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Weather Systems
Introduction
Air Masses
Frontal Systems
Mid-latitude Cyclones
Thunderstorms
Tornadoes
Hurricanes
Summary
A great thunderstorm; an extensive flood; a desolating hurricane; a sudden
and intense frost; an overwhelming snowstorm; a sultry day, - each of
these different scenes exhibits singular beauties even in spite of the
damage they cause. Often whilst the heart laments the loss to the citizen,
the enlightened mind, seeking for the natural causes, and astonished at the
effects, awakes itself to surprise and wonder.
St. John de Crévecoeur
Introduction
•
•
•
•
Weather is the state of the atmosphere at a given time and
place.
Extreme weather events threaten lives, disrupt
transportation systems, and cause destruction.
The National Weather Service makes over a million daily
weather observations that will be used in forecasts by
media sources nationwide.
Advanced technology such as weather satellites and
Doppler radar allow the accurate prediction of dangerous
weather systems like tornadoes and hurricanes.
No scientific phenomena concern us as much as the daily
evolution of weather systems. We live in a culture where
weather, the state of the atmosphere at a given time and place,
helps us define regional cultural variations. States such
as California and Florida are defined in part by their warm,
sunny weather. Seattle, Washington, is known for its rain,
North Dakota and Minnesota for cold winter temperatures, and
Oklahoma for tornadoes. Superimposed on the regular rhythms
of the atmosphere are more extreme events that threaten lives,
disrupt transportation systems, and cause destruction (Figs. 1,
2). Approximately 90% of presidential disaster declarations
are weather related. In the dozen years between 1988-1999,
there were 38 U.S. weather disasters that generated at least $1
billion in damages. In 1998 alone there were seven billiondollar weather disasters (Fig. 2).
Figure 1.
Extreme weather
events in the
lower 48 states.
Note hurricanes
are found along
Atlantic and Gulf
coasts, and
tornadoes are
most common
over the Great
Plains and upper
Midwest.
2
The first half of the chapter is divided into three sections that
describe how common weather systems develop across much
of the nation. Weather in any region is influenced by the
atmospheric changes that occur when masses of air with
contrasting properties interact. The characteristics of air
masses vary with location ranging from dry and cold to warm
and humid. The daily clash of air masses over North America
generates our common weather patterns characterized by highand low-pressure systems bounded by warm and cold fronts.
These frontal systems are relatively narrow, curvilinear zones
that mark a transition from one air mass to another. Weather
experienced over much of the central and eastern U.S. is the
result of the west-to-east migration of regional-scale lowpressure systems, termed mid-latitude cyclones, and their
associated warm and cold fronts. Mid-latitude cyclones affect
much of the continental landmass for up to a week at a time.
Meteorologists attempt to predict the path of these mid-latitude
cyclones and their frontal systems by monitoring their
associated atmospheric conditions such as moisture,
temperature, pressure, and wind direction. Using these
characteristics they can predict the potential weather for two to
five days in the future. However, these dynamic systems are
subject to change, and the short-term, relatively accurate
forecast becomes a long-term calculated guess as the forecast
extends beyond two or three days.
Figure 2. Top:
Cost of damages
associated with
weather events,
1998. Total cost
was over $16
billion. Bottom:
Proportion of
fatalities
associated with
specific weatherrelated
phenomena.
3
Such is our devotion to understanding how future weather
patterns will affect us that millions daily tune in to the cable
weather news station, the Weather Channel. Regardless of
where we get our information on the weather forecast, almost
all of it comes from the same place, the National Weather
Service (NWS). The NWS processes over one million weather
observations per day. These basic observations may be
reprocessed by commercial weather companies (e.g.,
Accuweather) to generate maps and graphics for public
distribution to a variety of media sources.
Figure 3. Early
national weather
map, created
September 1,
1872, shows a
high-pressure
system over the
Northeast. Image
courtesy of NOAA
photolibrary.
The NWS began life on February 9, 1870, as part of the Signal
Service Corps in the Department of War. It initially had the
unwieldy title of The Division of Telegrams and Reports for
the Benefits of Commerce and was given the charge “to
provide for taking meteorological observations at the military
stations in the interior of the continent and at other points in
the States and Territories . . . and for giving notice . . . of the
approach and force of storms.” The fledgling service made its
first simultaneous observations at just 24 sites on November 1,
1870. Within two years it was creating national weather maps
(Fig. 3), and by 1878 daily observations were being collected at
284 sites and relayed cross-country by telegraph.
