Monday, Feb. 11, 2019

Lucius "Woman" (4:45), "Until We Get There" (4:59), "Almost Makes Me Wish for Rain" (4:05), "How Loud Your Heart Gets" (6:30), "Turn it Around" (4:10),
"Two of Us on the Run" (4:51)

We'll use page 148c, page 148d, page 41, and page 42 from the ClassNotes.  We'll also go back to page 39c.

I'm adding a scanned copy of the figure that we drew at the start of class today.  It reviews what we learned about cold fronts in class last Friday and compares that with what we will be learning about warm fronts in class today.



You should be able to start with a blank sheet of paper and create something like this.

The upper part of the figure is a top view, a map view.  Draw in a big L to represent a surface center of low pressure.  Add a few isobars, contours of pressure and give them reasonable sea level pressure values.  Add a few arrows showing the winds that develop around a Low (counterclockwise spinning winds that blow inward (converge) toward the center of the Low).

Winds blow from north to south on the west side of the Low.   Because it is coming from higher latitude this will probably be colder air.  Draw in a cold front boundary at the leading edge of this cold air.   A cross sectional view of the advancing cold air mass is shown below the cold front.  Note the blunt, rounded, fist like shape of the leading edge.  Warm low density air ahead of the advancing cold air is forced upward.  If the warm air contains moisture, the rising motions could cause enough cooling for clouds to form. 

Warm air is advancing northward on the eastern side of the Low.  You'll find a warm front where this warm air encounters colder air.  The colder air ahead of the advancing warm front is slowly moving out of the way.  Now we're look at the back, retreating, edge of cold air.  The back edge is drawn out and has a distinctly different shape, it has more of a ramp shape (shown in the cross section).  The warm catches the retreating ramp of cold air and, because it is less dense, over runs the cold air.   If the warm air contains any moisture, the rising air will expand, cool, and clouds can form.  Because the warm air rises more gradually, a wide variety of cloud (and precipitation) types are found out ahead of the warm front.

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 page 148c is shown below, a somewhat cleaner version is in the ClassNotes.




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


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 148d in the ClassNotes.


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).

Lets go back to the surface map we began this section with


We were trying to figure out what was causing the cloudy & rainy weather in the NE part of the map and why a rain shower was being reported along the Gulf Coast.  There's pretty clear evidence of both a warm and cold front on this map.  They're shown in the figure below


The stormy weather in the NE appears to be largely caused by the warm front.  The rain shower along the Gulf Coast was associated with a cold front.


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 42 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 41 in the ClassNotes.  The older figures below are similar but the ridges and troughs are in different parts of the map.

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.