Wed., Aug. 23 notes

Wednesday Aug. 23, 2006

No, not very much. Not much physics, math, biology, or geology either. But, stay open minded, you might find that these subjects are more interesting than you might have thought and not necessarily that difficult either.

Having said that, here is a little more chemistry.

The earth's first atmosphere was composed mainly of hydrogen and helium. These light-weight gases escaped into space and were lost. The next atmosphere was built up of gases emitted during volcanic eruptions, mostly water vapor, carbon dioxide, and nitrogen. As the earth began to cool the water vapor condensed and began to create oceans. Carbon dioxide dissolved in the oceans and was slowly turned into rock. Much of the nitrogen remained in the atmosphere.

Note the volcanoes didn't add oxygen to the atmosphere.

The oxygen is thought to have first come from photodissociation of water vapor and carbon dioxide by ultraviolet light (the high energy radiation splits the H₂0 and CO₂ into pieces). The O and OH react to form O₂ and H.

Once O₂ begins to accumulate in the air it can react with O to form ozone, O₃. The ozone then begins to absorb ultraviolet light, life forms can move from the oceans (which would absorb UV light in the absence of ozone) onto land. Eventually plants and photosynthesis would become the main source of atmospheric oxygen.

Here's another question that someone asked

Does the dew point temperature have anything to do with relative humidity? They are related in the sense that they both tell you something about moisture in the air.

In the figure above the air temperature changes from 75 F in the morning to 95 F in the afternoon. The air's temperature (as we will see when we get to Chapter 4 later in the semester) determines how much water vapor the air can potentially contain.

The dew point temperature remains constant in the figure above. The actual amount of water vapor in the air doesn't change.

The relative humidity tells you how close the air is to being "filled to capacity" with water vapor.

If the early morning temperature had been 65 F, the same as the dew point, the relative humidity would have been 100%. It would have been foggy.

The relative humidity really tells you whether a cloud or fog or dew is about to form. The RH also gives you an idea of how well your evaporative cooler will work (it cools more effectively when the RH is low). It is also hard for your body to cool by perspiring when the RH is high (see heat index on p. 86 in the textbook).

Still another question from a student that came to my office

Many people think that the term monsoon just means thunderstorm. We will learn a fair amount about thunderstorms in this class.

The term monsoon really means a seasonal change in the direction of the prevailing winds (we'll learn a little bit about what causes that too). For most of the year winds in the Arizona come from the west and are dry. For two or three months in the summer the winds pick up an easterly component and are moister. When there is sufficient moisture thunderstorms can form. In an average year Tucson gets about half of its yearly precipitation during the summer monsoon season. The website maintained by the Tucson office of the National Weather Service has a lot of additional information about the summer monsoon.

There is a tropical storm (Debby) off the east coast of the US and a strong hurricane (Ileana) off the west coast. You can learn more about these tropical systems at the National Hurricane Center webpage
This was a good time to introduce the Saffir-Simpson scale used to rate hurricane strength or severity.

With sustained winds of 120 MPH, hurricane Ileana is currently a category 3 hurricane, a major hurricane. The hurricane center expects some strengthening (perhaps to category 4) followed soon by rapid weakening. Moisture from tropical storms and hurricanes is sometimes pulled into southern Arizona. This can lead to an increase in thunderstorm activity and heavy rainfall.

We finally covered some new material, found on p. 1 in the photocopied Class Notes.

Carbon dioxide is one of several greenhouse gases (H₂O, CH₄, N₂O, CFCs are some of the others)
The natural greenhouse effect is beneficial. The average global annual surface temperature on earth without greenhouse gases would be about 0^o F. The presence of greenhouse gases raises this average to about 60^o F.

Increasing the concentrations of greenhouse gases in the atmosphere could enhance the greenhouse effect and cause global warming. This could have many detrimental effects such as melting polar ice and causing a rise in sea level and flooding of coastal areas, changes in weather patterns and changes in the frequency and severity of storms.

The evidence for increasing CO₂ concentration is shown in the two graphs below

The top "Keeling" curve shows measurements of CO₂ that were begun in 1958 on top of the Mauna Loa volcano in Hawaii. Carbon dioxide concentrations have increased from 315 ppm to about 375 ppm during this period. The small wiggles show that CO₂ concentration changes slightly during the year.

Once scientists saw this data they began to wonder about how CO₂ concentration might have been changing prior to 1958. But how could you now, in 2006, go back and measure the amount of CO₂ in the atmosphere in 1906? Scientists have found a very clever way of doing just that. It involves coring down into ice sheets that have been building up in Antarctica and Greenland for hundreds of thousands of years.

As layers of snow are piled on top of each other year after year, the snow at the bottom is compressed and eventually turns into a layer of solid ice. The ice contains small bubbles of air trapped in the snow at the time it originally fell. Scientists are able to date and then take the air out of these bubbles and measure the carbon dioxide concentration. A book, The Two-Mile TIme Machine, by Richard B. Alley discusses ice cores and climate change. This is one of the books available for checkout should you decide to write a book report instead of an experiment report.

Using the ice core measurements scientists have determined that atmospheric CO₂ concentration was fairly constant at 280 ppm between 1000 AD and the mid-1700s when it started to increase. The start of rising CO₂ coincides with the "Industrial Revolution." Combustion of fossil fuels needed to power factories adds CO₂ to the atmosphere.

The figure above lists processes that add CO₂ to and remove CO₂ from the atmosphere.
We can use this information to better understand the yearly variation in atmospheric CO₂ concentration.

Atmospheric CO₂ peaks in the late winter to early spring. Many plants die or become dormant in the winter. With less photosynthesis, more CO₂ is added to the atmosphere than can be removed. The concentration builds throughout the winter until the rate of photosynthesis increases and brings things back into balance in the spring.

Similarly in the summer the removal of CO₂ by photosynthesis exceeds release. CO₂ concentration decreases throughout the summer and reaches a minimum in late summer to early fall.