Geography 140
Introduction to Physical Geography

Lecture: Storms as an Element of Weather

--------------------
  V. Storms are the last of the weather elements and, in a manner of speaking, 
     they are really nothing more than particular configurations of the other 
     three elements. Because of their importance, people have always been 
     trying to predict them and understand them. A big breakthrough toward 
     that goal was made in WWI by Jacob Bjerknes and his colleagues in Norway. 
     They developed something called "Air Mass Analysis."
     A. Air masses
        1. These are defined as bodies of air with uniform temperature 
           structures and humidity characteristics.
        2. These properties derive from their "source regions," The source 
           regions yield a two way system of simple air mass analysis: 
           a. The first dimension of the classification system is the latitude 
              zone of source region, which governs the temperature 
              characterisitics of an air mass.  There are four of these zones 
              in the Bjernkes system:
                i. Equatorial (E)
               ii. Tropical (T)
              iii. Polar (P)
               iv. Arctic or Antarctic (A or AA)
                   [ Bjerknes' air mass map ]
           b. The underlying surface of the source region also affects the 
              characteristics of the resulting air masses:
                i. Maritime or oceanic surfaces (m), which create relatively 
                   humid air masses
               ii. Continental or land surfaces (c), which create relatively 
                   dry air masses
           c. So, in Bjerknes' system, there are six basic air mass types:
                i. Arctic or Antarctic air (abbreviated A or AA):
                   a. Extremely cold
                   b. Very dry (because of the extreme cold)
               ii. Polar continental air (abbreviated cP or Pc):
                   a. Very cold, having developed over northern Canada and 
                      Russia
                   b. Very dry, partly because of the cold and partly from 
                      having developed over land.
                   c. Yes, the terminology is a little confusing: "Polar" air 
                      isn't the air mass that develops over the poles, because 
                      THAT air is called Arctic or Antarctic....
                   d. Somehow I think of a Lincoln in Ottawa .....
              iii. Polar maritime air (abbreviated mP or Pm):
                   a. Very cool because of the high latitude, but not as cold 
                      as polar continental air because of the moderating 
                      influence of the sea and the warm currents at these 
                      latitudes (the North Pacific Drift that continues the 
                      Japan or Kuroshio Current and the North Atlantic Drift 
                      that continues the Gulf Stream)
                   b. Rather dry because of the coolness but not as dry as the 
                      polar continental air because of evaporation from the 
                      drifts
               iv. Tropical continental (a Lincoln in Dallas? abbreviated cT 
                   or Tc):
                   a. Warm because of the lower latitude
                   b. Dry because it formed over land
                v. Tropical maritime (abbreviated mT or Tm):
                   a. Very warm because of the lower latitude
                   b. Very humid because of the tropical waters below it
               vi. Equatorial (abbreviated E)
                   a. Hot!
                   b. Extremely humid
                   c. Continental is not differentiated from maritime here 
                      because there's relatively little land along the equator 
                      and much of that land is covered with tropical 
                      rainforest, which evapotranspires as much water vapor 
                      into the air as an equivalent area of ocean water.
        3. Air masses usually have sharp boundaries between themselves. Air 
           masses of different characteristics, therefore, tend not to mix. 
           This boundary is called a "front" (the rather militaristic 
           vocabulary here may have had something to do with the pressure of 
           circumstances!).
     B. Fronts
        1. It is the fronts between air masses that create storms. This 
           happens when one mass pushes into another along their front and one 
           is forced upwards. This forced uplift produces adiabatic cooling 
           and so precipitation and storms.
        2. There are three basic types of front in air mass analysis:
           a. A cold front, where the invading air mass is colder and, 
              therefore, denser than the one it invades.
                i. This produces fast uplift.
               ii. Uplift produces a "squall line" of cumuliform clouds.
              iii. Precipitation, then, is intense, due to the sharp uplift -- 
                   and it can fall on both sides of the front's contact with 
                   the ground.
               iv. Because cold air is denser than the warm air it is 
                   invading, it can push the more buoyant warm air out of the 
                   way very fast. Cold fronts are, therefore, fast moving.
           b. A warm front, in which the invading air mass is warmer and, 
              therefore, lighter than the one it's invading.
                i. This produces a slow, low angle, gradual uplift.
               ii. This kind of gentle uplift creates stratiform clouds 
                   blanketing huge areas. At the forward edge, there are 
                   cirrus clouds of various types, which are replaced by alto-
                   stratus, stratus, and finally nimbo-stratus clouds.  Now, 
                   you can see why the appearance of certain kinds of clouds 
                   is associated with the advent of stormy weather.
              iii. These stratiform clouds yield less intense precipitation in 
                   a large area ahead of the front's contact with the ground.
               iv. Because warm air, particularly warm humid air, is lighter 
                   than the air it's moving into, it meets strong resistance. 
                   Warm fronts are, accordingly, slow moving.
                v. Warm fronts are found east of cold fronts.

