Fri., Feb. 10 notes

Friday, Feb. 10, 2012
click here to download today's notes in a more printer friendly format

I had hoped to be able to play four songs from The Beatles before class today ("I'll Cry Instead", "Things We Said Today", "You Can't Do That", and "I'll Be Back" all from the "A Hard Day's Night" album) but in the end there was time for only two and a half.

The 1S1P Radon reports have been graded and were returned in class today.

Experiment #2 materials were distributed in class today for the first time. There were a few extra sets left over, I'll bring them to class next Monday.

A quick review of where we left off last Wednesday.

Fronts are boundaries between air masses with different temperatures. Fronts (both warm and cold fronts) are a second way of causing air to rise (convergence - winds spiraling into the center of low pressure is another way of causing air to rise). Rising air is important because rising air cools and can form clouds and precipitation.

The storm above is referred to as a middle latitude storm or an extratropical cyclone.

We'll be looking in more detail at the structure of warm and cold fronts and the weather changes that can occur as they approach and pass through. We'll also look at how you might go about locating fronts on a surface weather map.

A vertical slice through a cold front is shown below at left. Pay particular attention to the shape of the advancing edge of the cold air mass. Friction with the ground causes the front edge to "bunch up" and gives it the blunt shape it has. You'd see something similar if you were to pour something thick and gooey on an inclined surface and watch it roll downhill.

The cold dense air mass behind a cold front moves into a region occupied by warm air. The warm air has lower density and will be displaced by the cold air mass. In some ways its analogous to a big heavy Cadillac plowing into a bunch of Volkswagens.

The VWs would be thrown up into the air by the Cadillac.

A sort of 3-dimensional 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 (warm) considerably. The air right behind the front will have traveled the furthest and warmed 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.

We watched a couple of short video segments at this point. The first used colored liquids with slightly different densities (a water/glycerin mixture) to show how a cold air mass can lift a warmer, less dense air mass. The second video was a time lapse movie of an actual cold front that passed through Tucson on Easter Sunday, April 4, in 1999. It actually snowed for a short time during the passage of the cold front (hard to imagine cold weather and snow on a day as warm and nice as it is today). Click here to see the cold front video (it may take a minute or two to transfer the data from the server computer in the Atmospheric Sciences Dept., be patient). Remember the video shows a time lapse movie of the frontal passage. The front seems to race through Tucson in the video, it wasn't moving as fast as the video might lead you to believe. Cold fronts typically move 15 to 25 MPH.

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 Surface Weather Map 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. 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.

We didn't have time to locate the warm front on the following surface map.

We'll do that on Monday.