Tue., Sep. 25 notes

Tuesday Sept. 25, 2012
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You heard just a single song ("Oaxaca") from an interesting group called Maserati before class today.

Quiz #1 has been graded and was returned in class today. Please check the grading carefully and see that the points are added up correctly.

The 1S1P reports on the 2012 North American Drought have also been graded. There is now a link on the class homepage where you can keep track of report grading status.

Several sets of experiment materials were returned today so I should have 5 or 6 sets of Expt. #2 materials that I can checkout in class on Thursday. I am planning on having the Expt. #1 reports graded in time to return in class on Thursday also.

Threee new Optional Assignments are going to be appearing in the next couple of classes. First, a "Surface Weather Map Analysis" assignment is 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 Tuesday (Oct. 2). There'll be another optional assignment on Thursday where you'll have the opportunity to earn extra credit or a "Green Card". And finally an Optional Assignment made during class today that will be due at the start of class on Thursday (Sept. 27). This latter assignment has two parts as shown below.

You should find everything you need to know for Assignment #3 in today's lecture notes.

Before the quiz we learned that drawing isobars on a surface weather map allows you to locate centers of high and low pressure. Here's a pictorial summary of what we learned about the winds that blow around centers of high and low pressure.

The pressure pattern will also tell you something about where you might expect to find fast or slow winds. In this case we look for regions where the isobars are either closely spaced together or widely spaced.

A picture of a hill is shown above at left. The map to its right is a topographic map that depicts the hill (the numbers on the contour lines are altitude). Below that is a weather map with pressure contours (isobars). The center of high pressure on a weather map could have exactly the same appearance as the topographic map.

On a topographic map, closely spaced contours indicate a steep slope or steep grade. If you were hiking and happened to fall you quickly roll downhill. The contours are further apart on the right side of the topographic map because the slope is more gradual. Here you'd roll downhill more slowly.

On a weather map closely spaced isobars means pressure is changing rapidly with distance. This is known as a strong pressure gradient and produces fast winds.

The winds are spinning clockwise and spiraling outward slightly as they blow around the high pressure center. Note the different wind speeds (30 knots and 10 knots plotted using the station model notation). Fast winds where to contours are close together and slower winds where they are further apart.

Winds spin counterclockwise and spiral inward around low pressure centers. The fastest winds are again found where the pressure gradient is strongest, that is where the contour lines are close together.

Now you can test yourself.

This figure is found at the bottom of p. 40 c in the photocopied ClassNotes. You should be able to sketch in the direction of the wind at each of the three points and determine where the fastest and slowest winds would be found. (you'll find the answer at the end of today's notes).

Next we'll see that once the winds start to blow they can affect and change the temperature pattern. The figure below shows the temperature pattern you would expect to see if the wind wasn't blowing at all or if the wind was just blowing straight from west to east. The bands of different temperature are aligned parallel to the lines of latitude. Temperature changes from south to north but not from west to east.

This picture gets a little more interesting if you put centers of high or low pressure in the middle.

In the case of high pressure, the clockwise spinning winds move warm air to the north on the western side of the High. The front edge of this northward moving air is shown with a dotted line (at Pt. W) in the picture above. Cold air moves toward the south on the eastern side of the High (another dotted line at Pt. C). The diverging winds also move the warm and cold air away from the center of the High. Now you would experience a change in temperature if you traveled from west to east across the center of the picture.

The transition from warm to cold along the boundaries (Pts. W and C) is spread out over a fairly long distance and is gradual. This is because the winds around high pressure blow outward away from the center of high pressure. There is also some mixing of the different temperature air along the boundaries.

Counterclockwise winds move cold air toward the south on the west side of the Low. Warm air advances toward the north on the eastern side of the low. This is just the opposite of what we saw with high pressure.

The converging winds in the case of low pressure will move the air masses of different temperature in toward the center of low pressure. The transition zone between different temperature air gets squeezed and compressed. The change from warm to cold occurs in a shorter distance and is more abrupt. Solid lines have been used to delineate the boundaries above. These sharper and more abrupt boundaries between are called fronts.

A cold front is drawn at the front edge of the southward moving mass of cold air on the west side of the Low. Cold fronts are generally drawn in blue on a surface weather map. The small triangular symbols on the side of the front identify it as a cold front and show what direction it is moving. The fronts are like spokes on a wheel. The "spokes" will spin counterclockwise around the low pressure center (the axle).

A warm front (drawn in red with half circle symbols) is shown on the right hand side of the map at front edge of the northward moving mass of. A warm front is usually drawn in red and has half circles on one side of the front to identify it and show its direction of motion.

Both types of fronts cause rising air motions. Fronts are another way of causing air to rise. Rising air expands and cools. If the air is moist and cools enough, clouds can form.

The storm system shown in the picture above (the Low together with the fronts) is referred to a middle latitude storm or an extratropical cyclone. Extra tropical means outside the tropics, cyclone means winds spinning around low pressure (tornadoes are sometimes called cyclones, so are hurricanes). These storms form at middle latitudes because that is where air masses coming from the polar regions to the north and the more tropical regions to the south can collide.

Large storms that form in the tropics (where this mostly just warm air) are called tropical cyclones or, in our part of the world, hurricanes.

And speaking of hurricanes. You can find the latest information about Hurricane Miriam at the National Hurricane Center website.

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 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.

The 2nd video was another cold front passage that occurred last spring on February 12.

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.

The next two figures weren't shown in class. I've included them here to give you a little bit better idea of how you going about drawing in isobars. 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.

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.

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. This is the first part of the Optional Assignment.

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.

This is already way too many notes for one class, so I've moved the Fall Equinox material to a separate location. You'll need to have a quick look at that if you want to answer the second question on the Optional Assignment.

There was a question earlier in today's notes about wind directions and wind speeds. Here's the answer to the question.

The winds are blowing from the NNW at Points 1 and 3. The winds are blowing from the SSE at Point 2. The fastest winds (30 knots) are found at Point 2 because that is where the isobars are closest together (strongest pressure gradient). The slowest winds (10 knots) are at Point 3. Notice also how the wind direction can affect the temperature pattern. The winds at Point 2 are coming from the south and are probably warmer than the winds coming from the north at Points 1 & 3.