Thursday Apr. 16, 2015

Celtic music from Gaelic Storm "Johnny Jump Up", "Heart of the Ocean", "Before the Night is Over" my personal favorite, and from De Dannan "Hibernian Rhapsody" played before most of the 9:30 students had arrived.

The 1S1P reports on Rainbows, Mirages and the Green Flash were collected today.

I came to class having graded all of the Experiment, Scientific Paper and Book Reports in my possession.  Those of you picking up reports that were turned in last Thursday before the quiz or Tuesday this week have to weeks to revise and resubmit your report if you want to (it's not required).  Revised reports are due by Thu., Apr. 30.

Step #9 - Friction and surface winds

Upper level winds blow parallel to the contour lines.  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 and points 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.


The top figure shows upper level winds blowing parallel to straight contours.  The PGF and CF point in opposite directions and have the same strength.  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 (H or L pressure, S or N hemisphere, Rising or Sinking air)

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.



Winds blow into the centers of low pressure and outward away from centers of 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).

Upper level winds review
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.


Remember 1st that stationary air will start moving toward low pressure.  The dots in the figure above show this initial movtion.  Then the wind will turn to the right or left depending on the hemisphere.  This is the effect of the Coriolis force, it 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.

The same approach can be used with H pressure.  The initial motion is again toward L pressure which is on the outside of the picture.  The winds move away from the center then turn to the right or left depending on the hemisphere. 



The left figure above shows winds blowing around H pressure in the northern hemisphere.

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, as we have seen, cause winds to spin in opposite directions around large scale high and low pressure centers in the northern and southern hemisphere. 

Situations where the PGF is stronger than the Coriolis force
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. 


He we have clockwise and counterclockwise spinning motions around both H and L pressure.  The direction of the PGF is shown in all four cases (the PGF always point toward L pressure).  Remember you need an inward pointing force in order to keep something moving in a circular path.  The PGF can provide this needed force so either clockwise or counterclockwise motion is possible around L pressure. 

Spinning motion around H pressure isn't possible when just the PGF is present because there isn't any inward pointing force. 

Water draining from a sink or toilet - direction of spin
This is what happens when water drains from a sink or toilet.  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). 

We can check out this idea that water can spin in either direction when draining from a sink or toilet with an Optional Experiment/Assignment.   Basically you'll need to find and flush a toilet and watch what direction the water spins.  Then report your observation back to me by 5 pm this Sunday (April 19) (you'll need to send me an email).  Please let me know which class section you are in (8 am = Sect. 2, 9:30 am = Sect. 3).  I'll tabulate the results and let you know how things turn out next week.

What if just the Coriolis force were present?
The following figure was on the back of the class handout.

You'll find the answer at the end of today's notes.

1-cell model of the earth's global scale circulation
We can use the basic concept of a thermal circulation to learn about global scale pressure and wind patterns.  Ordinarily you couldn't apply a small scale phenomena like a thermal circulation to the much larger global scale.  However if we make some simplifying assumptions, particularly if we assume that the earth doesn't rotate or only rotates slowly, we can ignore the Coriolis force, and a thermal circulation could become established. 

Some additional simplifications are also made and are listed below.
 


Because the earth isn't tilted, the incoming sunlight shines on the earth most directly at the equator.  The equator will become hotter than the poles.  By allowing the earth to rotate slowly we spread this warmth out in a belt that circles the globe at the equator rather than concentrating it in a spot on the side of the earth facing the sun.  Because the earth is of uniform composition there aren't any temperature differences created between oceans and continents. 


3-cell model of the earth's global scale circulation
Next we will remove the assumption concerning the rotation of the earth.  We won't be able to ignore the Coriolis force now.

This isn't something we can easily work out, we need a computer to predict what would happen.  Things are pretty much the same at the equator in the three cell and one cell models: surface low pressure and rising air.  At upper levels the winds begin to blow from the equator toward the poles.  Once headed toward the poles the upper level winds are deflected by the Coriolis force.  There end up being three closed loops in the northern and in the southern hemispheres.  There are surface belts of low pressure at the equator (the equatorial low) and at 60 degrees latitude (the subpolar low). There are belts of high pressure (the subtropical high) at 30 latitude and high pressure centers at the two poles (the polar highs).

We will look at the 3-cell model surface features (pressure belts and winds) in a little more detail because some of what is predicted, even with the unrealistic assumptions, is actually found on the earth.

Surface wind and pressure belts
Here's a map view of the region between 30 S and 30 N latitude.


There's a lot of information on this picture, but with a little study you should be able to start with a blank sheet of paper and reproduce this figure.  I would suggest starting at the equator.  You need to remember that there is a belt of low pressure found there.  Then remember that the pressure belts alternate:  there are belts of high pressure at 30 N and 30 S.

