Friday, Feb. 09, 2018

Sergio Mendoza y La Orkesta (currently Orchesta Mendoza) recorded by NPR Music in Austin, Tx, during the 2014 SXSW Festival.
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 (p. 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


Our "job" is to try to make some sense of this data.  To do that we'll draw in an isotherm or two.  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.

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 find the answer at the end of today's notes)

What can you begin to learn about the weather once you've mapped out 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.

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.






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.

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


This figure is found at the bottom of p. 40 c in the photocopied ClassNotes.


  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.

The next section on temperature patterns and fronts has been moved to the Mon., Feb. 12 notes.

Here are answers to a couple of questions embedded in today's notes



Pressures lower than 1002 mb are colored purple.  Pressures between 1002 and 1004 mb are blue.  Pressures between 1004 and 1006 mb are green and pressures greater than 1006 mb are red.  The isobar appearing in the question is highlighted yellow and is the 1004 mb isobar.  The 1002 mb and 1006 mb isobars have also been drawn in (because isobars are drawn at 4 mb intervals starting at 1000 mb, the 1002 mb and 1006 mb isobars wouldn't normally be drawn on a map)







Winds from the NW at 20 knots at Point #1, SE winds at 30 knots at Point #2, and NW winds at 10 knots at Point #3.
The southerly winds in the middle of the picture at Point #2 would probably be the warmest.  You would find colder air coming from the north at Points #2 and #3.