Wednesday Feb. 8, 2012

Music from a group that I just learned about, The Little Willies.  The group is named after Willie Nelson; the female vocalist was Norah Jones.  You heard "It's Not You, It's Me", "Best of All Possible Worlds", and "Night Life".

The Quiz #1 Study Guide is now available.  Quiz #1 is one week from today (Wed., Feb. 15) and will cover material on both the Practice Quiz Study Guide and the new Quiz #1 Study Guide.

Experiment #2 materials should be available in class on Friday.

A bunch of weather data has been plotted (using the station model notation) on a surface weather map in the figure below (p. 38 in the ClassNotes). 



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.  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, temperature contour lines, are usually drawn at 10o 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 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).  Temperatures generally decrease with increasing latitude: warmest temperatures are usually in the south, colder temperatures in the north.


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.    The isobars also enclose areas of high pressure and low pressure.  Isobars are generally drawn at 4 mb intervals (starting with a base value of 1000 mb).  Isobars also connect points on the map with the same pressure.  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 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 (this figure wasn't shown in class).


Is this the 1000, 1002, 1004, 1006, or 1008 mb isobar? (you'll find the answer at the end of today's notes)

Now we'll look at what you can learn about the weather once you've drawn in some isobars and mapped out the pressure pattern.

1.
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 (we'll learn more about the Coriolis force 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, don'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.  Rising air expands (because it is moving into lower pressure surroundings at higher altitude), the expansion causes it to cool.  If the air is moist and it is cooled enough (to or below the dew point temperature) clouds will form and may then begin to rain or snow.  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).  You often see cloudy skies and stormy weather associated with surface low 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. 

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 and wasn't shown in class.




2.
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. 


Closely spaced contours means pressure is changing rapidly with distance.  This is known as a strong pressure gradient and produces fast winds.  It is analogous to a steep slope on a hillside.  If you trip walking on a hill, you will roll rapidly down a steep hillside, more slowly down a gradual slope.

The winds around a high pressure center are shown above using both the station model notation and arrows. The winds are spinning clockwise and spiraling outward slightly.  Note the different wind speeds (25 knots and 10 knots plotted using the station model notation)



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


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).

3.
The pressure pattern determines the wind direction and wind speed.  Once the winds start to blow they can affect and change the temperature pattern.  The figure below shows the temperature pattern you would expect to see if the wind wasn't blowing at all or if the wind was just blowing straight from west to east.  The bands of different temperature are aligned parallel to the lines of latitude.  Temperature changes from south to north but not from west to east. 



This picture gets a little more interesting if you put centers of high or low pressure in the middle.



In the case of high pressure, the clockwise spinning winds move warm air to the north on the western side of the High.  The front edge of this northward moving air is shown with a dotted line (at Pt. W) in the picture above.  Cold air moves toward the south on the eastern side of the High (another dotted line at Pt. C).  The diverging winds also move the warm and cold air away from the center of the High.  Now you would experience a change in temperature if you traveled from west to east across the center of the picture. 

The transition from warm to cold along the boundaries (Pts. W and C) is spread out over a fairly long distance and is gradual.  This is because the winds around high pressure blow outward away from the center of high pressure.  There is also some mixing of the different temperature air along the boundaries.


Counterclockwise winds move cold air toward the south on the west side of the Low.  Warm air advances toward the north on the eastern side of the low.  This is just the opposite of what we saw with high pressure.

This is as far as we got in class today.  I've added a little bit of more material below to finish this topic.

The converging winds in the case of low pressure will move the air masses of different temperature in toward the center of low pressure.  The transition zone between different temperature air gets squeezed and compressed.  The change from warm to cold occurs in a shorter distance and is more abrupt.  Solid lines have been used to delineate the boundaries above. These sharper and more abrupt boundaries between are called fronts.



A cold front is drawn at the front edge of the southward moving mass of cold air on the west side of the Low.  Cold fronts are generally drawn in blue on a surface weather map.  The small triangular symbols on the side of the front identify it as a cold front and show what direction it is moving.  The fronts are like spokes on a wheel.  The "spokes" will spin counterclockwise around the low pressure center (the axle).

A warm front (drawn in red with half circle symbols) is shown on the right hand side of the map at front edge of the northward moving mass of.  A warm front is usually drawn in red and has half circles on one side of the front to identify it and show its direction of motion.

Both types of fronts cause rising air motions.  Fronts are another way of causing air to rise.  Rising air expands and cools.  If the air is moist and cools enough, clouds can form.

The storm system shown in the picture above (the Low together with the fronts) is referred to a middle latitude storm or an extratropical cyclone (extra tropical means outside the tropics, cyclone means winds spinning around low pressure).   These storms form at middle latitudes because that is where air masses coming from the polar regions to the north and the more tropical regions to the south can collide.

Large storms that form in the tropics (where this mostly just warm air) are called tropical cyclones or, in our part of the world, hurricanes. 

I apologize for the poor time management at the end of class.  We were left with about 6 minutes of classtime to watch almost 8 minutes of video tape.  It was the last of the "Piccard videos".

Here you saw the launch of the Cable & Wireless Balloon (Feb. 17, 1999 from Almeria, Spain) with Andy Elsen and Colin Prescot aboard. 

The Breitling Orbiter 3 balloon was launched almost a week later (Mar. 1, 1999 from Chateau d'Oex, Switzerland) with Brian Jones and Bertrand Piccard in command. 

The Cable and Wireless balloon had almost a 10-day lead on the Piccard balloon as it headed out over the Pacific Ocean.  But the Cable & Wireless balloon ran into some severe weather.  The balloon "iced up" which means it became coated with ice.  The ice made the balloon so heavy that it crash landed in the Sea of Japan (both pilots were quickly picked up by search and rescue boats). 

The Breitling Orbiter stayed in the air and was able, on Mar. 20, 1999, to complete the first non-stop trip around the globe in a balloon.

Here are the answers to the questions found earlier in the 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.

And here's the answer to the question about wind directions and wind speeds.


The winds are blowing from the NNW at Points 1 and 3.  The winds are blowing from the SSE at Point 2.  The fastest winds (30 knots) are found at Point 2 because that is where the isobars are closest together (strongest pressure gradient).  The slowest winds (10 knots) are at Point 3.  Notice also how the wind direction can affect the temperature pattern.  The winds at Point 2 are coming from the south and are probably warmer than the winds coming from the north at Points 1 & 3.