Tuesday, September 25, 2018

Playing for Change "Groove in G" (4:55), "Clandestino" (3:47), "With My Own Two Hands" (4:06), "Satchita" (4:16), "Tenia Tanto Que Darte" (4:31)

We'll be using page 148a, page 148b, page 149a, and page 149b in class today. These weren't included in the ClassNotes packet at the Bookstore, I'll provide copies in class.  We may also use page 41 and page 42.  Those are in the ClassNotes packet.

3-dimensional structure of cold fronts
A 3-dimensional cross-sectional view of a cold front is shown below.   We've jumped to page 148a in the online version of the ClassNotes (an older version is shown below, I'll bring copies of the newer figure to class).



The person in the figure is positioned ahead of an approaching cold front.  Time wise, it might be the day before the front actually passes through.  There are 3 fairly important features to notice in this picture.

1.    The front edge of the approaching air mass has a blunt, rounded shape.  A vertical slice through a cold front is shown below at left.



Friction with the ground causes the edge to "bunch up" and gives it the blunt shape it has.  You'd see something similar if you were to pour something thick and gooey on an inclined surface and watched it roll downhill.   Or, as shown in class, you can lay your arm and hand on a flat surface.
   



Slide your arm to the right. 



Your fingers will drag on the table surface and will curl up and your hand will make a fist. 

2.  A cold front, the leading edge of a cold air mass is kind of like a fist slamming into a bunch of warmer air.  Because it is denser, the cold air lifts the warm air out of the way.






The cold dense air mass behind a cold front moves into a region occupied by warm air.  The warm air has lower density and will be displaced by the cold air mass.  In some ways its analogous to a big heavy Cadillac plowing into a bunch of Volkswagens.

At this point, just 15 to 20 minutes into today's class, we're in a position to better appreciate a video recording of the cold front passing through Tucson.  The first video is a time lapse movie of a cold front that came through Tucson on on Easter Sunday morning, April 4, 1999.  Click here to see the cold front video (it may take a minute or two to transfer the data from the server computer in the Atmospheric Sciences Dept., be patient).  Remember this is a time lapse movie of the frontal passage.  The front seems to race through Tucson in the video, it wasn't moving as fast as the video might lead you to believe.  Cold fronts typically move 15 to 25 MPH. 

The 2nd video was another cold front passage that occurred on February 12, 2012.

In the past I've had trouble playing the videos using Firefox on the classroom computer.    If that is the case, you can right click on each link, then click on the Save Link As... option, and choose to save to the Desktop.  Then double click on the icon on your desktop to view the video.  If you use Chrome or Internet Explorer you should be able to watch them.

3.    Note the cool, cold, colder bands of air behind the cold front.



The warm air mass ahead of the front has just been sitting there and temperatures are pretty uniform throughout.  Cold fronts are found at the leading edge of a cold air mass.  The air behind the front might have originated in Canada.  It might have started out very cold but as it travels to a place like Arizona it can change (warm) considerably.  The air right behind the front will have traveled the furthest and warmed the most.  That's the reason for the cool, cold, and colder temperature bands (temperature gradient) behind the front.  The really cold air behind a cold front might not arrive in Arizona until 1 or 2 days after the passage of the front.



Weather changes that precede and follow passage of a cold front
Here are some of the specific weather changes that might precede and follow a cold front. 

Weather variable
 Behind
 Passing
 Ahead
Temperature
 cool, cold, colder*
warm
Dew Point
 usually much drier**
may be moist (though that is often
not the case here in the desert southwest)
Winds
 northwest
 gusty winds (dusty)
 from the southwest
Clouds, Weather
 clearing
 rain clouds, thunderstorms
in a narrow band along the front
(if the warm air mass is moist)
 might see some high clouds
Pressure
 rising
 reaches a minimum
 falling

*  as mentioned above, the coldest air might follow passage of a cold front by a day or two.
**nighttime temperatures drop much more quickly in dry air than in moist or cloudy air.  This is part of the reason it can get very cold a day or two after passage of a cold front.

Gusty winds and a shift in wind direction are often one of the most obvious change associated with the passage of a cold front in Tucson.

The pressure changes that precede and follow a cold front are not something we would observe or feel but are very useful when trying to locate a front on a weather map.


3-dimensional structure of warm fronts
We've learned a fair amount about cold fronts: cross-sectional structure, weather changes that precede and follow passage of a cold front, and how to locate a cold front on a surface weather map.  Now we have to do the same for warm fronts.  An older version of p. 149a is shown below, copies of the newer version of page 149a will be provided in class.