                   [ cold front and warm front, Jim Pejsa, 1999 ]

           c. An occluded front occurs when a fast moving cold front to the 
              west catches up with a slower moving warm front to the east. 
                i. The cold air behind the cold front links up with the cold 
                   air ahead of the warm front.
               ii. The warm air behind the warm front is caught in between 
                   and, being more buoyant, is forced up off the ground 
                   completely.  You can't perceive it from the ground anymore:  
                   It is hidden (hence the name, "occluded," for "hidden").
              iii. This sudden uplift leads to intense rain.
               iv. But then the warm air cools adiabatically and eventually 
                   stabilizes, stops rising, and so the storm dies. 
     C. There are different types of storm.  The variations are related to 
        latitude, as well as severity.  At first, it was thought that the 
        mechanisms of air mass analysis could explain all of them, but it 
        turned out that there is too little difference between the air masses 
        that produce tropical and equatorial storms to account for their 
        nature in air mass analytic terms.  So, I'll discuss storm types by 
        latitude and severity, starting with the mid-latitudes that most 
        conform to air mass analysis.
        1. Mid-latitude traveling cyclones, sometimes called extratropical 
           cyclones.
           a. Mid-latitude wave cyclones.
                i. This is far and away the most common type of storm in the 
                   middle latitudes.
               ii. It develops along the polar front, which separates polar 
                   air masses (Pc, Pm) from tropical air masses (Tc, Tm)
              iii. As such, the polar front corresponds to the subpolar low in 
                   the world pressure and wind system and is, therefore, a 
                   powerful storm generator, because the air masses it 
                   separates are very, very different in their temperature and 
                   humidity characteristics. 
               iv. It also is associated with the polar jet stream, sometimes 
                   called the polar front jet stream, a high speed westerly 
                   wind in the upper troposphere, which forms where the 
                   decline in thickness of the troposphere is the steepest 
                   (remember? the troposphere is quite thick over the tropics 
                   and thins towards the pole).  The path of the jet stream is 
                   unstable, sometimes being a fairly straight zonal west-east 
                   flow and other times having a strong meridional north-south 
                   component to it.  This is important as it affects the paths 
                   and the strengths of storms.
                v. Mid-latitude traveling cyclones go through four different 
                   stages of development and death.
                   a. Stage A:  Situation normal. All along the polar front 
                      the tropical air masses, propelled by the westerlies, 
                      are confronting the polar air masses, themselves 
                      propelled by the polar easterlies. For a while, this is 
                      an even match along the front, and the polar front forms 
                      a rather straight path from west to east.
                   b. Stage B: Sooner or later, this front develops bulges 
                      north and south as first one air mass, then the other is 
                      stronger at a given point. 
                      1. This means a warm front sector is formed where warm 
                         air pushes into polar air mass territory and a cold 
                         front sector forms behind it, pushing into tropical 
                         air territory. This, then, is the beginning of a 
                         storm, with its distinctive fronts established.
                      2. On weather maps, it is conventional to mark the cold 
                         front sector with a thick line with small triangles 
                         on it pointing in the direction of the front's 
                         advance;  the warm front is marked by a heavy line 
                         with small semicircles or bumps on it, again pointing 
                         in the direction of the front's advance.  The 
                         occluded sector is shown by alternating points and 
                         semicircles, all pointing in one direction. A section 
                         of a front not moving, a stationary front, is also 
                         shown with alterating triangles and semi-circles, but 
                         they point in opposite directions. Precipitation 
                         starts in with the uplift associated with the moving 
                         fronts.

                         [ sample weather map, Jim Pejsa, 1999 ]

                      3. At this point, the global circulation of Prevailing 
                         Westerlies converging with Polar Easterlies at the 
                         Subpolar Low gives way to the regional circulation of 
                         a storm, with air spiraling into the deepest part of 
                         the cyclone (where the cold front touches the warm 
                         front).  
                         A. In the Northern Hemisphere, the winds in front of 
                            the cold front will be coming from the south or 
                            southwest; after the cold front passes, there will 
                            be a wind shift, with the wind now coming in from 
                            the west or northwest.  
                         B. You can see that in the weathervane symbols used 
                            to show wind direction and strength in the map 
                            above: The reporting station is shown as a circle 
                            (black for cloudy and clear for, well, clear 
                            weather) and the vane points in the direction from 
                            which the wind is blowing.  The number of angled 
                            lines indicates wind speed.  
                         C. Anyhow, look at the stations to the east of the 
                            cold front and you'll see the winds are from the 
                            southwest; for those west of the cold front, the 
                            winds are coming from the northwest.
                   c. Stage C: The cold front sector moves faster, so it 
                      catches up with the warm front sector, attacking it from 
                      behind in a sort of rotational motion, lifting the warm 
                      tropical air mass clear off the ground. This is the 
                      beginning of the occluded stage.


                      [ cold front and warm front as map, Jim Pejsa, 
1999 ]


                      Here you see the cold and warm fronts, with two 
                      transects, or trips we'll take in cross-section, from E 
                      to A across the American South (the two kinds of front) 
                      and from G to F across the upper Midwest to see the 
                      situation above occlusion. 