The current NWS mission is to provide “weather, hydrologic,
and climate forecasts and warnings for the United States, its
territories, adjacent waters and ocean areas, for the protection
of life and property and the enhancement of the national
4
Figure 4.
Geostationary
satellites
generate
thousands of
images per
day. Image
courtesy of NOAA
photolibrary.
economy.” Today the NWS uses sophisticated satellite
technology to keep tabs on developing weather systems
worldwide. The Geostationary Operational Environmental
Satellite (GOES) Program began in 1968 and today has two
satellites in synchronous orbit above Earth that provide weather
coverage for 60% of the planet's surface (Fig. 4). The NWS
has over one hundred Doppler radar sites nationwide that are
used to track rapid changes in regional storms. The nationwide
expansion of Doppler radar installations resulted in an increase
in the warning times given for sudden weather phenomena
such as tornadoes and flash floods that claim hundreds of lives
annually.
The latter half of the chapter is divided into three sections that
review extreme weather events in the U.S., thunderstorms,
tornadoes, and hurricanes. Thunderstorms form as warm,
humid air is forced aloft, either in advance of cold fronts that
are migrating toward the east or as a result of differential
warming of air near Earth's surface. The high winds, hail,
heavy rains, and lightning associated with these storms claim
approximately a hundred lives a year in the U.S. Furthermore,
over the much of central and eastern U.S., thunderstorms
produce even more violent tornadoes (Fig. 5), the highest
velocity winds on Earth. The use of new technology, such as
Doppler radar, has increased the average lead time for tornado
warnings in the U.S. from 5 minutes (1986) to 12 minutes in
1998.
Figure 5. A
tornado near
Dimmit, west
Texas, 1995.
Image courtesy
of NSSL's photo
album.
5
Dangerous weather phenomena such as tornadoes and
hurricanes cannot be stopped but with detailed observations
meteorologists can provide timely warnings to protect people
from the onslaught of these hazardous winds. The sheer sizes
of hurricanes, hundreds of kilometers across and bigger than
most states, mean that they will have significant impact on
people and property when they come on shore. The most
expensive natural disaster in U.S. history occurred in 1992
when Hurricane Andrew wrecked havoc across southern
Florida, causing $30 billion in damages (Fig. 6). Damages
could easily have been doubled if the storm had made landfall
in the highly developed areas further north. Continued coastal
development makes a future $50 to $100 billion disaster
inevitable.
Figure 6.
Property damage
in southern
Florida resulting
from Hurricane
Andrew, 1992.
Image courtesy of
NOAA.
Think about it . . .
Examine the map at the end of the chapter that illustrates
the distribution of extreme weather events for the
conterminous U.S. during 2000. What patterns can you
identify in the weather characteristics displayed on the
map?
Air Masses
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6
Air masses are large regions of the lower atmosphere with
uniform characteristics that are originally defined by a
source area.
Air masses are identified by temperature (polar vs. tropical)
and the nature of the source area (continental vs. maritime)
North American weather patterns are dominated by
continental polar and maritime tropical air masses.
•
Air masses are modified as they move over areas with
different temperatures or topography than the source area.
Air masses represent large regions (1,000s km2) of the lower
troposphere with relatively uniform properties (temperature,
moisture content). The characteristics of individual air masses
are dependent upon the attributes of a source area and the
modification of the air mass that occurs as a result of
movement from the source region. Weather in any region is
influenced by the changes that occur in the air mass over time
and the interactions that occur at fronts, the boundaries
between contrasting air masses.
Source Areas
An air mass develops when the atmosphere is located above a
relatively uniform land or water surface for several days. The
lower atmosphere assimilates some of the properties of the
underlying surface. Air masses are identified by their
temperature (polar/tropical) and the character of the
underlying surface (continental/maritime). The latter property
is a proxy for moisture content. Air masses that develop
above oceans contain much more moisture than those formed
over land.
Figure 7.