Let's start at 30 S.  Winds will begin to blow from High pressure at 30 S toward Low pressure at the equator.  Once the winds start to blow they will turn to the left because of the Coriolis force.  Winds blow from 30 N toward the equator and turn to the right in the northern hemisphere (you need to turn the page upside down and look in the direction the winds are blowing).  These are the Trade Winds (northeasterly trade winds north of the equator and southeasterly trades south of the equator).  They converge at the equator and the air there rises (refer back to the crossectional view of the 3-cell model). This is the cause of the band of clouds that you can often see at or near the equator on a satellite photograph.  If that link doesn't work try this one.

The Intertropical Convergence Zone or ITCZ is another name for the equatorial low pressure belt.  This region is also referred to as the doldrums because it is a region where surface winds are often weak.  Sailing ships would sometimes get stranded there hundreds of miles from land.  Fortunately it is a cloudy and rainy region so the sailors wouldn't run out of drinking water (they might well have run out of rum though which they probably felt was worse).
  
Hurricanes form over warm ocean water in the subtropics between the equator and 30 latitude.  Winds at these latitudes have a strong easterly component and hurricanes, at least early in their development, move from east to west.  Middle latitude storms found between 30 and 60 latitude, where the prevailing westerly wind belt is found, move from west to east.

You find sinking air, clear skies, and weak surface winds associated with the subtropical high pressure belt.  This is also known as the horse latitudes.  Sailing ships could become stranded there also.  Horses were apparently either thrown overboard (to conserve drinking water) or eaten if food supplies were running low.  Note that sinking air is associated with the subtropical high pressure belt so this is a region on the earth where skies are clear (Tucson is located at 32 N latitude, so we are strongly affected by the subtropical high pressure belt).

The winds to the north of 30 N and to the south of 30 S are called the "prevailing westerlies."  They blow from the SW in the northern hemisphere and from the NW in the southern hemisphere. The 30 S to 60 S latitude belt in the southern hemisphere is mostly ocean.  Because there is less friction over the oceans, the prevailing westerlies there can get strong, especially in the winter.  They are sometimes referred to as the "roaring 40s" or the "ferocious 50s" (the 40s and 50s refer to the latitude belt they are found in).

This is as far as we got in class today and is as far as we will go on this topic.  I've put in some more information about the 3-cell model below which I will put in the category of Optional Reading





Here's the other surface map, it's a little simpler (it's a redrawn version of what was done in class).  We're just looking from about 30 N to a little bit past 60 N.  Winds blowing north from H pressure at 30 N toward Low pressure at 60 N turn to the right and blow from the SW.  These are the "prevailing westerlies."   The polar easterlies are cold winds coming down from high pressure at the north pole.  The subpolar low pressure belt is found at 60 latitude.  This is also a convergence zone where the cold polar easterly winds and the warmer prevailing westerly winds meet.  The boundary between these two different kinds of air is called the polar front and is often drawn as a stationary front on weather maps.  A strong current of winds called the polar jet stream is found overhead.  Strong middle latitude storms will often form along the polar front.

Ocean currents
The 3-cell model predicts subtropical belts of high pressure near 30 latitude.  What we really find are large circular centers of high pressure.  In the northern hemisphere the Bermuda high is found off the east coast of the US, the Pacific high is positioned off the west coast.  High pressure centers are found east and west of South America in the southern hemisphere.  Since I can't remember their names, you don't have to either.

Circular low pressure centers, the Icelandic low (off the east coast near Iceland and Greenland in the picture below) and the Aleutian low (off the west coast near the southern tip of Alaska), are found near 60 N.
 

The winds that blow around these large scale high pressure centers create some of the major ocean currents of the world.  If you remember that high pressure is positioned off the east and west coast of the US, and that winds blow clockwise around high in the northern hemisphere, you can determine the directions of the ocean currents flowing off the east and west coasts of the US.  The Gulf Stream is a warm current that flows from south to north along the east coast, the California current flows from north to south along the west coast and is a cold current.  A cold current is also found along the west coast of South America; winds blow counterclockwise around high in the southern hemisphere. 

The SW monsoon




Tucson gets about 12 inches of rain in a normal year (we are at about half of normal this year).  About half of this comes during the "summer monsoon" season.  The word monsoon, again, refers to a seasonal change in wind direction.  During the summer subtropical high pressure (the Pacific high) moves north of its normal position near 30 N latitude.  Winds on the southhern side of the subtropical high have an easterly component.   Moist air originating in Mexico and the Gulf of Mexico blows into Arizona.  The sun heats the ground during the day, warm moist air in contact with the ground rises and produces convective thunderstorms.

The close proximity of the Pacific high, with its sinking air motions, is what gives California, Oregon, and Washington dry summers.

In the winter the subtropical high moves south of 30 N latitude.  Winds to the north of the high blow from the west.  Air originating over the Pacific Ocean loses much of its moisture as it crosses mountains in California (remember the rain shadow effect).  The air is pretty dry by the time it reaches Arizona.  Significant winter rains occur in Arizona when storms systems are able to draw moist subtropical air from the southwest Pacific ocean into Arizona.
Here's the answer to the question on the back of the handout distributed 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.