                      [ cold front and warm front as cross-section, 
Jim Pejsa, 1999 ]


                      The cross-section above shows the distinct cold and warm 
                      front sectors west of D and east of C.

                      
                     [ later occlusion in cross-section, Jim Pejsa, 
1999 ]


                      This cross section shows the area above occlusion, where 
                      the warm air has been snapped up off the ground.

                   d. Stage D: The chilling of the warm air above the 
                      occlusion, which puts a gradual end to the occluded 
                      front and to the storm itself.  The deceleration in 
                      uplift eases up on the lower pressure of the cyclone, 
                      which exerts less influence on regional air flow:  This 
                      allows the regional cyclonic circulation to give way to 
                      the global pattern of converging westerlies and 
                      easterlies. The end of occlusion, thus, re-establishes 
                      Stage A, normalcy, until the next ripple comes along.
               vi. The whole storm system, as it develops and dies, tracks 
                   generally eastbound, under the path of the polar front jet 
                   stream. The jet stream is affected by the Rossby wave 
                   circulation around the poles, which is not perfectly 
                   circular (west-east). The Rossby circulation is sometimes 
                   nearly zonal (west-east) and other times markedly 
                   meridional (north-south).  So, the amplitude of the Rossby 
                   waves shrinks and swells with time in a roughly three to 
                   eight week cycle called the index cycle.
                   a. Low index means low zonal component
                   b. High index means high zonal component
                   c. During low index phases, the jet stream will curve 
                      markedly (and the paths of the storms below it will, 
                      too) and storm severity can be exaggerated as the curves 
                      in the jet stream accentuate the clockwise spiraling of 
                      surface highs and the counterclockwise spiraling of 
                      surface lows in the Northern Hemisphere, as shown in 
                      this graphic from USA Today:

                      [ jet stream amplification of surface highs 
and lows, Chad Palmer, 1999 ]

              vii. These mid-latitude wave cyclones are the source of 
                   California's winter storms. Such storms are spawned in 
                   great numbers by the Aleutian Low as it and the polar front 
                   are displaced south in winter with the migration of the sun 
                   into the Southern Hemisphere. Remember, the Hawai'ian High 
                   is also displaced south and weakened, taking our protection 
                   from these frontal cyclones away.  We have experienced 
                   winters with a markedly low index, which has brought the 
                   jet stream right over California, clobbering us with volley 
                   after volley of mid-latitude traveling cyclones.  Other 
                   times, we experience high index, with the jet stream and 
                   its Aleutian storms tracking well north of us and giving us 
                   a dry winter.
           b. Tornadoes -- much more "entertaining"

              [ tornado in Dimmitt, Texas, USA Today ]

                i. Also mid-latitude phenomena, these are the most violent 
                   storms on our planet.
               ii. They're an occasional by-product of very fast moving cold 
                   fronts in the mid-latitudes. When these fronts invade very 
                   moist maritime tropical air, the warm, wet air is forced 
                   suddenly aloft. This low is deepened further by the latent 
                   heat suddenly released by the intense condensation of the 
                   wet air racing aloft.
              iii. These fronts are moving so fast that they tend to run ahead 
                   of themselves:  The winds on the ground are impeded by 
                   friction with the ground, while the air aloft pulls ahead 
                   of the front's contact with the ground.  The front in 
                   cross-section would be even blunter than shown in the 
                   cross-section above.
               iv. This creates a pipeline of rolling air on the ground, kind 
                   of like that pipeline that surfers love so much just ahead 
                   of a tumbling wave crest, which is sort of what this is.
                v. At some point, the horizontally-rolling vortex of air can 
                   detach and tilt upward on one side and permit extremely 
                   fast spiraling airflow up into the cumulo-nimbus cloud 
                   above, which creates extremely low pressure. 
                   Voilà! -- a funnel cloud.  In a manner of 
                   speaking, a tornado is sort of like a cumulo-nimbus cloud 
                   on steroids.
               vi. As an extremely low pressure cyclone, this rogue cloud 
                   sucks air into it up to 400 km/h!
              vii. It's a very narrow storm: 100-500 m.
             viii. The tall, narrow vacuum with the twisting funnel that 
                   marks it writhes back and forth, touching ground 
                   frequently. Its dark color is that of sudden condensation 
                   plus all the stuff it yanks up off the ground.
               ix. A tornado is immensely destructive. It's so low a cyclone 
                   that it causes houses and buildings in its path to explode 
                   when the vacuum and the extremely high speed winds hit 
                   them. Also, its vortex tends to yank things up: cars, 
                   animals, people, trees, parts of houses -- in addition to 
                   its explosive effect.  These flying objects are lethal to 
                   anything they hit at these speeds:  Even a piece of straw 
                   becomes a bullet.
                x. At heart, a tornado is a cumulo-nimbus cloud gone very, 
                   VERY bad.  It exhibits all the damage of a regular cumulo-
                   nimbus cloud, only much worse:  Unbelievably torrential 
                   rain, hail, and lightning (as well as really weird aurora-
                   like electrical effects:  funny glowing colors, some pink, 
                   but mainly puke chartreuse or yellow).  I was in Worcester, 
                   Massachusetts, when a tornado came through in 1979.  I was 
                   unaware of what it was, because I did not see the funnel 
                   cloud:  All I was aware of was a staggering amount of rain 
                   pouring out of a black sky edged with barf-colored green-
                   yellow clear sky.  The noise from lightning and wind was 
                   astonishing, too.  It was all over about 15 minutes later.  
                   I didn't realize what it was until I read about some 
                   children in a campground nearby who'd been killed -- by a 
                   tornado. In Massachusetts, of all places.
               xi. About their geography:  They tend to be largely New World 
                   phenomena (look at the topography of the Western 
                   Hemisphere:  The mountains tend north-south, affording no 
                   block between polar and tropical air masses.  The Old World 
                   has the Himalayas and Caucasus and Zagros and Taurus and 
                   other east-west ranges to keep Siberian air out of the 
                   tropics).  

