Next Quiz Thursday 18 April, covering Chapters 15 and 16.

Chapter 15:  Thunderstorms and Tornadoes.  Read all, except Focus on p.423.

Note the major difference in the time and space scales of convective
phenomena such as thunderstorms from the large-scale systems we have talked
about in the previous 3 chapters.  The building block of the thunderstorm
is the "cell", which may be several kilometers across, and go through its
life cycle in 30-60 minutes.  Do understand the life cycle of the cell, and
appreciate how multi-cell thunderstorms (the most common type) normally
consist of several cells, in various stages of their life cycle.

Whether thunderstorms consist of scattered ordinary multicells, or whether
organized in a squall line, they may range in intensity from rather weak to
extremely severe.  The organized structure shown in Fig. 15.6 and 15.7
should be understood; note how the downdraft "feeds" the cold air outflow,
which in turn can lift the ambient warm, moist air, thereby creating new
storm cells.  This process may repeat itself for many hours in a
well-organized squall line.

The downdraft (sometimes called downburst or microburst) has been
responsible for many air crashes on takeoff and landing. It is not the
downward-moving air which is the fatal element, but the sudden loss in
airspeed experienced as the aircraft suddenly experiences a change from a
strong headwind to strong tailwind flying through the strongly diverging
winds near the ground.  It is NOT necessary for the winds to be "severe"
for the microburst to be a problem for an aircraft!

What determines severity of a thunderstorm is not an easy problem.  While
most thunderstorms require very unstable air masses (warm and moist at low
levels; cooler and usually drier aloft), the degree of instability is
imperfectly correlated with intensity.  Stronger storms seem to require
stronger wind shear; that is, a marked change of wind direction and speed
between the lowest layers (say 0-1 km) and the mid-troposphere (say 3-5
km).  When both instability and low-to-mid-level wind shear are great,
those thunderstorms which form are more likely to be severe, and more
likely to take the form of rotating storms called supercells.  When strong
tornadoes occur, they are usually associated with supercells (but not all
supercells spawn tornadoes).

The environment favorable for severe thunderstorms is illustrated in plan
view by Fig. 15.14 and 15.32, and in sounding form by Fig. 15.33 (you may
need to review conditional and convective instabilty in Chapter 7...this is
where you apply what you learned then!).  These favorable conditions are

(1) conditional and/or convective instability
(2) a stable layer ("lid") atop the warm moist air.  (This may seem a
paradox because the stable layer prevents convection.  The issue is that it
prevents premature release of instability in the form of ordinary
convective storms and permits large instability and wind shear to develop.)
(3) strong wind shear in the lowest 3-5 km (see paragraph above)
(4) a mechanism to LIFT THE AIR in an organized manner, thereby eliminating
the lid and releasing the pent-up instabilty in the form of powerful
storms.
        **Note:  It does not matter what the lifting agent is.  Fig. 15.14
illustrates the dryline, which is an important lifting agent in Texas and
Oklahoma, especially in Spring.  But ANYTHING which lifts the warm moist
air sufficiently to eliminate the stable layer and thereby permit
convection to grow will do.  Other possibilities include cold fronts,
strong upper-air troughs, outflow from nearby storm systems, or sea
breezes.  Anything which creates strong low-level convergence and lifting
of the low level air (remember "in-up-out") is a candidate.**

Distribution of thunderstorms.  Compare Fig. 15.18, 15.19, and 15.28.  The
conditions for "ordinary" multicell thunderstorms are most frequent in
Florida, those for hail in the High Plains, and those for tornadoes in the
central plains and through the Gulf coast states.
Chapter 16:  Hurricanes.  Read all, plus a handout you will get Tuesday
16th.  EXCEPTION:  While you should read the section in hurricane formation
(pp. 433-434), please do not try to distinguish between the "organized
convection" theory and the "heat engine" theory.  The book presents them as
alternative theories, but the truth is that they are both necessary parts
of the story!

Read the "Anatomy" section carefully; I will supplement this with a handout
which will help you visualize how the winds of a hurricane change as one
proceeds from the outside through the center and out the other side.
Understand the difference between the eye and the eyewall.  Also you should
appreciate that the wind speed of a hurricane is greater where its forward
speed of motion is additive with the winds blowing around the relatively
calm eye.  Therefore, in the Northern Hemisphere the winds are stronger on
the right semicircle than on the left semicircle, with respect to the
direction of forward motion of the hurricane.  (See Fig. 16.8 and the
accompanying discussion.)  This fact is of great importance when
considering the impacts of the hurricane at landfall.  The winds and the
storm surge will be far worse to the right of the location where the eye
makes landfall.

We will review the conditions for hurricane formation, and why the tracks
in Fig. 16.7 are present in some parts of the tropical oceans and not
others.  Also you should understand why hurricanes weaken rapidly over
land.

Examples of tracks and storm surges associated with individual hurricanes
will be given in class, including some famous Texas storms.  Special
circumstances in Texas include large areas with low-lying land which will
flood hours in advance of arrival of the storm center.  You should become
familiar with the stages of a hurricane (p.434), watches and warnings (p.
439), and the Saffir-Simpson scale (p. 444).