Thursday Sep. 19, 2019

Sia: "California Dreamin' " (3:37), "Chandelier" (4:14), "Chandelier" (4:05), "Midnight Decisions" (3:44), "Breathe Me" (4:55),  "The Girl You Lost to Cocaine" (3:58)

 We'll be using page 38a, page 38b, page 39a, page 39b, page 39c, page 40a, page 40b, page 40c, page 40d,
(these are mostly pictures, we'll add some written details in class)

The Troposphere and Stratosphere Optional Assignment has been graded and was returned today.  If you don't see a grade marked at the top of your paper you earned full credit on the assignment (0.35 extra credit pts).  Here are answers to the questions on the assignment.

An In-class Optional Assignment was handed out at the start of class today and was collected at the end of class.  If you weren't in class and would like to do the assignment you can download the assignment, answer the questions, and turn in your work at the start of class next Tuesday.

The Experiment #1 reports are due next Tuesday.  If you haven't returned your materials yet you can come by my office in Harsbarger 220 today, Friday, or next Monday.  You'll find a box outside my office door where you can leave the materials.  A copy of the Supplementary Information handout will be nearby.  It is important that you return the materials by next Tuesday at the latest because we need the glass cylinders for Expt. #2.  I am planning on checking out the Expt. #2 materials next Thursday before the quiz.

The Quiz #1 Study Guide is now available.  Quiz #1 is Thursday next week (Sep. 26).
There are a couple of tropical storms, Lorena and Mario, located off south of Baja California  that may affect our weather this weekend and early next week.



Both are storms are expected to move north north west in a track that parallels the west coast of Mexico.  (the image above and the two images below come from the National Hurricane Center web page  www.nhc.noaa.gov)


The predicted path of tropical storm Lorena.  Lorena is expected to intensify to a hurricane (H) then weaken back to tropical storm strength (S) in the figure above.  (D) in the figure indicates a tropical depression which is a step below a tropical storm.
 Tropical storm Mario is expected to follow a similar path though a little further offshore.

We should expect to see an increase in moisture and clouds as these storms move northward.  It is possible that we may also get some rain early next week. 


The current weather forecast is shown below (the images above and below are from the Tucson office of the National Weather Service https://www.weather.gov/twc/.  The increase in rain chances for early next week are due to the influx of moisture from Lorena.


Here are some weather observations from earlier this morning.


The dew point temperature and the air temperature (48 F and 49 F, respectively) being observed in Portland were nearly equal.  The relative humidity must have been near 100%.

I was surprised to see such a difference in the temperatures and the dew point values being reported in Flagstaff and Tucson.  The air in Flagstaff was quite a bit colder (40 F vs 70 F) and also drier (dew points of 35 F and 52 F) than in Tucson. 

We haven't learned what the numbers highlighted in yellow are.  That is pressure information.  That is what we will be starting with today.

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  (9:45 pm in Greenwich)

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 UT the next day

to convert from UT (Z) 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 --->   02:00 Z + 24:00 = 26:00 Z
next we will subtract the 7 hour time zone correction ---> 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

Surface weather map analyses
A bunch of weather data has been plotted (using the station model notation) on the surface weather map in the figure below (page 39a in the ClassNotes).  A couple of stormy regions have been circled in green.



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 along the Gulf Coast.  Some additional analysis is needed. 

1st step in surface map analysis: draw in some contour lines to reveal the large scale pressure pattern
Pressure contours =  isobars
( note the word bar is in millibar, barometer, and now isobar ,
they all have something to do with pressure)

Temperature contours = isotherms

A meteorologist would usually begin by drawing some contour lines of pressure (isobars) to map out the large scale pressure pattern.  We will look first at contour lines of temperature, they are a little easier to understand (the plotted data is easier to decode and temperature varies across the country in a more predictable way). 

Isotherms



Isotherms, temperature contour lines, are usually drawn at 10o F intervals. They do two things:

isotherms (1) connect points on the map with the same temperature
                      (2) separate regions warmer than a particular temperature
                      from regions colder than a particular temperature

The 40o F isotherm above passes through a city which is reporting a temperature of exactly 40o (Point A).  Mostly it goes between pairs of cities: one with a temperature warmer than 40o (41o at Point B) and the other colder than 40o (38o F at Point C).  The temperature pattern is also somewhat more predictable than the pressure pattern: temperatures generally decrease with increasing latitude: warmest temperatures are usually in the south, colder temperatures in the north.

Here's another example starting with just a bunch of temperature numbers (you'll find this figure on the in-class Optional Assignment)


Our "job" is to try to make some sense of this data.  To do that we'll draw in 2 or 3 isotherms (40 F, 50 F isotherms and maybe a small segment of a 60 F isotherm).  Colors can help you do this.



There is one temperature below 40 it has been colored blue, temperatures between 40 and 50 are green and temperatures in the 50s are colored yellow.   It should be pretty clear where the isotherms should go.



The isotherms have been drawn in at right; not how the isotherms separate the colored bands.  Note how the 40 F isotherm goes through the 40 on the map.  There is one city with a temperature of exactly 60 F so a little piece of a 60 F isotherm is drawn through that city.

 Isobars
These are a little harder to draw because you have to be able to decode the pressure data


isobars (1) connect points on the map with equal pressure
                                                (2) separate regions of high pressure from regions with lower pressure
                                 and identify and locate centers of high and low pressure

Here's the same weather map with isobars drawn in.  Isobars are generally drawn at 4 mb intervals (above and below a starting value of 1000 mb). 

The 1008 mb isobar (highlighted in yellow) passes through a city at Point 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 1009.7 mb at Point B and 1006.8 mb at Point C.  You would expect to find 1008 mb somewhere in between those two cites, that is where the 1008 mb isobar goes.