Warm air approaching and colliding with a cold air mass is like a fleet of Volkswagens overtaking a Cadillac



 The VWs are still lighter than the Cadillac.  What will happen when the VWs catch the Cadillac?







They'll run up and over (overrun) the Cadillac.


The same kind of thing happens along a warm front.  Warm air is overtaking some colder air that is also moving to the right.
The approaching warm air is still less dense than the cold air and will overrun the cold air mass. 

There's one key difference between cold and warm front boundaries

1.  The back edge of a retreating cold mass that the warm air overtakes has a much different shape than the advancing edge.  The advancing edge bunches up and is blunt.  The back edge gets stretched out and has a more gradual ramp like shape. 

You can use your hand and arm again.


You start with your fingers curled up then move your arm and hand to the right.




As your arm moves to the right, friction uncurls your fingers. 

The warm air rises more slowly and rises over a much larger area out ahead of the warm front.  A variety of cloud types form and spread out over a large area ahead of the warm front.  This is an important difference between warm and cold fronts.



Weather changes that precede and follow passage of a warm front

Here are the kinds of weather changes that usually precede and follow passage of a warm front. 

Weather Variable
 Behind (after)
 Passing
 Ahead (before)
Temperature
 warmer
cool
Dew point
 may be moister
drier
Winds
 SW, S, SE
from the East or SE, maybe even the S
Clouds, Weather
 clearing
wide variety of clouds that may precede arrival of the front by a day or two
clouds may produce a wide variety of types of precipitation also
(snow, sleet, freezing rain, and rain)
Pressure
 rising
 minimum
 falling


You won't need to worry about the remaining material (Locating cold and warm fronts on surface weather maps) for this week's quiz.

Locating a cold front on a weather map

In the next figure we started with some weather data plotted on a surface map using the station model notation.  We'll try to make a little more sense of this data and eventually locate a cold front.  Study this example carefully because you will have an opportunity to do a surface weather map analysis of your own and will be able to earn some 1S1P points or Extra Credit points.

Step #1 - draw in some isobars and ocate the low pressure center

In some respects fronts are like spokes on a wheel - they rotate counterclockwise around centers of low pressure.  It makes sense to first locate the center of low pressure.  To do that we need to draw in a few isobars and map out the pressure pattern.  A copy of the new version of page 148b will be provided in class.


Isobars are drawn at 4 mb increments above and below a starting value of 1000 mb.  Some of the allowed values are shown on the right side of the figure (992, 996, 1000, 1004, 1008 etc).  The highest pressure on the map is 1003.0 mb, the lowest is 994.9 mb.  You must choose from the allowed list of isobar values and pick only the values that fall between the high and low pressure values on the map.  Thus we only need to draw in  996 mb and 1000 mb isobars.
 
Step #1 cont'd
Color coding the plotted pressure values may be helpful.  In the figure below stations with pressures lower than 996 mb have been colored in purple.  These will be enclosed by the 996 mb contour.  Pressures between 996 and 1000 mb have been colored blue.  These stations will lie outside the 996 mb contour but inside the 1000 mb isobar.  Finally stations with pressures greater than 1000 mb have been colored green.  The 1000 mb isobar will separate the blue stations from the green stations.







End of Step #1
 The map below shows the same picture with the 996 mb and 1000 mb contours drawn in (it is always a good idea to label the isobars when you draw them in).
 


Step #2 - locate the warm air mass
The next step was to try to locate the warm air mass in the picture.  I'll start with a new map for clarity that keeps the isobars.  The colors now will represent different air temperatures.





Temperatures are in the 60s in the lower right portion of the map; this area has been circled in orange.  Cooler air to the west of the Low pressure center has also been identified.  Do the green, blue, purple (cool, cold, colder) bands look familiar?  Based on just the temperatures we have a pretty good idea where a cold front would be found.

 Step #3 - draw in a tentative location for the cold front
Locating and drawing in the cold front.


Step #4 - double check the front location
We should double check the front location using some of the other weather changes (wind shift, dew point, pressure change etc.) that precede and follow a cold front.


The air ahead of the front (Pts. B & C) is warm, moist, has winds blowing from the S or SW, and the pressure is falling.  These are all things you would expect to find ahead of a cold front.

Overcast skies are found at Pt. B. very near the front. 

The air behind the front at Pt. A is colder, drier, winds are blowing from the NW, and the pressure is rising.  That is just what you would expect behind a cold front.  So our location of the front looks pretty good.