Approximate
locations of air
masses developed
over the Northern
Hemisphere (left)
and Southern
Hemisphere (right)
in July. cA continental
Arctic/Antarctic; cP
- continental polar;
cT - continental
tropical; mP maritime polar; mT
- maritime
tropical. Original
The distribution of air masses is relatively intuitive. Arctic and
polar air masses are located at high latitudes (+50o) in the
Northern Hemisphere and tropical air masses are located closer
to the equator (Fig. 7). Continental air masses are found over
land, maritime air masses over ocean. The boundaries between
individual air masses vary with seasons. Polar air creeps further
south during winter and retreats northward during summer.
globes courtesy of
NOAA's National
Geophysical Data
Center.
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Heavy rains in the Midwest can result from the interaction
between the continental polar air mass and the maritime
tropical air mass that pushes northward across much of the
eastern U.S. during summer.
Meteorologists use a form of scientific shorthand to label the
most common types of air masses (Fig. 7). For example a
maritime tropical air mass (warm, moist air formed over the
ocean) is identified by mT (m = maritime; T = tropical). The
characteristics of five types of air masses are summarized
below.
•
cA - continental Arctic/Antarctic air forms at high
latitudes around the poles above permanently snow-covered
ground (or pack ice). These air masses are characterized by
extremely cold, dry air that may sweep south across Canada
and produce days of bone-chilling cold temperatures over
much of the central and eastern U.S. during winter.
•
cP - continental polar air forms over the northernmost
portions of North America, Europe, and Asia. It shares its
basic characteristics (cold, dry) with cA air, without the
exceptional cold temperatures. High "lake effect" snowfalls
in the upper Midwest are the result of the dry cP air masses
picking up moisture as they cross the warmer waters of the
Great Lakes (Fig. 8).
Figure 8. Lake
effect snows in
Michigan and
northern Illinois
and Indiana,
January 1997.
Image courtesy of
NASA GOES.
•
cT - hot, dry continental tropical air forms over
continental interiors such as the dry lands of northern
Mexico and southwestern U.S. (Fig. 9). This air mass
disappears from North America in winter but brings
scorching summers to the southwest. It will decay as it
moves east, absorbing moisture and cooling as it goes.
•
mP - maritime polar air masses form in the northern
Atlantic and Pacific Oceans and are characterized by cool,
moist air that affects states bordering the Atlantic shore of
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the Northeast and most of the Pacific coastline (Fig. 9).
Temperatures at the ocean surface are less extreme than on
land (less cold) so mP air is warmer than cP air.
•
mT - high temperatures and high humidity distinguish the
maritime tropical air masses that move inland from the
tropical Pacific, Gulf of Mexico, and tropical Atlantic
Ocean (Fig. 9). The mT air brings hot, humid summers to
southeastern states and can form at any time during the
year.
Figure 9. Principal
source areas for
air masses that
influence weather
patterns across
North America.
Original globe
courtesy of NOAA's
National
Geophysical Data
Center.
Modification of Air Masses
The initial characteristics of air masses will inevitably change
as the mass of air moves out of its source area and passes over
regions with contrasting attributes. The principal factors that
will cause modification are the temperature and topography
of the underlying surface. Air masses will be heated or cooled
from below depending upon the relative temperatures of the
original air mass and underlying surface. Heating (cP air
moving south) will lead to instability as air near the ground
surface rises, mixing the air column. Cooling (mT air moving
north) has the opposite effect, because cold air cannot rise but
remains in a stable configuration near the land surface.
Orographic lifting forces maritime air upward over mountain
ranges in the western U.S., leading to condensation and
precipitation that converts the formerly humid air to a much
dryer air mass.
9
Think about it . . .
Create a concept map that summarizes the characteristics
of the principal air masses and their influence on weather
patterns in North America.
Frontal Systems
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•
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•
Frontal systems form along the boundaries between
colliding air masses of contrasting properties.
Cold fronts and warm fronts are associated with the
interaction of continental polar and maritime tropical air
masses over the central U.S.
Heavy rainfall, decreasing temperatures, decreasing
humidity, and changing wind directions are associated with
passage of a cold front.
Light to moderate rain, warmer temperatures, increasing
humidity, and changing wind directions follow passage of a
warm front.
An occluded front forms when a cold front overtakes a
warm front.
