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   Circumzenithal Arc by Matthew Gingerich

      Winter is a good time to look for many of the optical phenomena which occasionally brighten our skies One of the more rare sights we might see is that of the circumzenithal arc, pictured above.  This is rather like an upside down rainbow.
     The circumzenithal arc is centered on the zenith and is located at least 46 degrees above either the sun or moon.  It is always high up in the sky when the sun is low and is formed when light is bent, or refracted, as it passes through 90 degree prisms of ice crystals.  Light enters these crystals through their horizontal bases and passes out through their vertical sides.
     Under usual conditions, the ice crystals responsible for the formation of the circumzenithal arc are rather large, oriented, six-sided (hexagonal) plates.  The arc pictured above is unusually bright, as such arcs are more often faint and usually short-lived.  The ice crystals which formed this arc compose some very thin cirrus clouds.


Sun pillar at sunrise by Nikki Brubaker
 
     Another rather rare sight is the sun pillar.  It is in the form of a column, or pillar, of light which may extend either above or below the sun.  The one pictured extends from the rising sun to some cumulus clouds in the center of the picture.
     A sun pillar is formed when sunlight is reflected by the sides of ice crystals which are shaped like little columns and are falling with their long axes horizontal.  Sometimes, pillars of light similar to a sun pillar are also formed by street lights.  These phenomena are known simply as light pillars. 
 


Louisville, Kentucky Radar image of a Thunderstorm Squall Line
 
     Thunderstorms are usually categorized as being either of the air mass or frontal type.  Air mass thunderstorms are those which pop up in a warm, humid air mass when no frontal boundary or low pressure system is nearby.  Frontal thunderstorms are those which form in connection with a front such as a warm, cold, stationary, or occluded front.
     However, thunderstorms may form into squall lines where a number of individual storms march side by side in a more or less solid line such as in the radar image above.  Another type of thunderstorm formation is the mesoscale convective complex, or MCC.  Mesoscale refers to a system which is smaller than the usual low pressure areas with their associated frontal systems but larger than a single individual thunderstorm.
     An MCC is a group of thunderstorms linked together in a more or less circular area which may be as large as or larger than an individual state.  Thunderstorms in an MCC interact with each other and affect the atmospheric circulation over quite a large area.  They produce a general, but by no means uniform, rainfall over the entire area covered by the MCC.
     Such a group of thunderstorms may be as much as 1,000 times the size of an individual thunderstorm, and the movement of an MCC is rather slow, averaging about 20 mph.  Thus, the area over which the complex passes gets a good soaking which may last from 12 to 24 hours.  Providing atmospheric conditions are right, an MCC can even redevelop the next day.
     Flash flooding is often a strong possibility with an MCC, as rainfalls from such a complex can approach hurricane rainfalls at times.  Somewhere between about 30 and 80 of these mesoscale convective complexes occur accross the eastern two-thirds of the United States every year, and the majority of these form at night. 
     Actually, an MCC is a kind of subclassification of a mesoscale convective system.  One other form such a system may take is that of a derecho.  Technically, a derecho consists of a family of strong thunderstorm clusters which produce downburst winds - strong downdrafts which often produce considerable damage.




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Bow echo
 
     Still another form thunderstorms may take is that of a bow echo as seen on radar such as the one shown above.  A bow echo is a line of storms which takes on the shape of a bow.  Bow echoes can range anywhere from about 12 to 120 miles long and usually last between 3 and 6 hours.  Damaging winds and even small tornadoes are frequently associated with bow echo thunderstorms.
     Bow echoes feature a strong current of air flowing into them from the rear - a rear-inflow jet.  This rear-inflow jet is actually a response to the tilting of the storm updrafts which result in a circulation that draws air from midlevels into the storms.  Such a rear-inflow jet brings potentially cold, dry air into the storms at midlevels, and this results in the production of strong updrafts.
     Another feature of bow echo thunderstorms is what is known as book-end vortices (little whirls at the ends of the bow echo line).  On the northern end of the line, the book-end vortex usually spins counterclockwise (cyclonically), but it often spins clockwise (anticyclonically) on the southern end of the line.  These vortices are usually at peak strength between 1 and 21/2 miles above the ground but have been observed near the surface and also as high as five miles above ground.





Missouri Ice Storm of 1949
 
     Many different kinds of precipitation may fall during the autumn, winter and early spring months.  One of those forms is freezing rain, which forms a coating of glaze on all cold outdoor surfaces.  In order for precipitation to fall in the form of freezing rain, there must be a layer of warmer air aloft overlying a fairly shallow layer of colder (32 degrees Fahrenheit or lower) surface air.  Also, there must be relatively few of the submicroscopic and microscopic freezing nuclei in the air because these nuclei cause water drops to turn into ice crystals.
     Freezing rain may begin aloft as either ordinary rain or as snowflakes which melt in their descent and become liquid water drops.  However, as the liquid water drops fall through the shallow layer of colder surface air, the drops become supercooled (cooled below the freezing point without turning to ice.  When these supercooled drops strike a cold surface such as a tree limb or sidewalk, they immediately turn to ice and glazing begins to take place.  If the freezing rain continues long enough, an ice storm occurs.  Sometimes, coatings of ice two inches or more thick will form, resulting in damage such as that shown in the photo above.
     Sometimes, however, if numerous freezing nuclei are present and if the layer of cold air at the surface is deeper, ice pellets, commonly called sleet, fall.  Ice pellets are formed from the freezing of raindrops or refreezing of melted snowflakes.
     Ice pellets, or sleet, are composed of translucent or transparent pellets of ice which bounce when they hit the ground.  These ice pellets are usually irregular in shape but may be spherical and even, in rather rare cases, conical.  In order to be called ice pellets, the grains of ice must be .20 inch or smaller in size.  Anything larger than this is called hail.
     Occassionally, snow pellets, which are often known as tapioca snow, become encased in a thin layer of ice either from melting and refreezing of their surface layer or from coming in contact with supercooled water drops.  Snow pellets with this thin outer layer of ice are also classed as a form of ice pellets and are still sometimes referred to as small hail.  Small hail has a diameter of less than one-quarter inch.
     Graupel is something similar to small hail but consists of snow particles .20 inch or less in size and heavily coated with a layer of opaque, granular rime from coming into contact with supercooled water crops.  Unlike ice pellets, graupel usually does not bounce upon hitting the ground.