                   [ NOAA map of tornadoes, 1989-1998 ]

                x. Uhhh, check out California.  We get more than most 
                   Californians think!  We are in denial, folks:  People came 
                   here from Kansas to get away from them.  When we get them, 
                   the newspapers will call them "freak windstorms," 
                   "waterspouts" (even up in Pasadena), and the current 
                   favorite, "microbursts."  According to Warren Blier of the 
                   Weather Service, our tornado incidence is much higher than 
                   locals perceive, the difference being that we get tornadoes 
                   of smaller intensity than the Midwest does.  We get F0s, 
                   F1s, and the rare F2s on the Fujita Scale, while Texas and 
                   Oklahoma have gotten them as high as F5s. 
               xi. The Fujita Scale (sometimes called the Fujita-Pearson 
                   Scale) is a way of representing the intensity of a tornado, 
                   judging from the specific patterns of damage it does. 
                   a. Weak Scale Class
                      1. F0 -- Gale tornado -- winds 64-115 km/h (40-72 mph) 
                         -- Some damage to chimneys; breaks branches off 
                         trees; pushes over shallow-rooted trees; damages sign 
                         boards. 
                      2. F1 -- Moderate tornado  -- winds 116-179 km/h (73-112 
                         mph) -- Moderate Damage: Surface of rooves peeled 
                         off, mobile homes pushed off foundations or 
                         overturned, outbuildings demolished, moving autos 
                         pushed off the roads, trees snapped or broken; 
                         beginning of hurricane-speed winds. 
                   b. Strong Scale Class
                      1. F2 -- Significant tornado  -- winds 180-251 km/h 
                         (113-157 mph) -- Considerable Damage: Roofs torn off 
                         frame houses, mobile homes demolished, frame houses 
                         with weak foundations lifted and moved, large trees 
                         snapped or uprooted, light-object missiles generated.
                      2. F3 -- Severe tornado  -- winds 252-330 km/h (158-206 
                         mph) -- Severe Damage: Roofs and some walls torn off 
                         well-constructed houses; trains overturned; most 
                         trees in forest uprooted, heavy cars lifted off the 
                         ground and thrown, weak pavement blown off the roads.
                   c. Violent Scale Class  
                      1. F4 -- Devastating tornado -- winds 331-416 km/h (207-
                         260 mph) -- Devastating Damage: Well-constructed 
                         houses leveled, structures with weak foundations 
                         blown off the distance, cars thrown and 
                         disintegrated, trees in forest uprooted and carried 
                         some distance away. 
                      2. F5 -- Incredible tornado -- winds 417-509 km/h (261-
                         318 mph) -- Incredible Damage: Strong frame houses 
                         lifted off foundations and carried considerable 
                         distance to disintegrate, automobile-sized missiles 
                         fly through the air in excess of 300 feet, trees 
                         debarked, incredible phenomena will occur. 
                      3. F6 -- Inconceivable tornado -- 510-606 km/h (319-379 
                         mph) -- These winds are very unlikely. The small area 
                         of damage they might produce would probably not be 
                         recognizable along with the mess produced by F4 and 
                         F5 wind that would surround the F6 winds. Missiles, 
                         such as cars and refrigerators would do serious 
                         secondary damage that could not be directly 
                         identified as F6 damage. If this level is ever 
                         achieved, evidence for it might only be found in some 
                         manner of ground swirl pattern, for it may never be 
                         identifiable through engineering studies.  
        2. Tropical weather systems aren't too similar to mid latitude storms: 
           The contrast between air masses (e.g., mT and E) is much less.  
           They do have tremendous capacities to cause precipitation through 
           convection, however.  Remember, tropical air is very moist. Once 
           frontal or convectional uplift gets going, the latent heat energy 
           released by all that condensation accelerates uplift, leading to 
           intense precipitation. 
           a. Easterly waves are the most common source of stormy weather in 
              the tropics.
                i. They are slow moving (300-500 km/day) low pressure troughs 
                   in the Trades, about 5-30° N or S.
               ii. A trough is an area of low pressure, stretched out along a 
                   line or arc, in this case pointing poleward from the 
                   equatorial area.  If you examined the pressure gradient in 
                   this area, you would see that, for the most part, the 
                   isobars trend east-west.  If the area of an easterly wave, 
                   however, they bend poleward to create a trough:

                   [ easterly wave isobar map, University of East 
Anglia ]

              iii. You can understand the "trough" expression by drawing a 
                   straight line east-west right across the middle of the 
                   trough.  Mark the places your line crosses isobars and note 
                   the reading there.  Copy the line and its tick marks onto 
                   another sheet of paper. Now, construct a Y axis going up 
                   from this line and mark it with isobar readings from, in 
                   this case, 1010 to 1020 mb (or hPa).  Above each tick mark 
                   on the X axis, put a dot across from its isobar reading.  
                   Now, connect the dots with a smooth curve and you'll see it 
                   dips in the middle:  That dip in your curve is the trough, 
                   or area of low pressure.
               iv. Air at the surface will cross the isobars from high 
                   pressure to low pressure, slightly deflected to the right 
                   in the Northern Hemisphere (to the left in the Southern 
                   Hemisphere).  This means the wind is bent so that it 
                   converges along the trough and just east of it, which means 
                   it tends to rise there, and it diverges (and sinks) on the 
                   west side and far to the east.
                v. So, as the easterly wave approaches, you would first enjoy 
                   clear weather.  Then, as the trough passes over you, you'd 
                   experience the scattered showers and thunderstorms 
                   associated with the convergence and uplift of hot, humid 
                   tropical air for a day or so.  Then, it would clear with 
                   the return of slightly divergent and subsiding air behind 
                   the easterly wave.
           b. Tropical cyclones (aka hurricanes in the Atlantic, typhoons in 
              the Northwestern Pacific, and cyclones in the Indian Ocean, and, 
              sometimes, chubascos along the west coast of Mexico)
                i. These often form when the low pressure of an easterly wave 
                   deepens (the isobars would become "pointier" in their bend 
                   poleward -- as in the dashed lines on the sketch map 
                   above), as uplift is accelerated by unusual heat release 
                   during condensation.
               ii. This becomes a self expanding process, because of the great 
                   amount of water vapor held in mT air over oceans. The 
                   uplift/condensation of that huge amount of water is a 
                   powerful source of heat energy to speed up the uplift and 
                   deepen the low pressure.
              iii. A hurricane is seen on weather maps when the isobars keep 
                   bending until they cut themselves off into concentric 
                   circular patterns (instead of wavy zonal patterns) and when 
                   they attain hurricane force winds (118 km/h) -- they're 
                   called "tropical depressions" or "tropical storms" when 
                   they develop the circular isobar pattern but before their 
                   winds hit the requisite speed.
               iv. Description:
                   a. A hurricane is a circular center of very low pressure 
                      (a few have been known to plunge below 920 mb)
                   b. This profound low draws in high speed winds: 120-200 
                      km/h
                   c. The uplift of these swirling winds creates heavy rain 
                      (through convergent and convectional uplift).
                   d. The storm is relatively small, about 150-1000 km in 
                      diameter.
                   e. Hurricanes usually move about 25-30 km/h, though they 
                      can stall and just sit there, creating devastating 
                      amounts of rain and storm surge.  On the other hand, 
                      they have been known to race along at up to 100 km/h!
                   f. Perhaps the greatest peculiarity is the eye. This is a 
                      small area of absolutely clear weather right in the 
                      center of the storm.  The eye is usually about 30-60 km 
                      across and can last an hour or so.  The eyes are clearly 
                      visible in the images of the 1992 Hurricane Andrew on 
                      the left (the most expensive hurricane disaster ever to 
                      hit the United States, doing at least $20 billion of 
                      damage) and the 1998 Hurricane Mitch on the right (which 
                      devastated Central America, killing over 10,000 people):

[ Hurricane Andrew, NOAA, NASA, 1992 ]   [ Hurricane Mitch, NOAA, NASA, 1998 ]
                      The eye is produced by subsidence of air at the core of 
                      the spiraling storm.  Air races towards the center of a 
                      hurricane, spiraling to form a large vortex in the 
                      center, made up of rapidly uplifted air (and 
                      condensation).  This forms the eyewall, the wall of 
                      immense cumulo-nimbus clouds around the eye.  Here is an 
                      image of the eyewall seen from just inside the eye:

                      [ Katrina Eyewall, NOAA, Dewie Floyd, Flight 
Engineer ] 

                      The wind at the top of the storm spirals outward for the 
                      most part, but some, trying to diverge, manages to head 
                      inward where it converges and most of it sinks.  The 
                      subsiding air, of course, brings dry clear weather at 
                      the heart of the storm.

