Friday Feb. 8, 2008

The Practice Quiz has been graded.  Papers were returned in class today.  Answers to the questions have been posted online.

The first Optional Assignment and the Experiment #1 reports are due next Monday.  If you haven't returned your materials, please do so on Monday so that the graduated cylinders can be clearned in time to be checked out next Wednesday for Experiment #2.

The "weather weenie" handout was distributed in class today.  On one side you'll find the symbols used for a variety of low, middle, and high altitude clouds.  The other side has just under 100 symbols used to describe a variety of weather conditions.
We started with some practice at decoding surface weather observations plotted using the station model notation.


The value of the relative humidity (RH) is not plotted.  When the air temperature and the dew point temperature are equal, however, you can say the relative humidity is 100%.

The pressure change data (how the pressure has changed during the preceding 3 hours) is shown to the right of the center circle.  We didn't mention this in class last Monday.  You must remember to add a decimal point.  Pressure changes are usually pretty small.   Some additional examples of decoding pressure change data can be found at the end of the Monday Feb. 4 online notes.

The pressure tendency shows visually how the pressure has been changing during the past 3 hours.  You'll find many examples in Appendix C in the textbook.

The pressure data requires quite a bit of further discusssion.

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.  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:

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 (the dot symbols are rain) and drizzle (the comma symbols) 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 40o F isotherm highlighted in yellow above passes through City A which is reporting a temperature of exactly 40o.  Mostly it goes between pairs of cities: one with a temperature warmer than 40o and the other colder than 40o (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.

In class on Monday we will find that winds spin in a counterclockwise direction around and spiral in toward the center of low pressure.  This inward motion causes the air in the center to rise.  Rising air is what can cause clouds to form.  Thus you often find stormy weather associated with surface low pressure.

Winds spin clockwise and spiral outward away from high pressure.  Air from above must sink to replace the outward moving air.  Sinking air motions generally results in clear skies.