Thursday Nov. 14, 2013

Celtic music before class today featuring De Danann: "Hibernian Rhapsody" and Gaelic Storm: "Black is the Colour", "Lover's Wreck", and "Human to a God".

The link to a list of students that have earned 45 1S1P pts is now active (students on this list don't need to write any additional reports).  I'll update the list periodically as we make our way through the many piles of reports.

I'm still hoping to have most of the rest of the work you have turned in graded in time to handout updated grade summaries next Tuesday.  If not Tuesday then Thursday.

We started class with Step #10 in our attempt to understand why winds blow the way they do - at upper levels and at the ground, around High and Low pressure, and in the northern and southern hemispheres.  Step #10 is shown on p. 129 in the ClassNotes.


You ought to be able to figure out (1) whether these are surface or upper level winds.  Upper level winds blow parallel to the contours on a weather map, surface winds blow across the contours.  These are surface winds.  And because the winds always blow across the contours toward low pressure it is easy to figure out (2) which of the figures are centers of low pressure (the wind blows inward toward the center of the picture)  The winds are spiraling inward in the top and bottom examples.  These must be surface centers of low pressure.  The winds are spiraling outward from the centers of high pressure (2 and 4).



I used to recommend that you not try to figure out (3) which of these are northern and which are southern hemisphere pictures.  I would suggest it might be best to just remember one of the pictures.  Early in the semester we learned that surface winds spin counterclockwise and spiral inward around centers of low pressure in the northern hemisphere.
 


Then remember that winds spin in the other direction and blow outward around high pressure in the northern hemisphere.  The spinning directions of the winds reverse when you move from the northern to the southern hemisphere.  Thus you find clockwise spinning winds and inward motion around low pressure and counterclockwise and outward spiraling winds around high pressure in the southern hemisphere.


Converging winds cause air to rise.  Rising air expands and cools and can cause clouds to form.  Clouds and stormy weather are associated with surface low pressure in both hemispheres.  Diverging winds created sinking wind motions and result in clear skies.

Somethings change when you move form the northern to the southern hemisphere (direction of the spinning winds).  Sometimes stay the same (winds spiral inward around centers of low pressure in both hemispheres, rising air motions are found with centers of low pressure in both hemispheres).

Next we examined a common misconception involving the Coriolis force.  You might have heard that water spins in a different direction when it drains from a sink or a toilet bowl in the southern hemisphere than it does in the northern hemisphere.  You might also have heard that this is due to the Coriolis force or the Coriolis effect. 

The Coriolis force does cause winds to spin in opposite directions around large scale high and low pressure centers in the northern and southern hemisphere.  The PGF starts the air moving (in toward low, out and away from high pressure) then the Coriolis force bends the wind to the right (N. hemisphere) or to the left (S. hemisphere).

Here's what you end up with in the case of low pressure. 



Just like a rock will always start to roll downhill and never uphill, air starts to move inward toward low pressure (the dots show this initial motion).  Then the Coriolis force causes it to turn to the right or left depending on which hemisphere you're in.  You should be able to say which of the pictures above is the northern hemisphere and which is the southern hemisphere picture.



The same kind of idea applies to high pressure except that the air starts moving outward.  The Coriolis force then turns it to the right or left.

Note that in all four cases above the inward pointing force is stronger than the force pointing outward.  This is what provides the net inward force needed for something to move in a circular path.

The figures above were actually on a handout distributed in class that looked like this.


Remember the term cyclone refers to winds spinning around a center of low pressure.  Anticyclone refers to high pressure.

There are situations where the PGF is much stronger than the CF and the CF can be ignored.  A tornado is an example.  The PGF is much much stronger than the CF and the CF can be ignored. 



You need an inward pointing force in order for wind to blow in a circular path.   It is possible for winds to spin around low pressure.  The inward pointing PGF provides the inward force that is needed.  The PGF points outward around centers of high pressure.  Winds can't spin around high pressure when there is just a pressure gradient force.  There's no inward force.

This is what happens when water drains from a sink or toilet.  The water can spin in either direction in either hemisphere.  What causes the inward pointing PGF?


The water at the edges of the spinning water is a little deeper than in the middle.  Since pressure depends on weight, the pressure at the outer edge of the spinning water is higher than in the center.  This creates the inward pointing pressure gradient (pressure difference) force.


Here's a picture of the "Old Sow" whirlpool in the Bay of Fundy and apparently the largest whirlpool in the Western Hemisphere (source). 

Water draining from a sink or toilet can spin in either direction.  It doesn't matter where you're located. 

But this something we should probably checkout for ourselves, so here is probably my favorite Optional Assignment of the semester.  You'll need to find and flush a toilet and watch what direction the water spins.  Then report back to me by midnight this Friday, Nov. 15 (you'll need to send me an email).

The following figure was on the back of the class handout.


What if there is just Coriolis Force and no pressure gradient force.  Which of the types of motion shown above would be possible.  This a good question.  It looks hard.  But by remembering and using just a couple of the basic concepts that we have been covering you can answer the question.  Two of the scenarios shown above are possible  Which two are they.  To be able to answer the question you need to remember the rule for the direction of the Coriolis foce and you need to remember what is needed to keep winds blowing in a circular path.  Think about this question for a minute or two.  You'll find the answer at the end of today's notes.

Next we went over a brief explanation of what causes the Coriolis force.  You'll find that stuck on the end of the notes from Tuesday's class.

Now back to surface winds again.  It's pretty easy to identify surface winds and to determine whether you're dealing with a center of high or low pressure.

If you remember that surface winds blow across the contours always in the direction of low pressure



you should be able to figure out that this shows surface winds blowing around a center of low pressure (surface winds always cross the contours toward low pressure).  But is it a northern or southern hemisphere situation?  I've come to the realization that this is also pretty easy to figure out also.