                      [ hurricane circulation, NOAA ] 

                   g. Another distinctive feature is the spiral rainbands that 
                      swirl outward from the eyewall, rather like the arms of 
                      a spiral galaxy.  These are made of cumulo-nimbus clouds 
                      formed from eddies or individual updrafts in the general 
                      inward rush of winds toward the center of the storm.  
                      The bands themselves spiral (counterclockwise in the 
                      Northern Hemisphere) slowly about the eyewall.  They can 
                      stretch out anywhere from 75 to 750 km from the eye.
                   h. Another feature of a hurricane is wind reversal after 
                      the eye passes. If you are in the direct path of a 
                      hurricane in the Northern Hemisphere, which is moving 
                      from east to west, the wind will first come at you from 
                      the north and you'll really get pounded, the winds 
                      accelerating as the eyewall approaches.  Then, the eye 
                      passes over you, and things are sunny and tranquil for 
                      half an hour or an hour or so.  Just when you start to 
                      relax, you'll be smashed by really fast winds, this time 
                      coming from the south.  If the storm is coming up from 
                      the south to the north in the Northern Hemisphere, then 
                      the wind reversal would be easterly winds before the eye 
                      and westerly after.  The opposite is true in the 
                      Southern Hemisphere.
                   i. Another feature of direct relevance to disaster planning 
                      is the notion of the (more) dangerous side.  The winds 
                      spiral into the storm at some high rate of speed but you 
                      have to remember the storm itself is moving. You have to 
                      add and subtract the storm's speed to get the effective 
                      speed of winds in any part of the hurricane.  In the 
                      Northern Hemisphere, the dangerous side is the right 
                      side; in the Southern, it's the left side.  So, if the 
                      storm were moving, say, 25 km/hr westbound in the 
                      Northern Hemisphere and the winds were coming in at, 
                      say, 150 km/h, the winds to the right would effectively 
                      be moving at 175 km/hr, while those on the left side 
                      would effectively be moving at 125 km/hr.  Here is a 
                      spiffy animation making the same point with a hurricane 
                      moving north into Louisiana and Mississippi at 30 mph 
                      with winds of 100 mph.

                      [ hurricane dangerous side, NOAA ] 

                   j. Direction of travel: generally westward at first, driven 
                      by the easterly Trades and the circulation around the 
                      oceanic subtropical high cell. That said, hurricanes 
                      pretty much go where they "want" to, driven by quirks in 
                      their own internal circulation and interactions with the 
                      regional circulation, which is itself affected by the 
                      position of the world pressure and wind belts, seasonal 
                      movements in these, and the land and sea pressure 
                      differential that is so prominent in the summer.  
                      Hurricanes will often initially start out westbound and 
                      then curve poleward along the isobars defining the 
                      Bermudas or Hawai'ian High.  If there is a marked low on 
                      land, hurricanes will head for land.  Here we see the 
                      North Atlantic hurricane tracks for 1999:

[ Atlantic hurricane tracks, NOAA, 1999 ]

                      1. If they manage their poleward turn out at sea, notice 
                         how long their tracks can be.  By staying out over 
                         water, they stay in contact with their power source: 
                         warm tropical water.  By power source, I mean they 
                         pick up enormous amounts of evaporated water and the 
                         latent heat it carries, the air rises, cooling 
                         adiabatically enough to create condensation (clouds 
                         and rain) and this releases the latent heat, which 
                         accelerates the original uplift. Hurricanes can ride 
                         the Gulf Stream or similar warm currents pretty far 
                         poleward and can eventually head east along the 
                         current, moving along those summer isobar patterns 
                         into the Westerlies belt.  Sooner or later, they 
                         degrade into lower power tropical or extratropical 
                         storms because they are slowly moving into ever 
                         cooler waters, which cuts them slowly off from their 
                         power source.
                      2. If they do their poleward swing onshore, they die 
                         suddenly.  Notice how generally short the tracks are 
                         that come ashore and how quickly they degrade into 
                         the yellow tracks of an ordinary tropical storm or 
                         the green tracks of a tropical depression.  This 
                         sudden flameout (if that's the right metaphor) is a 
                         result of the hurricane suddenly being cut off from 
                         access to its power source: warm tropical waters.
                   k. Global distribution.  From the foregoing, we can 
                      generally predict where and when hurricanes are going to 
                      occur globally.
                      1. They are largely summer phenomena, given the 
                         influence of the oceanic cells into which the 
                         Subtropical High break:  Hurricanes flow more or less 
                         along the curving isobars that encircle those oceanic 
                         peaks of high pressure.  Hurricane season in the 
                         North Atlantic and North Pacific, then, is largely 
                         July to October; in the North Indian Ocean, there are 
                         two peaks, one in May and another in November.  In 
                         the South Pacific and the South Indian oceans, it's 
                         more like late October to May.
                      2. Hurricanes are tropical but not equatorial.  Why 
                         don't they form right along the equator, where, 
                         presumably, the ocean water is warmest?  Because 
                         Coriolis Effect is non-existent.  There has to be 
                         some Coriolis Effect to induce the inspiraling so 
                         characteristic of these storms.  They generally 
                         originate somewhere between 8 and 15 degrees north or 
                         south of the equator.
                      3. They mainly afflict east coasts of continents because 
                         of their dependence on a warm current, such as the 
                         Gulf Stream.  We in California are protected from 
                         full-blown hurricanes by the cold California current.  
                         Occasionally, a hurricane will actually survive the 
                         short trip across Central America and wind up off the 
                         west coast, where they quickly degrade over the cold 
                         water.  These systems are sometimes called 
                         "chubascos."  We sometimes get the tropical moisture 
                         from a chubasco, usually in August or September, 
                         receiving thunderstorms in the summer which, 
                         otherwise, is pretty rare for us.  An interesting gap 
                         is present on this map of typical hurricane storm 
                         tracks:  the South Atlantic is free of them.  Current 
                         debates focus on ocean temperatures in the South 
                         Atlantic just south of the equator (they may not 
                         break the 26° C threshhold that seems necessary 
                         to trigger hurricanes) and on vertical wind shear in 
                         the troposphere there (too strong to support the 
                         vertical uplift of the eyewall).
 