Frontal systems represent the meteorological battle that
ensues when air masses of contrasting properties clash along
their boundaries. As air masses move across Earth's surface
they inevitably interact to create relatively narrow, curvilinear
10
Figure 10. Weather
patterns typically
encountered with cold
and warm fronts
associated with a
cyclone (low-pressure
system) over the
central U.S. The
occluded front formed
where cold and warm
fronts coalesced over
the northern plains.
Warm maritime tropical
air from the Gulf of
Mexico lies between
the two fronts. Note
that cloud cover occurs
in advance of the cold
front, adjacent to the
warm front, and around
the occluded front.
Lines A-B and C-D
represent sections
through the frontal
system (see Figs. 11
and 14).
zones that mark a front, a transition from one air mass to
another (Fig. 10). Advancing frontal systems bring clouds and
precipitation and are accompanied by changes in moisture,
temperature, pressure, and wind direction.
The clash between cP and mT air masses over the Great Plains
and Midwest is the most common source of frontal systems in
the U.S. (For more on the causes of this phenomenon, see the
section, Mid-latitude Cyclones.) Weather conditions change in
a predictable sequence as warm and cold fronts pass over an
area.
Weather Conditions Associated with a Passing Frontal System
Conditions
Before Warm Front
Between Warm and
Cold Fronts
After Cold Front
Pressure
Decreasing
Increasing
Winds
Temperature
Clouds
South, southeast
Cool
Cirrus, cirrostratus,
altostratus,
nimbostratus
Light-moderate,
increasing
Small decrease, then
small increase
Southwest
Warm
Cumulus,
cumulonimbus
Precipitation
Figure 11. Weather
conditions associated
with cross section AB on Figure 10. Warm
air (mT) lies between
the cold front and
warm front. The cold
front advances more
rapidly than the warm
front, forcing warm air
to rise, forming
thunderclouds and
heavy rains. Warm air
is forced to rise above
the more gently
sloping warm front,
resulting in the
formation of a series
of low to high clouds.
None, then heavy rains
prior to cold front
West, northwest
Cold
Cumulus, altostratus
Moderate-light,
decreasing
Cold Front
Cold, dense continental polar air replaces moist, warm
maritime tropical air across the cold front (Fig. 10). People
living downwind from the front experience decreasing
temperature and humidity and increasing atmospheric pressure
with the passage of the cold front. The cold front is pictured as
steep in Figure 11 but its actual inclination is ~1 degree toward
11
Figure 12. A squall
line highlighted by
intense
thunderstorms
associated with a
rapidly advancing
cold front, Gulf of
Mexico. Image
the warm air (not much, but twice as steep as a warm front).
Warm air will always rise over cooler air so both the cold and
warm fronts are inclined toward the warm air mass. Warm air
is pushed up and over the advancing cold front, causing
relatively rapid cooling and condensation that results in the
development of tall cumulonimbus clouds that host heavy but
relatively short-lived precipitation (Fig. 11). Rapidly advancing
cold fronts may be marked by the growth of a squall line of
thunderclouds (Fig. 12).
courtesy of NASA's
Johnson Space
Center Image
Services.
Warm Front
Changes following the passage of the warm front (Fig. 10) are
more benign than the storms that travel with the cold front.
Friction at Earth's surface causes the warm front to slope
gently (~½ degree inclination) toward the warm air mass (Fig.
11). Warm, humid air is transported upward over a distance of
approximately 1,000 km (625 miles). The first signal of an
approaching warm front is the appearance of light, upper-level
clouds (cirrus, cirrostratus). Up to 12 hours later, the high
clouds will be replaced by lower nimbostratus with associated
light to moderate precipitation. Rain associated with a warm
front may last longer than precipitation that accompanies a cold
front because the warm front typically moves more slowly and
extends over a larger area. Temperatures and humidity rise and
winds typically shift direction (from south to southwest) with
the passage of the warm front.
Occluded Front
The cold front moves more rapidly than the warm front (~ 10
km per hour faster) and will eventually close the gap between
the fronts, forcing the intervening warm air upward generating
additional precipitation (Figs. 13, 14). An occluded front is
represented by a combination of warm and cold front symbols
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Figure 13. An
occluded front forms
when a cold air
mass overtakes a
warmer air mass.
Figure 14.
