Tue., Feb. 5 notes

"Super Tuesday" Feb. 5, 2008

A busy week ahead. The first Optional Assignment of the semester was handed out. It is due in class next Tuesday (Feb. 12). You should complete the assignment before coming to class.

This coming Thursday is the first of the 1S1P Assignment #1 due dates. If you plan on doing 3 reports, at least one of them must be turned in Thu. this week. Thursday this week is also the date of the Practice Quiz.

The Experiment #1 reports are due one week from today. You should be finishing up the data collection so that you can return the materials this week and pick up the Supplementary Information sheet.

A couple of loose ends to finish up today before we move into some new material at the end of Chapter 1 (check the updated reading assignment page)

We used a stack of bricks to help understand air pressure and why it decreases with increasing altitude. Now we will imagine a stack of matresses to understand why air density decreases with increasing altitude.

Mattresses are compressible. The mattress at the bottom of the pile is compressed the most by the weight of all the mattresses above. The mattresses higher up aren't squished as much because there is less weight remaining above.

In the case of the atmosphere you can think of layers of air instead of mattresses.

1. You can first notice and remember that pressure decreases with increasing altitude.

Perhaps the subtlest point is that each air layer contains the same amount (mass) of air. You can tell because the pressure decrease as you move upward through each layer is the same (100 mb).

2. The densest air is found in the bottom layer because the air is squeezed into a thinner layer (into a smaller volume) than the other layers. This is new information - air density decreases with increasing altitude.

3. You again notice something that we covered earlier: the most rapid rate of pressure decrease with increasing altitude is in the densest air in the bottom air layer.

We spent a few minutes at the beginning of the period looking again at the upward pressure force demonstration that was conducted in class last Thursday. You'll find this discussion at the end of the Thurs., Jan. 31 online notes.

Next we looked at how air temperature changes with altitude.

The atmosphere can be split into layers depending on whether temperature is increasing or decreasing with increasing altitude. The two lowest layers are shown in the figure above. There are additional layers (the mesosphere and the thermosphere) above 50 km but we won't worry about them.

1. We live in the troposphere. The troposphere is found, on average, between 0 and about 10 km altitude, and is where temperature usually decreases with increasing altitude. [the troposphere is usually a little higher in the tropics and lower at polar latitudes]

The troposphere contains most of the water vapor in the atmosphere (the water vapor comes from evaporation of ocean water) and is where most of the clouds and weather occurs. The troposphere can be stable or unstable (tropo means to turn over and refers to the fact that air can move up and down).

2a. The thunderstorm shown in the figure indicates unstable conditions, meaning that strong up and down air motions are occurring. When the thunderstorm reaches the top of the troposphere, it runs into the stable stratosphere. The air can't continue to rise into the stable stratosphere so the cloud flattens out and forms an anvil (anvil is the name given to the flat top of the thunderstorm).   The flat anvil top is something that you can go outside and see and often marks the top of the troposphere. Not indicated on the picture is the tropopause, the boundary between the troposphere and the stratosphere.

2b. The summit of Mt. Everest is a little over 29,000 ft. tall and is close to the top of the troposphere.

2c.   Cruising altitude in a passenger jet is usually between 30,000 and 40,000, near or just above the top of the troposphere.

3. Temperature remains constant between 10 and 20 km and then increases with increasing altitude between 20 and 50 km. These two sections form the stratosphere. The stratosphere is a very stable air layer. Increasing temperature with increasing altitude is called an inversion. This is what makes the stratosphere so stable.

4.   A kilometer is one thousand meters. Since 1 meter is about 3 feet, 10 km is about 30,000 feet. There are 5280 feet in a mile so this is about 6 miles.

5.   Sunlight is a mixture of ultraviolet, visible, and infrared light. We can see the visible light.

5a. Much of the sunlight arriving at the top of the atmosphere passes through the atmosphere and is absorbed at the ground. This warms the ground. The air in contact with the ground is warmer than air just above. As you get further and further from the warm ground, the air is colder and colder. This explains why air temperature decreases with increasing altitude.

5b. How do you explain increasing temperature with increasing altitude in the stratosphere?

