Tuesday Sep. 23, 2014

A couple of songs ("The Soundmaker" and "Santo Domingo") from Rodrigo y Gabriela in a live KEXP performance.

The Experiment #1 reports were collected today.  It will take some time to get all these graded.  The hope is to have them graded in time to return in class on Thursday Oct. 2. 

Now that most of the Expt. #1 materials have come in the plan is to distribute materials for Experiment #2 before the quiz on Thursday.  It's first come first served, so come early.

An Optional Assignment was collected today.  Here are answers to the questions since you won't get this assignment back before the quiz on Thursday.

Class started with a brief explanation of the Galileo thermometer.  You'll find all of this stuck onto the end of the Thu., Sep. 18 online notes.



We're starting a new topic today - weather maps and some of what you can learn from them.

We began by learning how weather data are entered onto surface weather maps.

Much of our weather is produced by relatively large (synoptic scale) weather systems - systems that might cover several states or a significant fraction of the continental US.  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 meteorologists use.




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.  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; you can choose from close to 100 different weather symbols (on a handout distributed in class).  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. 

We worked through this material one step at a time (refer to p. 36 in the photocopied ClassNotes).  The figures below were borrowed from a previous semester or were redrawn and may differ somewhat from what was drawn in class.



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 (which you can quickly figure out).  5/8ths of the sky is covered with clouds in the lower example above.

In addition to the amount of cloud coverage, the actual types of clouds present (if any) can be important.  Cloud types can tell you something about the state of the atmosphere.  We'll learn to identify and name clouds later in the semester and will just say that clouds are classified according to altitude and appearance.


Positions are reserved above and below the station model center circle for high, middle, and low altitude cloud symbols.  Six cloud types and their symbols are shown above.   Purple represents high altitude in this picture.  Clouds found at high altitude are composed entirely of ice crystals.  Low altitude clouds are green in the figure.  They're warmer than freezing are composed of just water droplets.  The middle altitude clouds in blue are surprising.  They're composed of both ice crystals and water droplets that have been cooled below freezing but haven't frozen.

There are many more cloud symbols than shown here (click here for a more complete list of symbols together with photographs of the different cloud types)

We'll consider winds next.



A straight line extending out from the center circle shows the wind direction.  Meteorologists always give the direction the wind is coming from.  In the example above the winds (the finely drawn arrows) are blowing from the NW toward the SE at a speed of 5 knots.  A meteorologist would call these northwesterly 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.  The wind speed in this case 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, we will just consider one knot to be the same as one mile per hour.   It's fine with me in an example like this if you say the winds are blowing toward the SE as long as you include the word toward.



Winds blowing from the east at 20 knots.




A few more examples of wind directions (provided the wind is blowing) and wind speeds.  Note how 50 knots winds are indicated.




Here are four more examples to practice with.  Determine the wind direction and wind speed in each case.  Click here for the answers.

The air temperature and dew point temperature are found to the upper left and lower left of the center circle, respectively.



Dew point gives you an idea of the amount of moisture (water vapor) in the air.  The table below reminds you that dew points range from the mid 20s to the mid 40s during much of the year in Tucson.  The air is supposed to dry out next week.  You'll probably notice a change if dew points drop into the 30s or 40s. 

Dew points rise into the upper 50s and 60s during the summer thunderstorm season (the dew point reached 70 F last week as the remnants of Hurricane Odile were passing through, that's about as moist as it ever gets in Tucson.  Dew points are in the 10s, and may even drop below 0 during dry periods in Tucson.



Dew Point Temperatures (F)
70s
 common in many parts of the US in the summer
50s & 60s
 summer T-storm season in Arizona (summer monsoon)
20s, 30s, 40s
 most of the year in Arizona
10s or below
 very dry conditions

And maybe the most interesting part.



A symbol representing the weather that is currently occurring is plotted to the left of the center circle (in between the temperature and the dew point).  Some of the common weather symbols are shown.  There are about 100 different weather symbols that you can choose from.  There's no way I could expect you to remember all of those weather symbols.

The pressure data is usually the most confusing and most difficult data to decode.




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.  We'll look at this in more detail momentarily.

Pressure change data (how the pressure has changed during the preceding 3 hours) is shown to the right of the center circle.  Don't worry much about this now, but it may come up in a week or two.

The figures below show the pressure tendency, they are a record of how pressure has been changing during the past 3 hours.

 


Again this is something we might use when trying to locate warm and cold fronts on a surface weather map.  Don't worry too much about it now.


Here's what you need to know about the pressure data.


Meteorologists hope to map out small horizontal pressure changes on surface weather maps.  It is these small pressure differences 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 important but 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.  This sea level pressure estimate is the number that gets plotted on the surface weather map. 

Do you need to remember all the details above and be able to calculate the exact correction needed?  No.  You should remember that a correction for altitude is needed.  And the correction needs to be added to the station pressure.  I.e. the sea-level pressure is higher than the station pressure.