Nimbostratus
clouds generate
precipitation
along an
occluded front
(see section C-D
on Figure 10).
on weather maps (Fig. 10). The occluded front juxtaposes two
bodies of cold air; the warmer of the two masses is forced up
and over the other. Occluded fronts may be marked by the
occurrence of nimbostratus clouds.
Think about it . . .
Examine the map located at the end of the chapter and
answer the conceptest questions about frontal systems
based on the locations featured on the map. One or two of
the questions may require you to read the section that
follows on mid-latitude cyclones.
Mid-latitude Cyclones
•
•
•
•
Weather in the eastern U.S. is mainly the result of the
migration of regional-scale low-pressure systems, termed
mid-latitude cyclones.
Mid-latitude cyclones develop where continental polar and
maritime tropical air masses collide over the U.S. along the
polar front.
Converging surface winds associated with low-pressure
systems must be matched with divergent flow in the upper
atmosphere.
Cyclones develop from a waveform that originates where
irregularities at the surface cause local shearing that distorts
the polar front.
Much of the weather experienced over the eastern U.S. is the
result of the west-to-east migration of regional-scale lowpressure systems known as mid-latitude cyclones (Fig. 15).
13
Figure 15. A classic
comma-shaped
cloud pattern is
associated with a
mid-latitude cyclone
in the central United
States, Christmas
Eve, 1997. A lowpressure center is
located over the
lower Mississippi
Valley and a warm
front spirals over the
Gulf of Mexico.
These weather patterns are differentiated from tropical
cyclones formed over the warm tropical ocean waters that may
build into hurricanes. Mid-latitude cyclones (also called wave
cyclones) may be 1,000 to 2,000 km (625-1,250 miles) across
and can affect much of the continental land mass for periods of
three days to as much as a week.
Image courtesy of
NASA-Goddard Space
Flight Center, NOAA
GOES.
Mid-latitude cyclones develop where continental polar and
maritime tropical air masses collide over the U.S. along the
polar front. The position of the collision zone migrates south
during winter and moves north with summer. The boundary
between air masses is initially a stationary front, with airflow
in opposite directions on either side. Perturbations in the upper
airflow of the jet stream are necessary to promote the growth of
a surface low-pressure system. The converging surface winds
associated with low-pressure systems must be matched with
divergent flow aloft to maintain the cyclone (Fig. 16). (For
more on airflow within a cyclone, see Cyclones and
Anticyclones in the chapter, The Atmosphere.)
Cyclones develop from a waveform that originates where
irregularities at the surface cause local shearing that distorts
the polar front. Features that may induce shearing include
Figure 16. Lowpressure systems
(cyclones) form
below regions of
divergent flow in the
jet stream.
14
mountains and contrasting atmospheric properties at land/water
boundaries. The waveform becomes exaggerated as warm air
pushes northward and cold air moves south, generating
counterclockwise rotation typical of cyclones and forming the
pairing of warm and cold fronts discussed in the previous
section.
Warm air advances along the warm front, at rates of 15 to 20
km per hour (9-13 mph), moving over ground previously
covered by colder air. The cold front lies to the west and moves
about twice as fast as the warm front (Fig. 17). Warm air is
forced aloft as the cold front sweeps across the previously
warm sector. The cold front and warm front eventually merge
to form an occluded front, merging cold air masses and
producing a more stable, stratified atmosphere where cool air
lies below warmer air and resulting in the decay of the cyclone.
Figure 17. Three
stages in
cyclogenesis, the
development of a
mid-latitude
cyclone over the
U.S. Note west to
east track of the
cyclone and the
merging of warm
and cold fronts to
form an occluded
front in the final
image shortly
before the
cyclone decays.
15
Think about it . . .
Review the Frontal Systems and Mid-latitude Cyclones
exercise (see end of chapter) referred to following the
previous section. Would you change any of the answers
after reading the section above?
Thunderstorms
•
•
•
•
Thunderstorms form where warm, humid air is forced
upward at cold fronts or as a result of differential heating at
Earth's surface.
Latent heat, released during condensation, generates
updrafts that maintain upward movement.
Thunderstorms are most frequent over the southeastern
U.S.
The three stages (cumulus, mature, dissipating) in the life
cycle of a thunderstorm occur over approximately two
hours.
