Thursday Nov. 12, 2015
Being featured for the 1st time in ATMO 170, several songs from Tracy Chapman: "The Love That You Had", "Telling Stories", "She's Got Her Ticket", "Save Us All", "Change"

More or less up to date grade summaries were handed out in class today.  I haven't had time yet to grade the revised Expt. #2 reports yet so they aren't included.  Neither are the 1S1P reports on Fog and Satellite Photographs of Clouds. 

There is a new 1S1P Assignment available.  You can earn either 1S1P pts or Extra Credit pts.  The assignment is due next Thursday, Nov. 19.

One of the coolest nights of the semester last night, though it didn't get as cold as predicted in many parts of town.  The figure below shows one reason that may have been the case.

Clear skies, very dry air, and calm winds were expected overnight.  Those are ideal conditions for the development of a temperature inversion like is shown in the left figure above (temperature increases with increasing altitude in a temperature inversion).  Some light breezes developed early this morning in many parts of town.  What they do is mix air next to the ground with air higher up.  The three layers of 35 F, 40 F, and 45 F air in the left picture were mixed together.  The result was a thicker layer of slightly warmer, 40 F, air.

It is supposed to warm up somewhat (highs of 76 F and 77 F being predicted for Friday and Saturday) before another storm moves through the state.  A low of 33 F is being predicted for next Monday night, so it looks like another cold bicycle ride into campus next Tuesday morning for class.

Upper level winds review
Here's a quick review of much of what we covered in class on Tuesday.  Many of the figures below were on a handout distributed in class today.
Winds spin counterclockwise around L pressure in the northern hemisphere then switch direction and spin clockwise around L pressure in the southern hemisphere.   I think by just remembering a couple of things you can figure this out rather than just trying to memorize it.

The pressure gradient will start stationary air moving toward low pressure (just like a rock placed on a slope will start to move downhill)

The dots in the figure above show this initial motion and its in toward the center of the picture.  These must both be centers of Low pressure.  Then the wind will turn to the right or left depending on the hemisphere.  This is the effect of the Coriolis force, the CF turns wind to the right in the northern hemisphere and to the left in the southern hemisphere.   



The northern hemisphere winds are shown at left in the figure above, the southern hemisphere winds are shown at right.  The inward pointing force is always stronger than the outward force so that there is a net inward pointing force.

This initial motion is outward away from the center in the two figures below. 

Low pressure is on the outside edges of the picture.  High pressure must be found in the center of both pictures.



The outward moving air takes a right turn in the left figure above, a left turn in the right figure (you may need to rotate the picture so that you are looking downstream, in the direction the wind is blowing to clearly see the left hand turn).

Friction and surface winds

Next we'll try to understand why friction causes surface winds to blow across the contour lines (always toward low pressure).

With surface winds we need to take into account the PGF, the CF, and the frictional force (F).  That means we'll need some rules for the direction and strength of the frictional force.  Friction arises with surface winds because the air is blowing across (rubbing against) the earth's surface.



You're probably somewhat familiar with the effects of friction.  If you stop pedaling your bicycle on a flat road you will slow down and eventually come to a stop due to air friction and friction between the tires and road surface.  Friction always acts to slow a moving object it must point in a direction opposite the motion.

The strength of the frictional force depends on wind speed.  The faster you try to go the harder it becomes because of increased wind resistance.  It's harder to ride on a rough road than on a smooth road surface.  In the case of air there is less friction when wind blows over the ocean than when the air blows over land.  If the wind isn't blowing there isn't any friction at all.



The top figure (p. 128 in the ClassNotes) shows upper level winds blowing parallel to straight contours.  The PGF and CF point in opposite directions and have the same strength (the fact that there are only two forces present tells you these are upper level winds).  Note the CF is to the right of the wind, this is a northern hemisphere case.  The total force, the net force, is zero.  The winds would blow in a straight line at constant speed. 

We add friction in the second picture.  It points in a direction opposite the wind and acts to slow the wind down. 

Slowing the wind weakens the CF and it can no longer balance the PGF (3rd figure).  The stronger PGF causes the wind to turn and start to blow across the contours toward Low.  This is shown in the 4th figure. 


Step #10 - Surface winds blowing around H & L pressure in the N. & S. hemispheres.
I think you'll be surprised at how easy it is to determine whether each of the figures below (p. 129 in the ClassNotes) is a surface center of H or L pressure, found in the N or S hemisphere, and whether rising or sinking air motions/clear or cloudy skies would be associated with each figure.

Key point to remember: surface winds blow across the contours always toward low pressure.



