Monday, Feb. 4, 2019

First Aid Kit "To a Poet" (5:43), "Crazy on You" (4:55), "On the Road Again" (2:27), "When I Grow Up" (3:36),
   "Fireworks" (4:19), "Winter is All Over You" (4:49)

 We'll be using page 37a, page 37b, page 38a, and page 38b from the ClassNotes today.


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

Surface weather maps

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

Much of our weather is produced by relatively large scale (synoptic scale) weather systems - systems that might cover several states or a significant fraction of the continental US.  To be able to identify and locate 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.

 Station model notation


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; there are close to 100 different weather symbols that you can choose from.  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 an example of a surface map from the Dept. of Hydrology and Atmospheric Science web page.  This is the 2 pm map from last Thursday (Jan. 31), I'll try to show a current map in class.  Maps like this are available here.  The Arizona portion of the map is shown below.  The data for Tucson are circled and blown up in the lower right part of the figure.



In Tucson at 2 pm MST last Thursday the temperature was 70 F and the dew point temperature was 20 F (perhaps 28 F, it's a little hard to read).  The winds were from the SSE at 5 knots.  Clear skies were being reported.  The pressure (corrected to sea level altitude) was 1012.5 mb (this is derived from the 125 value to the upper right of the circle).

We'll work through this material one step at a time (refer to page 37a in the ClassNotes). 
Meteorologists determine how much of the sky is covered with clouds and try to identify the particular types of clouds that are present.

Cloud cover and cloud type

The center circle is filled in to indicate the portion of the sky covered with clouds (to the nearest 1/8th of the sky) using the code at the top of the figure (which I think you can mostly figure out).  5/8ths of the sky is covered with clouds in the example shown.

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 (thunderstorms indicate unstable conditions, for example).  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 center circle for high, middle, and low altitude cloud symbols.  Six cloud types and their symbols are sketched 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 and 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.  You can click on any of the cloud images to get a larger picture and additional examples of each cloud type)

Air temperature and dew point temperature
The air temperature and dew point temperature are found to the upper left and lower left of the center circle, respectively.  These are probably these easiest data to read.


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.  Dew points rise into the upper 50s and 60s during the summer thunderstorm season and the dew point was still pretty high this morning.  The summer thunderstorm should be coming to an end in the next week or so and we should notice the drop in humidity when that occurs.  



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


Wind direction and wind speed
We'll consider winds next.  Wind direction and wind speed are plotted (page 37b in the ClassNotes)





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.  If there's just a short barb it's positioned in from the end of the longer line (so that it wouldn't be mistaken for a 10 knot barb).

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 calm winds are indicated (the winds were calm in Tucson at class time this morning).  Note also how 50 knot 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.

Weather (that may be occurring when the observations were made)
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 (the link was corrected after class) that you can choose from. 

Pressure
The pressure data is usually the most confusing and most difficult data to decode (page 38a in the ClassNotes)





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.  That is done to save room on the surface map.   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 next week.

The figures below show the pressure tendency, the symbol following the pressure change value.  This is a visual 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.

Sea level pressure
Before being plotted on a surface map, pressure data must be corrected for altitude.


Some typical rates of pressure change are shown below



Meteorologists hope to map out small horizontal pressure differences on a surface map.  It is the small horizontal differences in pressure that cause the wind to blow and create storms.  If corrections for altitude were not made, the large vertical changes in pressure caused by altitude would dominate and would completely hide the horizontal pressure variations.

Here's an example of what would be done with a station pressure measurement made in Tucson.


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.  And actually there is one additional complication:


To save space only a portion of the full sea level pressure value is plotted on the map.  When reading a weather map you need to remember to replace the missing 9 or 10 and the decimal point.

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.
Coding and decoding pressure

Here are some examples of coding and decoding the pressure data (page 38b in the ClassNotes) 

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 1011.6 mb would be removed; 116 would be plotted on the weather map (to the upper right of the center circle).  Some additional examples are shown below.



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 131 could be either 913.1 or 1013.1 mb. You pick the value that falls closest to 1000 mb average sea level pressure. (so 1013.1 mb would be the correct value, 913.1 mb would be too low).  A couple more examples are shown below.



Here are a few more examples to try on your own (answers are at the end of today's notes): 422, 700, 990.  Caution: It is values like 990 where you are likely to make a mistake.  The 990 value looks reasonable, 990 mb.  But you do still have to add a leading 9 or 10. 

Time
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 (these may differ from the examples worked 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
then 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
15Z:
subtract the 7 hour time zone correction ---> 15:00 - 7:00 = 8:00 am MST 
        
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

Answers to the 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
700 ---> 970.0 mb or 1070.0 mb ---> 970.0 mb
990 ---> 999.0 mb or 1099.0 mb ---> 999.0 mb