                         [ global hurricane geography ]

                   l. There are several separate sources of hazard in a 
                      hurricane:
                      1. Extreme winds.
                      2. Torrential rain and the resulting freshwater flooding 
                         and mudslides (which is what killed most of the more 
                         than 10,000 people who died in Hurricane Mitch in 
                         1998).
                      3. Saltwater flooding from the storm surge and high 
                         winds.  
                         A. The extreme low pressure of a hurricane actually 
                            pulls up the ocean surface in the area under it!  
                            When this surge strikes the coast, waves can 
                            penetrate farther inland than they would 
                            otherwise, sort of the mother of all high tides in 
                            effect. 
                         B. Waves in water are a function of wind speed, 
                            duration, and fetch (the distance of open water in 
                            the direction the wind is blowing): Hurricanes 
                            generate really high wind speeds for a sustained 
                            period of time and the waves that come at a coast 
                            may have had a long stretch of open water.
                         C. So, you have really big waves and an elevated 
                            ocean surface and that translates into serious 
                            saltwater flooding along coastal lowlands.  
                         D. Human life is endangered both by the possibility 
                            of drowning and by the debris carried by these 
                            waves.
                      4. The dangerous side of hurricanes also often spawns 
                         tornadoes to make the misery complete.
                      5. For a hurricane to become a disaster, it needs people 
                         and property in the way.  The Red Spot on Jupiter, 
                         apparently some kind of hurricane, is not a natural 
                         hazard, because there are no people and assets at 
                         risk to it.  Over the course of this century, 
                         hurricanes have become costlier and costlier as the 
                         human population grows and as it concentrates on 
                         hurricane coasts.  A hurricane (called a "cyclone" 
                         locally) hit Bangladesh in 1970 and destroyed over 
                         500,000 human beings.  Hurricane Andrew did over $20 
                         billion dollars of damage in 1992, because there are 
                         now so many people and economic activities and assets 
                         in South Florida.
                   m. Like the Fujita Scale with tornadoes, there is a 
                      hurricane intensity scale, which is called the Saffir-
                      Simpson Scale.  It has five hurricane levels, 1-5, 
                      covering damage levels from minimal to catastrophic:

                      Saffir-Simpson Scale

      
                      Type     Damage       Press: mb  Wind: km/h  Surge: m
                                                                        
                      Depression (easterly wave develops circular isobars) 
                      Tropical storm (many hurricane traits, 
                               but not strong enough yet)
                      Hurr. 1  minimal         >980    <118        1.25-1.75       
                      Hurr. 2  moderate      965-980   118-154     1.75-2.75      
                      Hurr. 3  extensive     945-965   154-178     2.75-4.00      
                      Hurr. 4  extreme       920-965   178-210     4.00-5.50      
                      Hurr. 5  catastrophic   <920      >210         >5.50

