A situation that is noticeably favorable to tornado
activity is cold air advection aloft. When mP air moves
across the United States, it becomes heated in the low
levels in the western plateaus. The resulting density of
the now warm mP air is then equal to or less than that of
mT air moving northward over the Mississippi Valley.
The mP air rides up over the mT air. The mP air still
maintains low temperatures at higher altitudes causing
extreme instability.
The following conditions may indicate possible
tornado activity:
1.
Pronounced horizontal wind shear. (Wind
shear is the rate of change of wind velocity with
distance.)
2.
Rapidly moving cold front.
3.
Strong convergent flow at the surface.
4.
Marked convective instability.
5.
Dry air mass superimposed on a moist air mass
and abrupt change in moisture content, usually below
10,000 feet.
6.
Marked convection up to the minus 10°C
isotherm.
WATERSPOUTS
Waterspouts are tornadoes that form over ocean
areas. This phenomenon consists of two types: tornado
in origin and locally induced. The difference between
the two types is significant in that the tornado type has
potential for inducing substantial damage and injury
over a broad area, while the local type has potential for
causing only minor damage in a small area. The
following information is provided to help you to better
understand the two types of waterspouts.
Tornado Type
These waterspouts form at the cloud and extend
down to the surface. They originate from severe
convective cells associated with a cold front, squall
line, or large convective cluster. Whenever the
conditions for tornado development are present over
coastal areas and the triggering mechanism extends into
the
adjacent
maritime
area,
then
potential
for
waterspout
development
is
high.
The
tornado
waterspout has a relatively short life span and usually
stays over water. However, when one does come
ashore, there is potential for it to assume the
characteristics of a tornado; although its life span is
limited, the initial intensity is sufficient to cause
property damage and injury to personnel.
Local Type
These waterspouts originate from convective
clouds of moderate vertical extent which form a line or
a small cluster. Their existence is sensitive to wind and
temperature in that surface winds of 20 knots or greater,
or a cooling of the atmosphere by precipitation,
dissipates them. Additionally, when local waterspouts
come ashore, the friction induced by the land rapidly
dissipates them. The biggest threat posed by these
waterspouts is to small craft, recreational boating, and
to support facilities such as harbor operations and
marinas.
REVIEW QUESTIONS
Q5-1. Describe the major difference between rain
and drizzle.
Q5-2.
What altitude range do clouds occur in the
tropics?
Q5-3.
What is the altitude range of middle clouds in
the temperate regions?
Q5-4.
Describe the difference between sea fog and
steam fog.
Q5-5.
What criteria must be met for a hydrometeor
to be classified as blowing spray?
LITHOMETEORS
LEARNING OBJECTIVE: Identify the
characteristics of lithometeors (haze, smoke,
dust, sand, and dust devils).
Lithometeors comprise a class of atmospheric
phenomena of which dry haze and smoke are the most
common examples. In contrast to hydrometeors, which
consist largely of water, lithometeors are composed of
solid dust or sand particles, or the ashy products of
combustion.
HAZE
Haze is composed of suspended dust or salt
particles that are so small that they cannot be
individually felt or seen by the unaided eye. They
reduce visibility and lend a characteristic opalescent
appearance to the air. Haze resembles a uniform veil
over the landscape that subdues all colors. This veil has
a bluish tinge when viewed against a dark background
5-12