     The ozone layer is found in the stratosphere (peak concentrations are found near 25 km altitude). Absorption of ultraviolet light by ozone warms the air in the stratosphere and explains why the air can warm. The air in the stratosphere is much less dense (thinner) than in the troposphere. It doesn't take as much energy to warm this thin air as it would to warm denser air closer to the ground.

6. That's a manned balloon; Auguste Piccard and Paul Kipfer are inside. They were to first men to travel into the stratosphere (see pps 31 & 32 in the photocopied Class Notes). We'll see a short video showing part of their adventure before the Practice Quiz on Wednesday.

Now we ready to move into the last part of Chapter 1. This week and next we'll learn how weather data are entered onto surface weather maps and learn about some of the analyses of the data that are done and what they can tell you about the weather. We will also have a brief look at upper level weather maps.

Much of our weather is produced by relatively large (synoptic scale) weather systems. To be able to identify and characterize these weather systems you must first collect weather data (temperature, pressure, wind direction and speed, dew point, cloud cover, etc) from stations across the country and plot the data on a map. The large amount of data requires that the information be plotted in a clear and compact way. The station model notation is what meterologists use (you'll find the station model notation discussed in Appendix C in the textbook).

The figure above wasn't shown in class.
A small circle is plotted on the map at the location where the weather measurements were made. The circle can be filled in to indicate the amount of cloud cover. Positions are reserved above and below the center circle for special symbols that represent different types of high, middle, and low altitude clouds (a handout with many of these symbols was distributed in class). The air temperature and dew point temperature are entered to the upper left and lower left of the circle respectively. A symbol indicating the current weather (if any) is plotted to the left of the circle in between the temperature and the dew point (weather symbols were included on the class handout). The pressure is plotted to the upper right of the circle and the pressure change (that has occurred in the past 3 hours) is plotted to the right of the circle.

Here's the actual page we worked on in class.

This might be a little hard to unscramble so we will look at the picture one small portion at a time.

The center circle is filled in to indicate the portion of the sky covered with clouds (estimated to the nearest 1/8th of the sky) using the code at the top of the figure. Then symbols (not drawn in class) are used to identify the actual types of high, middle, and low altitude clouds (the symbols are on a handout that will be distributed in class on Friday).

The air temperature in this example was 98^o F (this is plotted above and to the left of the center circle). The dew point temperature was 59^o F and is plotted below and to the left of the center circle. The box at lower left reminds you that dew points are in the 30s and 40s during much of the year in Tucson. Dew points rise into the upper 50s and 60s during the summer thunderstorm season (dew points are in the 70s in many parts of the country in the summer). Dew points are in the 20s, 10s, and may even drop below 0 during dry periods in Tucson.

A straight line extending out from the center circle shows the wind direction. Meteorologists always give the direction the wind is coming from. In this example the winds are blowing from the SE toward the NW at a speed of 25 knots. A meteorologist would call these southeasterly winds. Small barbs at the end of the straight line give the wind speed in knots. Each long barb is worth 10 knots, the short barb is 5 knots. Knots are nautical miles per hour. One nautical mile per hour is 1.15 statute miles per hour. We won't worry about the distinction in this class, you can just pretend that one knot is the same as one mile per hour.

Here are some additional wind examples that weren't shown in class:

In (a) the winds are from the NE at 5 knots, in (b) from the SW at 15 knots, in (c) from the NW at 20 knots, and in (d) the winds are from the NE at 1 to 2 knots.

A symbol representing the weather that is currently occurring is plotted to the left of the center circle. Some of the common weather symbols are shown. There are about 100 different weather symbols (on the class handout) that you can choose from.

The sea level pressure is shown above and to the right of the center circle. Decoding this data is a little "trickier" because some information is missing. Decoding the pressure is explained below and on p. 37 in the photocopied notes.

Pressure change data (how the pressure has changed during the preceding 3 hours) is shown to the right of the center circle. You must remember to add a decimal point. Pressure changes are usually pretty small.

Here are some links to surface weather maps with data plotted using the station model notation: UA Atmos. Sci. Dept. Wx page, National Weather Service Hydrometeorological Prediction Center, American Meteorological Society.