Tue., Sep. 11 notes

Tuesday, Sept. 11, 2007

The first optional homework assignment is due at the beginning of class on Thursday.

The first 1S1P assignment is due next Tuesday (Sept. 18).

The Experiment #1 reports are also due next Tuesday (Sept. 18). You should bring in your materials this week so that you can pick up the Supplementary Information sheet for Experiment #1. The materials for Experiment #2 will probably be distributed in class on Thursday next week.

Today we started some new material. This week 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 may 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).

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 will be 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. The figure above wasn't shown in class.

Here is the example we studied 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 (estimate 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 was distributed in class)

The air temperature in this example was 88^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.

Meteorologists hope to map out small horizontal pressure changes on surface weather maps (that produce wind and storms). Pressure changes much more quickly when moving in a vertical direction. The pressure measurements are all corrected to sea level altitude to remove the effects of altitude. If this were not done large differences in pressure at different cities at different altitudes would completely hide the smaller horizontal changes.

In the example above, a station pressure value of 927.3 mb was measured in Tucson. Since Tucson is about 750 meters above sea level, a 75 mb correction is added to the station pressure (1 mb for every 10 meters of altitude). The sea level pressure estimate for Tucson is 927.3 + 75 = 1002.3 mb.

To save room, the leading 9 or 10 on the sea level pressure value and the decimal point are removed before plotting the data on the map. For example the 10 and the . in 1002.3 mb would be removed; 023 would be plotted on the weather map (to the upper right of the center circle). Some additional examples are shown above.

When reading pressure values off a map you must remember to add a 9 or 10 and a decimal point. For example
138 could be either 913.8 or 1013.8 mb. You pick the value that falls between 950.0 mb and 1050.0 mb (so 1013.8 mb would be the correct value, 913.8 mb would be too low).

Another important piece of information that is included on a surface weather map is the time the observations were collected. We covered this very briefly in class. Time on a surface map is converted to a universally agreed upon time zone called Universal Time (or Greenwich Mean Time, or Zulu time). That is the time at 0 degrees longitude. There is a 7 hour time zone difference between Tucson (Mountain Standard Time year round) and Universal Time. You must add 7 hours to the time in Tucson to obtain Universal Time.

Here are some examples of conversions to and from GMT.

8 am MST:

add the 7 hour time zone correction ---> 8:00 + 7:00 = 15:00 UT (3:00 pm in Greenwich)

2 pm MST:

first convert 2 pm to the 24 hour clock format 2:00 +12:00 = 14:00 MST
then add the 7 hour time zone correction ---> 14:00 + 7:00 = 21:00 UT (9 pm in Greenwich)

18Z:

subtract the 7 hour time zone correction ---> 18:00 - 7:00 = 11:00 am MST

02Z:
if we subtract the 7 hour time zone correction we will get a negative number. We will add 24:00 to 02:00 UT then subtract 7 hours

02:00 + 24:00 = 26:00
26:00 - 7:00 = 19:00 MST on the previous day
2 hours past midnight in Greenwich is 7 pm the previous day in Tucson

Now we will put what we have learned to use and plot a bunch of weather data on a surface map:

Plotting the surface weather data on a map is just the beginning. For example you really can't tell what is causing the cloudy weather with rain and drizzle in the NE portion of the map above or the rain shower at the location along the Gulf Coast. Some additional analysis is needed. A meteorologist would usually begin by drawing some contour lines of pressure to map out the large scale pressure pattern. We will look first at contour lines of temperature, they are a little easier to understand.

Isotherms, temperature contour lines, are drawn at 10 F intervals. They do two things: (1) connect points on the map that all have the same temperature, and (2) separate regions that are warmer than a particular temperature from regions that are colder. The 40^o F isotherm highlighted in yellow above passes through one City A reporting a temperature of exactly 40^o. Mostly it goes between pairs of cities: one with a temperature warmer than 40^o and the other colder than 40^o (such as near Point B).. Temperatures generally decrease with increasing latitude.

Now the same data with isobars drawn in. Again they separate regions with pressure higher than a particular value from regions with pressures lower than that value. Isobars are generally drawn at 4 mb intervals. Isobars also connect points on the map with the same pressure. The 1008 mb isobar (highlighted in yellow) passes through City A where the pressure is exactly 1008.0 mb. Most of the time the isobar will pass between two cities. The 1008 mb isobar passes between cities with pressures of 1006.8 mb and 1009.7 mb in the vicinity of Point B. You would expect to find 1008 mb about halfway between those two cites, that is where the 1008 mb isobar goes.

The pattern on this map is very different from the pattern of isotherms. On this map the main features are the circular low and high pressure centers.

What kind of weather can you expect in the vicinity of a low pressure center?

A pressure difference will first start air moving toward low pressure (imagine a rock sitting on a hillside that starts to roll downhill). Then something called the Coriolis force will cause the wind to start to spin (we'll learn more about the Coriolis force later in the semester). Winds spin in a counterclockwise (CCW) direction around surface low pressure centers. The winds also spiral inward toward the center of the low, this is called convergence. [winds spin clockwise around low pressure centers in the southern hemisphere but still spiral inward]

The convergence causes the air to rise at the center of the low. Rising air expands and cools. If the air is sufficiently moist clouds can form and then begin to rain or snow. Thus you often see cloudy skies and stormy weather associated with surface low pressure.

We didn't have time to look at what happens in the vicinity of a circular high pressure center. Here are the notes from the other section of the class.

It is pretty much the opposite situation with surface high pressure centers. Winds spin clockwise and spiral outward. The outward motion is called divergence. Air sinks in the center of surface high pressure to replace the diverging air. The sinking air is compressed and warms. This keeps clouds from forming so clear skies are normally found with high pressure.

Finally we had a quick look ahead at another topic that we will be covering on Thursday and early next week before the quiz.

We are going to try to understand why warm air rises and cold air sinks.
It is always a good idea to have a picture in mind, a hot air balloon for example.
Hot air balloons do sometimes fall from the sky; most everyone in the classroom would understand that gravity was the force responsible for bringing down a hot air balloon.
But what causes a hot air balloon to rise? We will see that it is a pressure difference force. Pressure decreases with increasing altitude. This creates a force that points upward from high toward low pressure.

Understanding rising and sinking air is a 3-step process. The first step is learning about the ideal gas law.

When you fill a balloon with air you don't really fill it with air. That is the inside of the balloon is mostly empty space. The balloon is kept inflated by the rapid motions of the air molecules which are zipping around inside the balloon and colliding with the walls of the balloon. The outward push from each collision is very weak but the collisions are so numerous and frequent that the total effect is large.

The ideal gas law equation (that we will learn about in class on Thursday) explains how pressure depends on variables such as the volume of the balloon, the temperature and number of air molecules in the balloon.