Monday Feb. 15, 2016

Laura Marling at the Great Wild Open Festival "The Muse" (4:15), "Devil's Spoke" (3:37), "Blackberry Stone" (3:48)

continuing from where we left off on Friday
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 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.

A quick look at upper level charts
We won't be spending much classroom time on upper level charts but there are a few features that you should be familiar with.

First the overall appearance is somewhat different from a surface weather map.  The pattern on a surface map can be complex but 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).  The figures and text below come from Upper Level Charts pt. 1, which is required reading.
 


A simple upper level chart pattern is sketched below (a pure top 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. 

An actual example of an upper level map is shown above at left.   Temperature data is shown in the figure at right.  Colder than normal temperatures at right match up well with an upper level trough on the map at left.  The warmer than average temperatures along the western US are associated with the eastern edge of an upper level ridge.  Tucson is expecting near record high temperatures this afternoon. 






A simple upper level pattern was in place last Wednesday (02/10/16).  The eastern half of the United States was under an upper level trough.  There is a ridge over the western half of the country.  (source of this image)

 This is a "temperature departure from normal" map.  The blue that covers much of the eastern part of the country indicates colder than normal.  Orange and red mean warmer than normal temperatures. (source of this chart)

A final point concerning winds on upper level and surface maps.  A comparison of upper level and surface winds is shown below.



Upper level winds blow parallel to the contour from west to east.  Surface winds blow across the contours always in the direction of low pressure (inward toward a center of low pressure and outward away from a center of high pressure.

We're done with weather maps for the time being.  Though if interesting weather appears imminent I'll try to mention it in class.