Imagine you're approaching the low pressure center and want to merge with the existing winds (kind of like approaching a traffic circle in your car and wanting to merge with the traffic that is already there).  Would you need to turn to the left or the right as you approach?  I think it's pretty clear you'd need to turn left.  So in addition to remembering that surface winds blow across the contours always toward low pressure if you remember that the Coriolis force acts to the left of the wind in the southern hemisphere and to the north of the wind in the northern hemisphere


you can quickly figure out this is a southern hemisphere map. 

We're getting close enough to the end of the semester that I have a pretty good idea what we will be covering between now and then.  Here are the topics that will lead up to Quiz #4.

Today:  Thunderstorms pt. 1
Tuesday (Nov. 19): Thunderstorms pt. 2 & Tornadoes pt. 1
Thursday (Nov. 21): Tornadoes pt. 2 & Lightning pt. 1
Tuesday (Nov. 26): Lightning pt. 2 & Hurricanes pt. 1
Thursday (Nov. 28): Thanksgiving
Tuesday (Dec. 3): Hurricanes pt. 2

Today and part of next Tuesday will be devoted to thunderstorms.  Here's a little bit of an introduction (found on p. 150 in the ClassNotes)



Thunderstorms come in different sizes and levels of severity.  We will mostly be concerned with ordinary single-cell thunderstorms (also referred to as air mass thunderstorms).  They form in the middle of warm moist air, away from fronts.  Most summer thunderstorms in Tucson are this type.   An air mass thunderstorm has a vertical updraft.  A cell is just a term that means a single thunderstorm "unit" (a storm with an updraft and a downdraft).

Tilted updrafts are found in severe and supercell thunderstorms.  As we shall see this allows those storms to get bigger, stronger, and last longer.  The tilted updraft will sometimes begin to rotate.  We'll see this produces an interesting cloud feature called a wall cloud and maybe tornadoes.  Supercell thunderstorms have a complex internal structure;  we'll watch a short video at some point that shows a computer simulation of the complex air motions inside a supercell thunderstorm.

We won't spend anytime discussing mesoscale convective systems except to say that they are a much larger storm system.  They can cover a large portion of a state.  They move slowly and often thunderstorm activity can persist for much of a day.  Occasionally in the summer in Tucson we'll have activity that lasts throughout the night.  This is often caused by an MCS.

The following somewhat tedious material was intended to prepare you to better appreciate a time lapse video movie of a thunderstorm developing over the Catalina mountains.  I don't expect you to remember all of the details given below.  The figures below are more carefully drawn versions of what was done in class.


Refer back and forth between the lettered points in the figure above and the commentary below.

The numbers in Column A show the temperature of the air in the atmosphere at various altitudes above the ground (note the altitude scale on the right edge of the figure).  On this particular day the air temperature was decreasing at a rate of 8 C per kilometer.  This rate of decrease is referred to as the environmental lapse rate (lapse rate just means rate of decrease with altitude).  Temperature could decrease more quickly than shown here or less rapidly.  Temperature in the atmosphere can even increase with increasing altitude (a temperature inversion).

At Point B, some of the surface air is put into an imaginary container, a parcel.  Then a meteorological process of some kind lifts the air to 1 km altitude (in Arizona in the summer, sunlight heats the ground and air in contact with the ground, the warm air becomes buoyant - that's called free convection).  The rising air will expand and cool as it is rising.  Unsaturated (RH is less than 100%) air cools at a rate of 10 C per kilometer.  So the 15 C surface air will have a temperature of 5 C once it arrives at 1 km altitude. 

Early in the morning "Mother Nature" is only able to lift the parcel to 1 km and "then lets go."  At Point C note that the air inside the parcel is slightly colder than the air outside (5 C inside versus 7 C outside).  The air inside the parcel will be denser than the air outside and the parcel will sink back to the ground. 

By 10:30 am the parcel is being lifted to 2 km as shown at Point D.  It is still cooling 10 C for every kilometer of altitude gain.  At 2 km, at Point E  the air has cooled to its dew point temperature, the relative humidity is now 100%,  and a cloud has formed.  Notice at Point F, the air in the parcel or in the cloud (-5 C) is still colder and denser than the surrounding air (-1 C), so the air will sink back to the ground and the cloud will disappear.  Still no thunderstorm at this point.


At noon, the air is lifted to 3 km.  Because the air became saturated at 2 km, it will cool at a different rate between  2 and 3 km altitude.  It cools at a rate of 6 C/km instead of 10 C/km.  The saturated air cools more slowly because release of latent heat during condensation offsets some of the cooling due to expansion.  The air that arrives at 3km, Point H, is again still colder than the surrounding air and will sink back down to the surface.

By 1:30 pm the air is getting high enough that it has become neutrally buoyant, it has the same temperature and density as the air around it (-17 C inside and -17 C outside).  This is called the level of free convection, Point J in the figure.

If you can, somehow or another,  lift air above the level of free convection it will find itself warmer and less dense than the surrounding air as shown at Point K and will float upward to the top of the troposphere on its own.  This is really the beginning of a thunderstorm.  The thunderstorm will grow upward until it reaches very stable air at the bottom of the stratosphere.

This was followed by a time lapse video showing a day's worth of work leading eventually to the development of a thunderstorm.

Here's the answer to the question about spinning winds that would be possible if just the Coriolis force was present.


What we've done is draw in the direction of the Coriolis force for each of the four examples above.  The CF is perpendicular and to the right of the wind (as you look downstream) in the Northern Hemisphere and to the left in the Southern Hemisphere.  The CF points inward in examples (b) and (d) and could supply the net inward force needed to keep air spinning in a circular path.  The winds in (a) and (c) would not be possible because is no inward pointing force.