Mon., Sep. 28 notes

Monday Sep. 28, 2009
click here to download today's notes in a more printer friendly format

Three songs ( "Turpentine," "My Song," and "The Story" )from Brandi Carlile before class today. She'll be in Tucson at the Rialto Theatre on Fri., Oct. 16.

The quizzes have all been graded and were returned in class today. Be sure to carefully check the grading for errors. Here are answers to questions on one of the versions of the quiz.

Today we'll learn about some of the weather conditions that precede and follow passage of warm and cold front. There is a change in the weather being forecast for Tucson starting about mid-week, perhaps a cold front or something resembling a cold front. Temperatures are expected to be significantly lower by the end of the week.

A crossectional view of a cold front is shown below:

Here are some of the specific weather changes

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.

We watched a couple of short video segments at this point. One attempted to show cold dense air displacing warm lower density air like you might see along a cold front (the video used mixtures of water and glycerin colored red and blue, by the way. Water with a lot of glycerin is denser than water with only a little glycerin). The second was a time lapse video of an actual cold front passing through Tucson on Easter Sunday, 1999. Clouds forming along the front edge of the front made the front visible.

In the next figure we started with some weather data plotted on a surface map using the station model notation.

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 beginning at 1000 mb. Some of the allowed values are shown on the right side of the figure. The highest pressure on the map is 1003.0 mb, the lowest is 994.9 mb. Thus we have drawn in 996 mb and 1000 mb isobars.

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 red.

The cold front on the map seems to be properly postioned. The air ahead of the front 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.

The air behind the front is colder, drier, winds are blowing from the NW, and the pressure is falling. 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.

Next we went through the same procedure with warm fronts.
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		from E or SE
Clouds, Weather	clearing		wide variety of clouds well ahead of the front, may be a wide variety of types of precipitation also.
Pressure	rising	minimum	falling

And here is the surface map analysis:

There was also pretty clear evidence of a cold front on this map. There's a very clear wind shift E and ESE ahead of the warm front becoming southerly behind the warm front. There's a big temperature change: 60s behind the warm front & 40s ahead of the warm front. There's also a fairly extensive region with overcast skies, rain and snow ahead of the front.

Note the cold front that was drawn in after class. Temperature go from the 60s ahead of the cold front to the 30s and 40s behind the front. Winds in the warm air ahead of the cold front are from the SW and turn to the NW behind the cold front.

Next we went back to the surface map example from Wed., Feb. 11. We were trying to figure out what was causing the clouds in the NE portion of the map and what was causing the rain shower along the Gulf Coast.

We've added a warm and cold front to the picture. The warm front is probably what is producing most of the widespread cloudiness and precipitation in the NE portion of the map (rising air motions caused by surface winds converging into the low pressure center are also contributing). The cold front is producing the showers along the Gulf Coast.