Tuesday, Sept. 29, 2015

Some Cajun music from the Lost Bayou Ramblers "Blue Moon Special" from a concert in Sweden, "Moi J'Connais Pas" from a performance in France, and "When The Saints".

Quiz #1 has been graded and was returned in class today.  The average grade was 75% for both this and the 9:30 am section of the class.  Be sure to carefully check to be sure your quiz was graded correctly.  Also keep any and all graded work that is returned to you until grades are submitted at the end of the semester.  If there is some question about your final grade, you might have to bring in all your work in order to resolve any discrepancies.

1S1P Assignment #2a is due Thursday this week.  A new Assignment, a surface weather map analysis is now available and is due next Tuesday (Oct. 6). 

Here's a quick review of where we left off last Tuesday.  The figure below shows the temperature pattern you would expect to see if the wind wasn't blowing at all.


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.  Cold air is found in the north, warmer air down south.

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



Winds spinning around High and Low pressure move cold air southward and warm air northward.

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 or boundary between different temperature air gets squeezed and compressed.  The change from warm to cold occurs in a shorter distance and is sharper and more distinct.  Solid lines have been used to delineate the boundaries above. These sharper and more abrupt boundaries are called fronts.  That's what we'll be working on today.

Warm and cold fronts, middle latitude storms (aka extratropical cyclones)

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.

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.  There are two additional types of fronts, stationary fronts and occluded fronts, that weren't discussed in class today.

The fronts are like spokes on a wheel.  The "spokes" will spin counterclockwise around the low pressure center (the axle).

Both types of fronts cause rising air motions.  Fronts are another way of causing air to rise.  That's important because rising air expands and cools.  If the air is moist and cools enough, clouds can form.   Both types of fronts can cause clouds, but there are differences in the pattern of clouds produced by warm and cold front. 

The storm system shown in the picture above (the Low together with the fronts) is referred to a middle latitude storm or an extra-tropical cyclone.  Extra-tropical means outside the tropics, cyclone means winds spinning around low pressure (tornadoes are sometimes called cyclones, so are hurricanes).  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.

We'll be learning about cold and warm fronts today and about the kinds of weather that precede and follow them.  The material that we cover today should give you everything you need to know to be able to complete the 1S1P Surface Map Analysis that is due next Tuesday (Oct. 6)).

3-dimensional structure of cold fronts
A 3-dimensional cross-sectional view of a cold front is shown below (we've jumped to p. 148a in the photocopied ClassNotes)



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. 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.

And maybe just 15 to 20 minutes into today's class you're in a position to appreciate a video recording of the cold front passing through Tucson.  The first video was 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.

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.  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.


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 be doing the same thing on the 1S1P surface weather map analysis assignment.

Step #1
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. 




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
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
Locating and drawing in the  cold front.


Step #4
We should 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.

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.




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. 

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.  This is an important difference between warm and cold fronts.

Weather changes that precede and follow passage of a warm front
Here's the 3-dimensional view again that's in the ClassNotes.


and the map view


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



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.


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
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




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
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
 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 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 didn't really go over the following three figures in class today but I'm going to include them in the notes anyway.
We need to go back to the figure where this section on surface weather maps all began.





After learning how weather data are plotted on a map using the station model notation we found that the data, by themselves, were not enough to really be able to say what was causing the cloudy, rainy weather in the NE and along the Gulf Coast.



We added some isobars to reveal the pressure pattern and to locate large centers of high and low pressure.  Winds converging into the center of low pressure cause air to rise and might be part of the explanation for the unsettled weather in the NE.  That would explain the rain shower along the Gulf Coast however.



Now we've added cold and warm fronts to the picture.  The approaching cold front is almost certainly the cause of the shower along the Gulf Coast.  The clouds in the NE are probably mostly being produced by the warm front.