It should be very easy to figure out which two of the figures above are surface centers of low and high pressure.




Next to determine whether each figure is in the northern or southern hemisphere we will imagine approaching the upper left figure in an automobile.  We'll imagine it's a traffic circle and the arrows represent cars instead of wind.


You're approaching the traffic circle, what direction would you need to turn in order to merge with the other cars.  In this case it's left.  That left turn is the Coriolis force at work and tells you this is a southern hemisphere map.

The remaining examples are shown below


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).

Situations (small scale) where the PGF is stronger than the Coriolis force
The situations we have been looking at so far are representative of large "country size" storm systems.  There are situations, though, where the PGF is much stronger than the CF and the CF can be ignored.  A tornado is an example.  Spinning water draining from a sink or toilet is another.  The PGF is much much stronger than the CF and the CF can be ignored. 





Large scale winds upper level winds blowing around Low pressure.  You  must take into account both PGF and CF forces. Winds only spin in a CCW direction around L in the northern hemisphere and change direction in the southern hemisphere.
 A net inward force is need to keep winds spinning in a circular path.  The inward pointing PGF provides the needed net inward force in this case and winds can spin in either direction around the L in either hemisphere.



Water draining from a sink or toilet - direction of spin
This is what happens when water drains from a sink or toilet.  The PGF is present, there is no CF.  The water can spin in either direction in either hemisphere.  It might not be obvious though what causes the inward pointing PGF in the case of spinning water.




If you look carefully at some spinning water you'll notice the surface has a "bowl" or "funnel" shape as sketched above.  The water at the edges is a little deeper.  That additional water has more weight and produces more pressure.  The water in the middle is shallower, doesn't weigh as much and the pressure is lower.  Thus there is a PGF pointing from the edges into the center of the vortex.


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

Here are the results from the toilet bowl flushing experiment and results from the past several semesters.  I've been conducting the toilet experiment for the past few years.  We should expect to see roughly equal numbers of reports of clockwise (CW) and counterclockwise (CCW) spin.  Some years, for some reason, one direction seems to prevail.  In Fall 2014 there were almost twice as many CCW reports as there were CW.

CW spin
 CCW spin
Fall 2013
 35
 29
Spring 2014
 24
 17
Fall 2014
 12
 24
Spring 2015
 17
 26
subtotal
 88
 96
Fall 2015
 experiment still in progress

This semester's experiment is still ongoing and the number of responses has increased a lot since class this morning.  I'll let you know the final results next week.

It isn't possible to find spinning winds around high pressure when the CF is not present




The CF plays an important role here, it is the force that provides the net inward force needed to keep the winds blowing in a circular trajectory.
 With just the PGF there's nothing to provide a net inward force.  Circular winds around centers of high pressure is not possible when there is no CF.


What if just the Coriolis force were present?
The following figure is on the back of the class handout.  Which of these would be possible if just the CF were present?

The CF is perpendicular and to the right of the wind in the northern hemisphere, to the left of the wind in the southern hemisphere.  A net inward force is needed to keep wind blowing in a circular trajectory.  That's all you need to know to be able to answer this question.  You'll find the answer at the end of today's notes.

Thunderstorms introduction




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.  In class I showed a gallery of storm images that were taken by Mike Olbinski.  The 1st and 5th images in the gallery show the base of a supercell thunderstorms photographed in Texas with wall clouds.  There are additional images further down in the gallery.

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 buildup to an air mass thunderstorm
The following somewhat tedious material is intended to prepare you to better appreciate a time lapse video movie of a thunderstorm developing over the Catalina mountains.  The newest 1S1P/Optional Assignment makes uses of a couple of the numbers below (the rates of cooling of rising parcels of unsaturated and saturated air).


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 air (RH is less than 100%) 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.  You can't see this because the air is clear, invisible.

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.  A dew point temperature of -5 C was used in this example.  It could be warmer or colder than that. 

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 kilometers altitude.  Saturated air 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, it doesn't need Mother Nature's help anymore.  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 the rising air will quickly become colder and denser than the surrounding air if it travels into the stratosphere).

We ended today's class with a time lapse video showing a day's worth of work leading eventually to the development of a thunderstorm over the Catalina mountains north of Tucson. 

Here's the answer to the question on the back of the handout distributed in class today (I forgot to show it in class today)

The Coriolis force is perpendicular to the wind and to the right in the northern hemisphere, perpendicular and to the left in the southern hemisphere.  Spinning motions would be possible in Figs.  b and d work because the Coriolis force is pointed inward.  An inward force is needed to keep something moving in a circular path.