Monday Sept. 26, 2011
click here to download today's notes in a more printer friendly format

I kind of liked a song that I heard during a Lowe's commercial on TV the other night.  Turns out it was "Don't Stop" and was sung by Gin Wigmore.  I downloaded a couple more of her songs for class today ("Oh My" and "Hey Ho")

The Experiment #1 reports have been graded and were returned in class today.  You are allowed to revise your reports if you want to (you don't have to).  The revised reports will be due in 2 weeks, on or before Mon., Oct. 10.  You only need to rewrite portions where you want to earn additional credit.  Please return your original report with your revised report.

A new Optional Assignment on upper level charts is now available.  You can earn extra credit and a Green Card.  It is due at the start of class on Wed. Oct. 5. 

Last Friday was the Fall Equinox; the sun rises exactly in the east and sets exactly in the west on that day.  If you're out driving on an east-west oriented street at sunrise or sunset, the sun will be right in your face (or in your rear view mirror).  There were a few parents in the class on Friday because it was also the start of Parent's Weekend.  One of the parents came up to the front after class and mentioned seeing the sun right at the end of 77th St. in New York City around this time of year.  That got me thinking that a picture of sunset at the end of one of the long streets with all the tall buildings might be spectacular.

When I started looking however I found that the major streets in Manhattan aren't oriented EW and NS.  You can see this on a Google map of Manhattan.  77th St. is oriented in more of a NW-SE direction.  You won't see the sun set at the west end of 77th St on the equinox.  However between the Spring and Fall Equinoxes sunset will move north of west.  It will be furthest north on the summer solstice (June 21).  Then I stumbled on the this Manhattanhenge map which shows the direction of sunset (the left, west, side of the map) and sunrise (the right, east, side of the map) at various times of the year.  On May 31 this year the sun did set right at the west end of 77th St. (and any other street with the same NW-SE orientation).  It happened again on July 12 or 13 (maybe both nights).  And apparently it is quite an event.  An article with several Manhattanhenge photographs from the May 31 event appeared in a story on the Business Insider webpage.  The "henge" part of the name comes from Stonehenge where the rising and setting sun aligns with stones on the solstices.


The remainder of today's class involved looking at some of the weather changes that precede and follow passage of cold and warm fronts and locating fronts on a surface weather map.

A crossectional 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.  It might be the day before the front actually passes through. 

The warm air mass ahead of the front has just been sitting there and temperatures are pretty uniform throughout.  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 considerably.  The air right behind the front will have traveled the furthest and changed the most.  That's the reason for the cool, cold, and colder temperature gradient behind the 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
narrow band along the front
(if the warm air mass is moist)
 might see some high clouds
Pressure
 rising
 reaches a minimum
 falling

*  the coldest air might follow passage of a cold front by a day or two.  Nighttime temperatures often plummet in the cold dry air behind a cold front. 

A temperature drop is probably the most obvious change associated with a cold front.  Here is southern Arizona, gusty winds and a wind shift are also often noticeable when a cold front passes.

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.

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.


Before trying to locate a cold front, we needed to draw in a few isobars and map out the pressure pattern.  In some respects fronts are like spokes on a wheel - they rotate counterclockwise around centers of low pressure.  It makes sense to first determine the location of the low pressure center.

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 have drawn in  996 mb and 1000 mb isobars (you can get a better idea of how to actually draw in the isobars by looking at the 1S1P Bonus Assignment Example Analysis.

The next step was to try to locate the warm air mass in the picture.  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.  Based on just the temperatures just should have a pretty good idea where a cold front would be found.


The cold front on the map seems to be properly postioned.  Note how the cold front is positioned at the leading edge of the cold air mass, not necessarily in front of the coldest air in the cold air mass.   3 of the stations from the bottom portion of the map have been redrawn below.


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. 

Next we'll have a look at a warm front.  We really didn't cover warm fronts last Friday, so we need to back track a little bit.



Here's a warm front in crossection (top) and the Volkswagen/Cadillac analogy.  First we see it is the warm air that is invading and catching up to some colder air that might be slowly moving out of the way.  We're looking at the back, retreating edge of the cold air mass now. 

The warm air still has lower density than the cold air.  So when the warm air catches the colder air it won't wedge its way underneath and lift the colder air.  The VWs are not going to throw the Cadillac into the air when they collide.


The VWs run up and over the Cadillac.  The warm air overruns the colder air mass.

Something else to notice is the shape of the back edge of the cold air mass which has a ramp like shape. 

A picture might make clearer the distinction between the front advancing edge of cold air that you see with a cold front and the back retreating edge of cold air found at a warm front. 

Imagine pouring something gooey like honey or syrup onto a surface with a slight slope.  The gooey stuff will ooze downhill.  Friction will cause the front edge to bunch up;  this is what you see with a cold front.  The back edge gets drawn out.  This is the gradual ramp like shape we see with a warm front. 

(If you were to try this experiment instead of imagining it and were to take a picture I'd be happy to give you a Green Card for your efforts)

Here's the crossectional view
Here ae the weather changes in advance of and following the frontal passage.
Weather Variable
 Behind (after)
 Passing
 Ahead (before)
Temperature
 warmer
cool
Dew point
 may be moister
drier
Winds
 from S or SW, maybe W
from S, SE, even the E
Clouds, Weather
 clearing
wide variety of clouds that may precede arrival of the front by a day or two
may be a wide variety of types of precipitation also (snow, sleet, freezing rain, and rain)
Pressure
 rising
 minimum
 falling

Probably the key difference between warm and cold fronts (other than a cold-to-warm rather than a warm-to-cold change) is the wide variety of clouds that a warm front cause to form cover a much larger area out ahead of the front.  Clouds associated with a cold front are usually found in a fairly narrow band along the front.

Now let's try to locate a warm front on the following surface map


We start by locating 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.

Note that the 992 and 996 mb isobars each went through a station with pressures of exactly 992.0 and 996.0 mb (highlighted in yellow).  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?

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).  Most of the cloud outlined in green are probably being produced by the warm front.  You can see how more extensive cloud coverage is with a warm front.  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.

Here's the picture again with something more drawn in.

There's pretty good evidence of a cold front on the left portion of the map.

One last picture and this is something that wasn't shown in class.  We go back to the map on p. 39 in the ClassNotes.

One of the questions we had was what might be causing the clouds, rain, and drizzle in the northeastern part of the country, and the rain shower along the Gulf Coast.  The cold front is almost certainly the cause of the rain shower and much of the wet weather in the NE is probably being caused by the warm front.