Thunderstorms form where warm, humid air is forced
upward to altitudes of up to 15 km. Condensation occurs as
the air cools, releasing latent heat and ensuring that the rising
air remains unstable (warmer than surrounding air).
Thunderstorms may occur as relatively isolated, short-lived
events or as longer-duration severe storms depending upon the
conditions that cause the air to rise.
Figure 18. Afternoon
thunderstorms form
over the Florida
peninsula as humid
maritime tropical air
moves over the
warmer landmass.
16
Isolated afternoon thunderstorms, or cells, are commonplace in
warm summer months where moist maritime tropical air
masses move over land (Fig. 18). The temperature of the land
surface rises to a maximum during the mid-afternoon, warming
overlying air parcels and causing them to become unstable
enough to rise, generating scattered thunderstorms. (For more
on this process, see Clouds and Cloud Formation in the
chapter, The Atmosphere).
Figure 19. Map of
the average
number of days
per year with
thunderstorms
featuring
damaging winds
or wind speeds of
more than 50
knots/hour, 19801994.
Severe storms, or supercells, are associated with frontal lifting
along the cold front between the continental polar and maritime
tropical air masses in mid-latitude cyclones. These storms are
most common during spring and early summer, when the
contrast in temperatures between air masses is greatest.
Because thunderstorms are associated with mid-latitude
cyclones it should come as no surprise that storms are most
common over the Great Plains and southeastern U.S. States
such as Florida may experience over 100 days a year with
thunderstorms whereas such storms are rare (less than 20 per
year) in Pacific Coast states (Fig. 19).
17
There are three stages during the life of a typical thunderstorm
that rarely lasts for more than two hours:
1. Cumulus stage - early cloud development when a cumulus
cloud expands laterally and vertically as air enters the cloud
mass at all levels. Cloud formation is rapid, requiring
approximately 15 minutes to grow vertically to heights of
10 km (6 miles; Fig. 20a). Updrafts (~ 4 m/sec near ground
surface to ~ 10 m/sec at high levels) within the cloud carry
humid air to higher, colder levels where condensation
occurs.
Figure 20a.
Cumulus stage in
the life cycle of a
thunderstorm is
characterized by
rapid vertical
growth of the cloud
and condensation.
2. Mature stage - top of cloud cell may be at altitudes up to
15 km (9 miles; Figs. 20b, 21). Rain and ice formed by
condensation become too large to be supported by updrafts
and fall to ground. Falling precipitation generates friction
within rising air, creating a zone of downdrafts. Descending
air warms up, resulting in evaporation that absorbs latent
heat and cools the cloud. This stage lasts for 15 to 30
minutes generating rainfall or hail (Fig. 22) at the surface
accompanied by gusty winds (downdraft).
Figure 20b.
Downdrafts and
precipitation
characterize the
mature stage in
the life cycle of a
thunderstorm.
18
Figure 21.
Developing
thunderstorm
cloud (top) and
mature supercell
(below). Images
courtesy of NOAA
Photolibrary.
Figure 22. Hail coats
the ground after a
storm (above right).
Thunderstorms can
generate individual
hailstones up to 15
cm (6 inches) in
diameter (above).
The hail above is ~6
cm across. Images
courtesy of NOAA
photolibrary.
3. Dissipating stage - cloud formation processes end as
moisture is expended and descending air cools the cloud
mass, returning stability to air. The final stage is
characterized by diminishing precipitation (light rain) as
the cell dissolves (Fig. 20).
Tornadoes
•
•
•
•
Tornadoes are narrow, funnel-shaped spirals of wind that
rotate at speeds of up to 500 km/hr because of extreme
pressure gradients.
Tornadoes are ranked from F0 (weakest) to F5 (strongest)
using the Fujita Intensity scale.
Most tornadoes move to the east or northeast at an average
speed of approximately 50 km/hr.
Tornadoes are associated with thunderstorms and develop
in association with mesocyclones within the thunderstorm
cell.
19
•
The U.S. experiences more tornadoes than any other nation
and most occur in tornado alley (Texas, Oklahoma, Kansas,
Nebraska, Iowa, Illinois, and Indiana).
