or snow its name is changed to nimbostratus. A nimbostratus cloud is also often somewhat thicker and lower than an altostratus cloud. Sometimes it might sneak into the low altitude category.

It is very hard to just look up in the sky and determine a cloud's altitude. You will need to look for other clues to distinquish between high and middle altitude clouds. We'll learn about some of the clues when we look at cloud pictures later in the class.

There is no key word for low altitude clouds. Low altitude clouds have bases that form 2 km or less above the ground. The summit of Mt. Lemmon in the Santa Catalina mountains north of Tucson is about 2 km above the valley floor. Low altitude clouds will have bases that form at or below the summit of Mt. Lemmon.

Clouds can have a patchy of puffy (or lumpy, wavy, or ripply) appearance. These are cumuliform clouds and will have cumulo or cumulus in their name. In an unstable atmosphere cumuliform clouds will grow vertically. Strong thunderstorms can produce dangerous severe weather.

Stratiform clouds grow horizontally and form layers. They form when the atmosphere is stable.

Cirrus clouds are sometimes considered to be a third type of cloud appearance.

The last key word, nimbo or nimbus, means precipitation. Only two of the 10 cloud types are able to produce (significant amounts of) precipitation. It's not as easy as you might think to make precipitation.

Nimbostratus clouds tend to produce fairly light precipitation over a large area. Cumulonimbus clouds produce heavy showers over localized areas. Thunderstorm clouds can also produce hail, lightning, and tornadoes. Hail would never fall from a Ns cloud.

While you are still learning the cloud names you might put the correct key words together in the wrong order (stratonimbus instead of nimbostratus or nimbocumulus instead of cumulonimbus). You won't be penalized for those kinds of errors in this class because you are putting together the right two key words.

Here's the cloud chart from earlier. We've added the three altitude categories along the vertical side of the figure and the two appearance categories along the top. By the end of the class we will add a picture to each of the boxes.

We'll look at slides and learn about some of the key characteristics of each of the 10 cloud types in class on Thursday.