Moon Corona
 
     One of the more (and rarer) sights you may find in the sky is the corona.  A corona is a set of one or more colored rings around either the sun or moon when thin clouds cover them.  The color sequence runs from bluish-white inside to reddish outside and may be repeated.  The inner part of the corona, which is a bluish-white disc around sun or moon, is known as the aureole and may often be seen by itself without the other colored rings which compose the corona.
     Coronas are caused by diffraction of light as it passes by the edges of water drops or dust particles.  As light waves pass by the edges of water drops, they are bent slightly, causing the light to break apart somewhat into different colors.  Best colors and most discernable rings are found in coronas when water drop sizes in any certain part of a cloud are nearly uniform in size.  Rings of a corona are also most nearly circular if the drops forming different parts of the corona are spaced about the same distance apart.  When there is quite a range of water drop sizes, corona rings may be clear, but the colors become rather faint.
     Size of corona rings depends on two main factors.  One of these is the wavelength of the light, and the other is drop size.  Smaller water drops produce larger rings, while larger drops produce smaller rings.  Cloud drops of less than .00006 inch are responsible for coronas.  Moon coronas, such as the one shown above, are more easily seen simply because the moon is less bright than the sun.
     Certain types of clouds are needed for good corona displays.  Among the best cloud types are lenticular clouds, certain types of altocumulus and cirrocumulus clouds and some cirrus clouds.  If cirrus and cirrocumulus clouds form when the jet stream happens to be passing almost overhead, conditions are about as ideal as they will get for corona formation.




            
Snowflake photo by Wilson Bently            Snowflake photo by Charles Russell
 
     Snow comes in many forms.  Most commonly, the type of snow usually coming down upon us is composed of broken single crystals, fragments of crystals, or clusters of these former two.  Snowflakes such as those pictured above are not so frequently found in snowstorms as more quiet air is needed for these.  Snowflakes composed of bunches, or aggregates, of ice crystals are rather common.  There may be over 100 ice crystals in one snowflake aggregate, and larger snowflake aggregates can have thousands of crystals in them.  These aggregates are composed of thin, platelike crystals or tiny needles.  Clouds filled with tiny water droplets give birth to these types of snowflake aggregates.
     Clouds filled with tiny ice crystals, rather than water droplets, give rise to a different kind of snowflake aggregate.  Aggregates which come from such clouds are composed of thick, platelike crystals or crystals which are shaped like columns. 
     Usually, snowflakes have six sides, but there have been occasions on which three-sided and five-sided snowflakes have been found.  The six-sided (hexagonal) snowflakes form in air temperatures of approximately 27 to 32 degrees, providing certain other conditions are met.  Between about 23 and 27 degrees, ice needles form, while hollow columnar prisms form from about 18-23 degrees.  Between about 10 and 18 degrees hexagonal plates of a different nature are formed.  Stars with a fernlike structure form  between approximately 3 and 10 degrees, more plates form from about -13 to 3 degrees, and below this the hollow columnar prisms again form.  The final form each snowflake assumes (hexagonal, columnar, plate, etc.) is determined by temperature, humidity and atmospheric pressure.
     No matter in what form the snowflakes come, a lot of those little crystals are needed to really cover the ground.  One estimate by Vincent J. Schaefer is that over 1,000,000 of them are needed to cover an area two feet square to a depth of 10 inches.  Despite all this, no two snowflakes have ever been found exactly alike.
     The basic atomic structure of a snowflake consists of one oxygen atom in the middle of two hydrogen atoms bonded by electric charges at 120 degree angles from the oxygen atom.  In just one snowflake, there can be as many as 100,000,000 molecules like this!
     Generally speaking, the largest and fanciest flakes form with cloud temperatures near or slightly below the freezing point when there is abundant moisture present.  Smaller and less intricate snowflakes form at lower temperatures in drier air.  However, the air never gets so cold that snow cannot fall.
     When a faling snowflake falls into air that is warmer than the snowflake, rays form outward until the crystal temperature reaches the air temperature.  Once that happens, another new pattern of growth begins.  Snowflakes moslty fall with their broadest surfaces almost horizontal, and they rotate so that the edges of the flakes come into the greatest contact with the water vapor.  Therefore, the growth on a snowflake takes place primarily on the edges.  Snowflakes become coated with rime when they fall through a layer of supercooled water droplets (droplets which have cooled below the freezing point but have not turned into ice).  The moment a cold snowflake comes into contact with a supercooled water droplet, the droplet freezes onto the snowflake, and soon a layer of rime is added to the snowflake.
     Snow is usually thought of as being white.  However, there can be pink, red, yellow, green, or blue snow.  These colors are usually due to algae, and there are some algae which live their whole lives in snowbanks.  Red snow may also be caused by particles of red dust.