Tornadoes are narrow, funnel-shaped spirals of rapidly
rotating air (Fig. 23) that form in association with
thunderstorms. Like hurricanes and mid-latitude cyclones,
tornadoes are near-circular low-pressure systems. However,
the pressure gradient is much more intense for tornadoes.
Pressure differences across mid-latitude cyclones are in the
range of 20 to 30 mb (millibars) over hundreds of kilometers
(pressure gradient, 0.02-0.03 mb/km). Hurricanes may
experience pressure gradients of more than 100 mb over
shorter distances (~0.2-2 mb/km) but large pressure differences
in tornadoes occur over distances measured in hundreds of
meters. Extreme pressure gradients of up to ~0.1 to 1 mb/m
are possible for tornadoes, generating the strongest natural
winds on Earth with wind velocities of up to 500 km/hr.
Tornadoes are classified using the Fujita Intensity scale which
places tornadoes in one of six categories (F0-F5) according to
level of destruction which is taken as a proxy for wind speed.
The scale divides tornadoes into three subgroups: weak (F0,
F1); strong (F2, F3), and violent (F4, F5). Wind speed cannot be
measured directly because the high winds that can blast apart
whole buildings (Fig. 24) would make short work of measuring
instruments. Scientists use the level of destruction to gauge
estimates of wind speed and thus determine the Fujita value for
a specific tornado. This makes it difficult to rank winds that
touch down in sparsely populated areas.
Figure 23.
Twisting, nearvertical funnelshaped tornado.
Image courtesy of
NSSL's photo
album.
Fujita Intensity Scale
Scale
F0
F1
F2
F3
F4
F5
Wind Speed
km/hr (miles/hr)
<116 (<72)
116-180 (72-112)
181-253 (113-157)
254-332 (158-206)
333-419 (207-260)
>419 (>260)
Damage
Description
Light
Moderate
Considerable
Severe
Devastating
Incredible
Tornado Class
% of U.S.
Tornadoes
Time on
Ground
Weak
69%
<10 minutes
Strong
29%
~20 minutes
Violent
2%
> 1 hour
The funnel of the tornado moves more slowly than the winds
that give it shape. Funnels are typically less than 600 m (2,000
feet) wide and average funnel velocities are approximately 50
km/hr, although velocities as high as 200 km/hr (125 mph)
have been recorded. Tornado paths follow the direction of
movement of their parent thunderstorms that are in turn
20
Figure 24.
Destruction
associated with a
violent tornado in a
suburb of Oklahoma
City, May 3, 1999.
There were 38
deaths from this
single tornado that
reached F5 strength
along part of its
path. Image courtesy
of FEMA.
associated with east to northeast-directed mid-latitude
cyclones. Scientists have been unable to observe the birth of a
tornado because of the difficulty in determining exactly where
tornadoes may originate. One hypothesis on tornado formation
considers tornado development in three stages (Fig. 25):
•
Early stage: Friction slows winds at the ground surface,
resulting in increasing wind velocity with elevation in the
lower troposphere. These contrasting vertical wind speeds
generate local winds that rotate about a horizontal axis.
•
Updraft stage: Updrafts below thunderstorm cell draw the
spiraling horizontal winds upward forming a mesocyclone
(a small cyclone) within the larger storm cloud
cell. Mesocyclones may be up to 10 km in diameter.
•
Tornado stage: Rotation within the mesocyclone forms
smaller, more intense spiraling winds within a tornado that
extend downward from a cloud base toward the ground
surface (Fig. 26).
Figure 25.
Stages in the
development
of a tornado.
Tornado paths are typically 5 to 25 km (3-16 miles) in length
but some larger tornadoes may remain on the ground for over
100 km (62 miles). Smaller funnels may skip across the surface
like a fickle avenger, destroying one home while leaving
neighboring properties undisturbed. Improvements in
21
forecasting methods have reduced the number of fatalities
associated with tornadoes (Fig. 27). Approximately two-thirds
of U.S. fatalities occur as a result of tornadoes destroying
homes; nonpermanent mobile homes are especially susceptible
(Fig. 28).
Figure 26. Left:
Waterspouts are
tornado-like
phenomena formed
over water. Right:
The latter stages in
tornado
development for an
example that
touched down in
Enid, Oklahoma.
Images courtesy of
NSSL's photo album.
