Tuesday Feb. 14, 2012
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4 short 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). 

The Optional Assignment was collected at the start of class.  You won't get your papers back before the quiz this week, so here are some answers to the questions on the assignment.

Several people brought back their Expt. #2 materials today and received a green card in addtion to the supplementary information handout.  If you're working on Expt. #2 and return your materials this week you can also earn a green card.  Additional opportunities for everyone else to earn a greencard will come up during the semester.

Quiz #1 is Thursday this week (you'll have the full class period for the quiz).  Consult the Quiz #1 Study Guide for more information and times and locations of this weeks reviews.

Finally here's something you may not have seen, a map drawing and analysis assignment that you can try (if you want to) and, in the process, earn some 1S1P pts.  I'll probably extend the due date (until Tue., Feb. 28) because we need to cover some of this in class and that won't happen until next week.

Here's about where we left off last week. 

We learned how to plot surface weather observations onto a map and then found one of most useful analyses that can be done is to draw in some isobars, contours of pressure, to reveal the large scale pressure pattern.

Now we'll look at what you can learn about the weather once you've done that.

1.
We'll start with the large nearly circular centers of High and Low pressure.  Low pressure is drawn below.  These figures are more neatly drawn versions of what we did in class.



Air will start moving toward low pressure (like a rock sitting on a hillside that starts to roll downhill), then something called the Coriolis force will cause the wind to start to spin (we'll learn more about the Coriolis force later in the semester). In the northern hemisphere winds spin in a counterclockwise (CCW) direction around surface low pressure centers.  The winds also spiral inward toward the center of the low, this is called convergence.  [winds spin clockwise around low pressure centers in the southern hemisphere but still spiral inward, don't worry about the southern hemisphere until later in the semester]


When the converging air reaches the center of the low it starts to rise.  Rising air expands (because it is moving into lower pressure surroundings at higher altitude), the expansion causes it to cool.  If the air is moist and it is cooled enough (to or below the dew point temperature) clouds will form and may then begin to rain or snow.  Convergence is 1 of 4 ways of causing air to rise (we'll learn what the rest are soon, and, actually, you already know what one of them is).  You often see cloudy skies and stormy weather associated with surface low pressure.

Everything is pretty much the exact opposite in the case of surface high pressure.


Winds spin clockwise (counterclockwise in the southern hemisphere) and spiral outward.  The outward motion is called divergence.

Air sinks in the center of surface high pressure to replace the diverging air.  The sinking air is compressed and warms.  This keeps clouds from forming so clear skies are normally found with high pressure.

Clear skies doesn't necessarily mean warm weather, strong surface high pressure often forms when the air is very cold. 

Here's a picture summarizing what we've learned so far.  It's a slightly different view of wind motions around surface highs and low.



2.
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. 


Closely spaced contours means pressure is changing rapidly with distance.  This is known as a strong pressure gradient and produces fast winds.  It is analogous to a steep slope on a hillside.  If you trip walking on a hill, you will roll rapidly down a steep hillside, more slowly down a gradual slope.

The winds around a high pressure center are shown above using both the station model notation and arrows. The winds are spinning clockwise and spiraling outward slightly.  Note the different wind speeds (25 knots and 10 knots plotted using the station model notation)



Winds spin counterclockwise and spiral inward around low pressure centers.  The fastest winds are again found where the pressure gradient is strongest.


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).

3.
The pressure pattern determines the wind direction and wind speed.  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).   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. 

Next we spent some time 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.

I wanted you to know something more about cold fronts because a pretty good example of a cold front passage would take place later in the day.

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. 


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.  Much more extensive cloudiness is found out ahead of a warm front that a cold front.  With the extensive and varied mix of clouds comes some very different types of precipitation (snow, sleet, freezing rain, rain etc). 

We look more at the weather changes associated with the approach and passage of a warm front and also at how to locate fronts on surface weather maps at the beginning of class next Tuesday.

And here is the answer to some questions found earlier in today's notes


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.