The isobars separate regions of high and low pressure.  The pressure pattern is not as predictable as the isotherm map.  Low pressure is found on the eastern half of this map and high pressure in the west.  The pattern could just as easily have been reversed.

This site (from the American Meteorological Society) first shows surface weather observations by themselves (plotted using the station model notation) and then an analysis of the surface data like what we've just looked at.  There are links below each of the maps that will show you current surface weather data.

Here's a little practice




A single isobar is shown.  Is it the 1000, 1002, 1004, 1006, or 1008 mb isobar? You'll need to decode the pressure data (add either a 9 or 10 and a decimal point).  You'll find the answer at the end of today's notes.

What can you begin to learn about the weather once you've draw isobars on a surface weather map and revealed the pressure pattern?

1a.  Surface centers of low pressure

We'll start with the large nearly circular centers of High and Low pressure.  Low pressure is drawn below.  These figures are more neatly drawn versions of what we did in class.



Air will start moving toward low pressure (like 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 (don't worry about the Coriolis force at this point, we'll learn more about it later in the semester).

In the northern hemisphere 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, we won't worry about the southern hemisphere until later in the semester]

When the converging air reaches the center of the low it starts to rise.


Convergence causes air to rise (1 of 4 ways)
 rising air e-x-p-a-n-d-s (it moves into lower pressure surroundings at higher altitude) 
The expansion causes the air to cool
 If you cool moist air enough (to or below its dew point temperature) clouds can form

Convergence is 1 of 4 ways of causing air to rise (we'll learn what the rest are soon, and, actually, you already know what one of them is - warm air rises, that's called convection).  You often see cloudy skies and stormy weather associated with surface low pressure.

We can see the counterclockwise spinning motions around low pressure on both radar and satellite observations of Hurricane Dorian.


Radar observations of Hurricane as it approached and then stalled over the Bahamas obtained from a radar in Nassau.  (credit: Brian McNoldy, Univ. of Miami Rosenthiel School of Marine and Atmospheric Science, source of the loop)
 Visible satellite photography of Hurricane Dorian on Sep. 1, 2019 (source of the loop)

Converging winds generally produce rising air motions in the very center of surface low pressure.  In the case of a hurricane the rising air motions are found in a ring of strong thunderstorms, the eye wall, that surrounds the center of the storm.  Air actually sinks in the center of a hurricane.  This sinking air produces clear skies and produces the distinctive huricane eye.

 1b.  Surface centers of high pressure
Everything is pretty much the exact opposite in the case of surface high pressure.

Winds spin clockwise (counterclockwise in the southern hemisphere) 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.

Clear skies doesn't necessarily mean warm weather, strong surface high pressure often forms when the air is very cold. 

Divergence causes air to sink
sinking air is compressed and warms
warming air keeps clouds from forming - clear skies


Here's a picture summarizing what we've learned so far.  It's a slightly different view of wind motions around surface highs and low that tries to combine all the key features in as simple a sketch as possible.




We were running short on time at this point.  So we will cover what follows at the beginning of class next Tuesday.

2.  Strong and weak pressure gradients - fast or slow winds
The pressure pattern will also tell you something about where you might expect to find fast or slow winds.  In this case we look for regions where the isobars are either closely spaced together or widely spaced.  I handed out a replacement for p. 40c in the ClassNotes (don't throw p. 40c away).


A picture of a hill is shown above at left.  The map at upper right is a topographic map that depicts the hill (the numbers on the contour lines are altitudes).  A center of high pressure on a weather map, the figure at the bottom, has the same overall appearance.  The numbers on the contours are different.  These are contours (isobars) of pressure values in millibars.

Closely spaced contours on a topographic map indicate a steep slope.  More widely spaced contours mean the slope is more gradual.  If you roll a rock downhill on a steep slope it will roll more quickly than if it is on a gradual slope.  A rock will always roll downhill, away from the summit in this case toward the outer edge of the topographic map.  Air will always start to move toward low pressure

On a weather map, closely spaced contours (isobars) means pressure is changing rapidly with distance.  This is known as a strong pressure gradient and produces fast winds (a 30 knot wind blowing from the SE is shown in the orange shaded region above).  Widely spaced isobars indicate a weaker pressure gradient and the winds would be slower (the 10 knot wind blowing from the NW in the figure).

Winds spin counterclockwise and spiral inward around low pressure centers.  The fastest winds are again found where the contour lines are close together and the pressure gradient is strongest.

The following figure is on the in-class Optional Assignment.
Contour spacing
closely spaced isobars = strong pressure gradient (big change in pressure with distance) - fast winds
widely spaced isobars = weak pressure gradient (small change in pressure with distance) - slow winds






You should be able to sketch in the direction of the wind at each of the three points and determine where the fastest and slowest winds would be found. (you'll find the answer at the end of today's notes).  Once you know which directions the winds are blowing you should be able to say whether the air at each of the points would be warmer or colder than normal.




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 

Here are a couple more questions embedded in the today's notes.



The isobar in the earlier figure is the 1004 mb contour.  It separates pressures less than 1004 mb (colored blue and violet) from pressures greater than 1004 (green and orange).  The 1002 mb and 1006 mb isobars have also been drawn in (isobars are normally drawn at 4 mb intervals, so the 1002 mb and 1006 mb contours wouldn't normally be included).




First the Low and High pressure centers have been labeled.  The brown arrows show the winds blowing counterclockwise and inward around the Low, clockwise and outward around the High.  Winds are shown using the station model notation at Points #1, #2, and #3 so that an idea of wind speed could be included.  The isobars are most tightly spaced (strong pressure gradient) at Point #3.  That's where the fast winds are shown.  The wind at Point #2 is coming from the south, that's where the warmest air would most likely be found.  Colder winds coming from the NW are found at Points #1 and #3.