Tuesday, Feb. 20, 2018

Monsieur Perine  "La Muerte" (3:31), "Llore" (6:31), "Suin Romanticon" (4:37), "Tu M'as Promis" (5:30), "Nuestra Cancion" (6:30)

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.




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'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.  We'll probably go through this fairly quickly in class.
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 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. 




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.


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).


Upper Level Charts - Basic Features
There is an Assignment that accompanies this 3-part reading material on Upper Level Charts.  You'll be able to earn extra credit points or points that will be added to your next quiz score.  We'll only cover Upper-level charts Pt. 1 in class.  Here are links to Pt. 2 and Pt. 3.

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).

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 below).  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 below (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".

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).).

Yesterday's rainy weather was associated with a trough that had formed over the western US.  The trough is the dominant feature in the figure above at left.  The figure at right shows 2 pm surface temperatures being reported early yesterday afternoon.  There does seem to be a tendency for cooler temperatures to be found under and in the vicinity of the trough.


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 first 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.