Locating a warm front on a weather map
We need to finish our study of surface weather maps by trying to located a warm front.  The figure below is on page 149b in the ClassNotes; a copy will be provided in class.


This is the map we will be working with (see p. 149b in the ClassNotes).  It's worth pausing and noting that you really can't make any sense out of this jumble of weather data at this point.

Step #1 - draw in some isobars and locate the low pressure center
We'll start by drawing some isobars to map out the pressure pattern.  A partial list of allowed isobars is shown at the right side of the map above (increments of 4 mb starting at 1000 mb).


We've located located the highest and lowest pressure values on the map.  Then we choose allowed isobar values that fall between these limits.  In this case we'll need to draw 992 mb and 996 mb isobars.

Here's the map with color coded pressures.  Pressures less than 992 mb are purple, pressures between 992 and 996 mb are blue, and pressures greater than 996 mb are green.  Note that station B has a pressure of exactly 992.0 mb, the 992 mb isobar will go through that station.  The 996 mb isobar will go through station A because it has a pressure of exactly 996.0 mb.


Here's the map with the isobars drawn in.  On the map below we use colors to locate the warm and cooler air masses.
Step #2 - locate the warm air mass




The warm air mass has been colored in orange.  Cooler air east of the low pressure center is blue.  Can you see where the warm front should go?

Step #3 - draw in a tentative warm front location
Here's the map with a warm front drawn in
(the map was redrawn so that the edge of the warm (orange) air mass would coincide with the warm front). 



The change in wind directions was probably more pronounced than the temperature change.  Most of the clouds outlined in green are probably being produced by the warm front.  You can see how more extensive cloud coverage is with a warm front. 

Step #4 - double check the front location
 Two of the stations near the right edge of the picture and on opposite sides of the front are redrawn below.



The station north of the front has cooler and drier air, winds are from the east, skies are overcast and light rain is falling.  The pressure is falling as the warm front approaches.  These are all things you'd expect to find ahead of a warm front.  Behind the front at the southern station pressure is rising, the air is warmer and moister, winds have shifted to the south and the skies are starting to clear.
In this case there is a Step #5 - have a look at the rest of the surface map

 Have a look at the left, western, side of the map.  There's pretty good evidence of a cold front.




There's a big temperature change (low 60s to low 40s and 30s) and a very noticeable wind shift (SW ahead of the cold front and NW behind).


We probably won't have time for the material that follows.  I'll leave it here for the time being nonetheless.

Upper Level Charts - Basic Features
We've been spending some time learning about surface weather maps.  Maps showing conditions at various altitudes above the ground are also drawn.  Upper level conditions can affect the development and movement of surface features (and vice versa).  The figure below is on page 41 in the ClassNotes.


 


In this first section we'll just learn 3 basic facts about upper level charts.  First the overall appearance is somewhat different from a surface weather map.  The pattern on a surface map can be complex and you generally find circular (more or less) centers of high and low pressure (see the bottom portion of the figure above).  You can also find closed high and low pressure centers at upper levels, but mostly you find a relatively simple wavy pattern like is shown on the upper portion of the figure above (sort of a 3-dimensional view).

A simple upper level chart pattern is sketched below (a map view).  There are two basic features: wavy lines that dip southward and have a "u-shape" and lines that bend northward and have an "n-shape".   See page 42 in the ClassNotes

The u-shaped portion of the pattern is called a trough.  The n-shaped portion is called a ridge.

Troughs are produced by large volumes of cool or cold air (the cold air is found between the ground and the upper level that the map depicts).  The western half of the country in the map above would probably be experiencing colder than average temperatures.  Large volumes of warm or hot air produce ridges.  We'll see why this is true in "Upper level charts pt. 2".






The 500 mb upper level chart for Monday Feb. 19, 2018.  Note the trough positioned over the western states.
 2 pm surface temperatures for Monday, Feb. 19 (18Z = 11 am MST).  At a given latitude, temperatures do seem to be somewhat cooler under the trough over the western third of the US (blue and green colors) compared to the eastern portion of the US (yellow and red isotherms).).



The winds on upper level charts blow parallel to the contour lines generally from west to east.  This is a little different from surface winds which blow across the isobars toward low pressure.  An example of surface winds is shown below.


That's it for this section.  Really all you need to be able to do is
1. identify troughs and ridges,
2. remember that troughs are associated with cold air & ridges with warm air, and
3. remember that upper level winds blow parallel to the contour lines from west to east.