           c. Polar outbreaks
                i. Cold air from polar regions breaks through to very low 
                   latitudes in the Western Hemisphere sometimes (remember in 
                   the discussion of tornadoes, I commented on how the New 
                   World's mountains generally trend north-south, allowing 
                   Arctic air to contact warm tropical air?  Well, the 
                   topography is relevant to this type of storm, too).
               ii. A regular "squall line" of cumulo-nimbus clouds heralds the 
                   cold front.
              iii. Strong, steady winds follow the squall line.
               iv. This brings unusually cool, clear weather to the tropics -- 
                   this can chill out many tropical crops grown on tropical 
                   highlands, such as coffee ("mountain-grown," as the ads 
                   have it!), wiping out much of the crop and raising the 
                   price of your morning cuppa joe.
                v. In the Caribbean and Central America, these outbreaks are 
                   called "nortes," because they come from the north; in South 
                   America, they're called "pamperos," because they blow in 
                   from Antarctica over the Pampas (the famous grassland in 
                   Argentina).
               vi. In the low-lying areas, this actually brings some of the 
                   most pleasant weather there: cooler and drier.
        3. Equatorial storms are really different from both mid-latitude and 
           from tropical storms.  They are often referred to as "weak 
           equatorial lows."
           a. There is almost no difference among air masses, which we saw 
              earlier in discussing tropical storms (e.g., very little to 
              distinguish mT from E air masses), so, like the tropical storms, 
              air mass analysis isn't helpful.
           b. Unlike the tropical storms, though, there is too little or no 
              Coriolis Effect to induce spiraling.
           c. These storms consist of individuals or groups of cumulo-nimbus 
              clouds scattered in an area.
           d. They are associated with the Inter-Tropical Cconvergence Zone 
              (ITCZ):  Basically, the converging Trades produce uplift, which 
              generates many individual convectional and convergent storms.
           e. A typical day along the ITCZ consists of day breaking clear and 
              sunny.  By afternoon, thunderheads are piling up, especially 
              wherever there's some relief in the landscape:  islands in the 
              open sea or mountains.  By late afternoon, it's "raining cats 
              and dogs" (or is it "pitchforks and hammer handles"?).  This 
              goes on for a short while, maybe 20 minutes to a couple of 
              hours, and then, as the sun sets, the clouds die down and 
              dissipate and the stars come out in a largely clear sky.  This 
              goes on for weeks at a time.  Pretty exciting, huh?
        4. High latitude storms.  Not much is known about the storms 
           experienced in the polar and circumpolar latitudes.  The climate is 
           pretty dry up there, given the low temperatures, so not much 
           precipitation happens most of the time (these are sometimes called 
           "the polar deserts").  But, when it does, it comes with great 
           suddenness and violence.
           a. Polar lows:  Some of these storms are similar to mid-latitude 
              wave cyclones, with front-like features and the asymmetry of the 
              precipitation bands we associate with classic mid-latitude 
              cyclones.  They may result from A or AA or cP air interacting 
              with mP air blowing in from over a warm current, such as the 
              Gulf Stream/North Atlantic Drift or the Japan Current/North 
              Pacific Drift.
           b. Others actually look suspiciously like hurricanes, of all 
              things, complete with an eye.  Some people call them even call 
              them "(Ant-) Arctic hurricanes" or "polar hurricanes."  
              Sometimes they're called "bomb cyclones," because the air 
              pressure in them drops catastrophically (like a bomb), at least 
              24 mb in 24 hours (there have been a few that dropped 60 mb or 
              hPa in 24 hours!!!).  
                i. So, even though they may look like a hurricane, they are 
                   still pretty different:
                   a. They have cold air and cold fronts, which would destroy 
                      a conventional tropical cyclone or hurricane.
                   b. They form under strong upper-level winds (remember one 
                      of the explanations for the South Atlantic being free of 
                      hurricanes is unusually strong vertical wind shear?).
                   c. They form in the winter, and tropical cyclones are 
                      summer affairs.
                   d. They form in the Northwest Pacfic, Northwest Atlantic 
                      ("the Perfect Storm"), and, rarely, even in the 
                      Mediterranean, while true hurricanes need warm tropical 
                      oceans to form and these bomb cyclones may have 
                      something to do with the famous, sudden-onset 
                      Nor'easters on the American East Coast. 
               ii. One possible explanation for these is that they are 
                   hybrids, formed when an unusually strong mid-latitude wave 
                   cyclone somehow connects up with a tropical cyclone moving 
                   into higher latitudes, allowing the mid-latitude cyclone to 
                   exploit the massive amounts of water vapor carried by the 
                   tropical cyclone.
           c. In short, not a lot is known about high latitude weather and 
              it's an area of active research and controversy, with many 
              opportunities for students who acquire the physical geography, 
              physics, chemistry, meteorology, or remote sensing background to 
              help out (and who actually like freezing in the field).
     D. Importance of storms -- a few wrap-up comments before we leave the 
        weather section.
        1. Storms liberate enormous amounts of latent heat into the 
           atmosphere.
        2. As such they are part of the global circulation carrying surplus 
           heat away from the low latitudes to the heat-deficient higher 
           latitudes, where the storms release the heat through condensation 
           and freezing.
        3. Storms supply precipitation and so largely determine fresh water 
           distribution and water resources.
        4. They create many environmental hazards:
           a. Lightning and lightning fires
           b. Hail and crop loss
           c. Tornadoes and hurricanes are just hazards by definition whenever 
              people or their "stuff" are in their paths.
           d. Floods (a little too much of that water resource!)
           e. Winds
        5. People try to minimize the hazardous aspect of storms (e.g., 
              hurricane seeding to drop wind velocity or dam and levee 
              building), but such efforts often backfire unpredictably.  
              Later, we'll see a common theme in research is that society 
              responds to recurrent, low level hazards in a way that sets it 
              up for much more catastrophic losses later, when the much rarer, 
              higher magnitude event happens along.  More on that later.

Well, th-th-that's about all for storms, folks.  Make sure you understand the 
basic idea of air mass analysis (air masses and fronts) and know that it was a 
major breakthrough in the understanding of mid-latitude wave cyclones, the 
most common storm in the mid-latitudes.  Be able to associate the major storm 
types discussed in this lecture with the general latitudes in which they occur 
(mid-latitude, tropical, equatorial, and high latitude).  Make sure you are 
familiar with the main characteristics and dynamics of the major storm types 
and that science still does not understand high latitude storm genesis and 
development.  Be aware of the various ways that storms are important in the 
earth system (and to us humans).

The next lecture will summarize major climate types. 


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Document and © maintained by Dr. Rodrigue
First placed on web: 10/22/00
Last revised: 06/19/07

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