Mon. Sept. 24, 2012

You heard about 2/3rds of "Oaxaca" from a group called Maserati before class today.

A new "Surface Weather Map Analysis" assignment has appeared online.  It's an optional assignment with a twist: you can earn either 1S1P points or extra credit.  The assignment is due at the start of class next Monday.  There'll be another optional assignment on Wednesday where you'll have to opportunity to earn extra credit or a "Green Card".

We learned a little bit about the structure of cold fronts last Friday.  Today we'll learn how to locate a cold front on a surface weather map and also learn about warm fronts.

Fronts are boundaries between air masses with different temperatures and are another way of causing rising air motions.

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 need to draw in  996 mb and 1000 mb isobars.

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.

The map below shows the same picture with the 996 mb and 1000 mb contours drawn in.

You'll find another example in the Surface Weather Map Analysis Example.that accompanies the assignment mentioned at the top of today's lecture notes.

The next step was to try to locate the warm air mass in the picture.  We've left the isobars in the figure below but now have used colors to identify air masses with different 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.  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.  Now we're looking at warm air overtaking a slowly retreating cold air mass.

In the case of the car analogy it is like the Volkswagen about to catch up with a Cadillac.

What will happen when they catch the Cadillac?

The Volkswagens are still not nearly as heavy as the Cadillac.  They'll overrun the Cadillac.

The same happens along a warm front.  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 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 ramp like shape.  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.

Here's the 3-dimensional view again that's in the ClassNotes.

And the weather changes found 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 SW, S, SE from the East or SE 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

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 we'll follow the same procedure and try to locate a warm front on a surface weather map.

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.

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 an additional front drawn in.

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

One last picture, we go back to the map on p. 39 in the ClassNotes.

This is one of the maps that we looked at at the start of this section of material.  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.