The calculation above is illustrated below.





To save room, the full 1002.3 mb value wouldn't be plotted on a surface map.  Here are some examples of coding and decoding the pressure data. 

First of all we'll take some sea level pressure values and show what needs to be done before the data is plotted on the surface weather
map.  Here are more examples than  we did in class.
Sea level pressures generally fall between 950 mb and 1050 mb.  The values always start with a 9 or a 10.  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 decimal pt 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.

Here are 3 more examples for you to try (you'll find the answers at the end of today's notes):

1035.6 mb
990.1 mb
1000 mb

You'll mostly have to go the other direction.  I.e. read the 3 digits of pressure data off a map and figure out what the sea level pressure actually was.  This is illustrated below. 

When reading pressure values off a map you must remember to add a 9 or 10 and a decimal point.  For example
118 could be either 911.8 or 1011.8 mb. You pick the value that falls closest to 1000 mb average sea level pressure. (so 1011.8 mb would be the correct value, 911.8 mb would be too low). 

Here are a few more examples to try (answers are at the end of today's notes)

422
800
990


Another important piece of information on a surface 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, the Prime Meridian. There is a 7 hour time zone difference between Tucson and Universal Time (this never changes because Tucson stays on Mountain Standard Time year round).  You must add 7 hours to the time in Tucson to obtain Universal Time.


Here are several examples of conversions between MST and UT that weren't done in class

to convert from MST (Mountain Standard Time) to UT (Universal Time)
10:20 am MST:
add the 7 hour time zone correction --->   10:20 + 7:00 = 17:20 UT (5:20 pm in Greenwich)

2:45 pm MST :
first convert to the 24 hour clock by adding 12 hours   2:45 pm MST + 12:00 = 14:45 MST
add the 7 hour time zone correction --->  14:45 + 7:00 = 21:45 UT (7:45 pm in England)

7:45 pm MST:
convert to the 24 hour clock by adding 12 hours   7:45 pm MST + 12:00 = 19:45 MST
add the 7 hour time zone correction ---> 19:45 + 7:00 = 26:45 UT
since this is greater than 24:00 (past midnight) we'll subtract 24 hours   26:45 UT - 24:00 = 02:45 am the next day

to convert from UT to MST
16Z:
subtract the 7 hour time zone correction ---> 16:00 - 7:00 = 9:00 am MST
         this is the example we worked in class
02Z:
if we subtract the 7 hour time zone correction we will get a negative number. 
So we will first 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


Finally I had several videos that I wasn't able to show in class last week.


A very short segment of a second video was shown mainly for the picture showing what would happen to a polystyrene head if it were exposed to 5000 psi (the pressure at a depth of 10,000 feet).  Auguste Piccard and his son Jacques traveled down to about 10,000 feet depth in the ocean in a trial run of the Bathyscaph Trieste.  Jacques would later travel with Lt. Don Walsh of the US Navy to the bottom of the Mariana Trench (35,800 feet deep).
Here's a National Geographic video describing film director James Cameron's much more recent solo dive to the Challenger Deep in the Mariana Trench on Mar. 12, 2012 (2:16).  (note mention of the 16,000 psi pressure on the submersible at the bottom of the ocean)

Bertrand Piccard, Jacques' son (Auguste's grandson) was part of the first two man team to circle the globe non-stop in the Breitling Orbiter 3 balloon (Mar. 20, 1999).  Brian Jones was the second team member (source of the left image above, source of the right image).  I showed a pretty good video summary of their trip (6:00) that I wasn't able to find online.  Here are three alternate videos of the event: short summary (1:40), longer summary (6:15 with music only, no commentary) and a full documentary (54:06).  




In the tape shown in class, another team, Andy Elsen and Colin Prescot in the Cable and Wireless Balloon,  launched Feb. 17, 1999 from a location in Spain and got out to a 10 day lead.  Bertrand Piccard and Brian Jones launched Mar. 1, 1999 from Switzerland.  The Cable and Wireless balloon was forced down in the sea off the coast of Japan.  The balloon "iced up."  It became coated with ice and was so heavy that it couldn't be flown.  Both teams were forced to fly around China because the Chinese government wouldn't allow them into Chinese airspace.

Answers to questions about coding and decoding surface weather map pressure data embedded in today's notes:
Coding pressures (you must remove the leading 9 or 10 and the decimal point.

1035.6 mb ---> 356
990.1 mb ---> 901
1000 mb = 1000.0 mb ---> 000


Decoding pressures (you must add a 9 or a 10 and a decimal point) and pick the value closest to 1000 mb.

422 ---> 942.2 mb or 1042.2 mb ---> 1042.2 mb
800 ---> 980.0 mb or 1080.0 mb ---> 980.0 mb
990 ---> 999.0 mb or 1099.0 mb ---> 999.0 mb