The U.S. is home to the majority of the world's tornadoes,
averaging about 1,000 a year. Tornadoes occur when
thunderstorm activity is at an optimum, during the late spring
and early summer across much of the nation. The highest
frequency of tornadoes per area occurs over the Great Plains
states (Texas, Oklahoma, Kansas, Nebraska) and parts of the
upper Midwest (Iowa, Indiana, Illinois), a region that has come
to be known as tornado alley (Figs. 29, 30).
Figure 27. U.S.
tornado fatalities
have declined as
forecasting
technology
improved to provide
better tornado
warnings.
Figure 28.
Proportion of U.S.
tornado fatalities by
location, 1985-1998.
The timing of tornado activity is tied to seasonal movement of
the polar front that drives mid-latitude cyclones. Tornadoes
move out of the Gulf Coast and southeastern states into the
22
Great Plains as the front retreats northward in late spring.
Summer sees tornado activity shifting to the northern Plains
states and the upper Midwest.
Figure 29.
Average annual
distribution of
strong and
violent
tornadoes,
1950-1995.
Figure 30.
Tornado
bearing down
across open
country. Image
courtesy of
NSSL's photo
album.
Think about it . . .
Use the Venn diagram located at the end of the chapter to
compare and contrast the characteristics of tornadoes and
hurricanes.
Hurricanes
•
•
•
Hurricanes are rotating storm systems hundreds of
kilometers across that strike the U.S. during summer and
early fall.
Hurricanes grow from tropical depressions in regions of
convergent winds and warm oceans, and are sustained by
divergent airflow in the upper troposphere.
Destruction from high winds, heavy rains, and coastal
flooding occurs when hurricanes make landfall.
23
•
Hurricanes are divided into five categories by wind speed
using the Saffir-Simpson hurricane intensity scale.
Hurricanes are cyclonic storm systems that form over tropical
oceans during summer and fall. Also known as typhoons
(Pacific Ocean) and cyclones (Indian Ocean), hurricanes are
characterized by high winds (more than 119 km/hr , 74 mph),
heavy rainfall (10-25 cm 4-10 inches), and storm surges
(sudden rise in sea level) along coastlines. Hurricanes are
smaller (approximately a third to half the size) and less
frequent than mid-latitude cyclones that govern most U.S.
weather patterns but they have much more powerful winds.
Unlike mid-latitude cyclones, hurricanes do not originate as a
result of perturbations at a boundary between contrasting air
masses. Instead they grow from tropical storms generated by
disturbances in the belt of equatorial trade winds.
Hurricanes can result in fatalities and substantial property
damages as winds and high seas cause structural damage to
ocean-going vessels and coastal developments, and heavy
rainfall leads to flooding as much as 200 km inland from the
coast. An estimated 300,000 people died in Bangladesh (Fig.
31) in 1973 when cyclone pushed onshore from the Bay of
Bengal, generating a 7 meter (22 foot) storm surge and
producing widespread flooding of the low-lying coastal plain.
The government of Bangladesh built nearly a thousand
concrete shelters in coastal communities with sufficient space
to shelter a million residents and improved communications
links to try to reduce the danger from future cyclones.
Hurricane Andrew became the costliest natural disaster to
affect North America when it decimated much of southern
Florida in 1992, generating up to $30 billion in damages.
Building a Hurricane
Hurricanes develop under a specific suite of conditions
including warm surface waters, cyclonic circulation, and
divergent flow in the upper troposphere.
•
24
The initial stage in the development of a hurricane is the
formation of a tropical depression (low-pressure system)
where the trade winds converge near the equator. The
location of the convergent winds changes with seasons,
lying north of the equator during summer in the Northern
Hemisphere and migrating to southern latitudes during our
winter. The rising air cools and condenses to form cumulus
Figure 31.
Location of
Bangladesh.
clouds that will develop into cumulonimbus cells if the
rising air is sufficiently warm and humid.
Figure 32.
Hurricanes
originate in
areas of the
world's oceans
where water
temperatures
are greater than
27oC.
Hurricanes in
the Northern
Hemisphere are
most common
during summer
and early fall.
Southern
Hemisphere
hurricanes are
frequent during
our winter (their
summer).
•
Water temperatures must be at least 27oC and should
extend downward for 50 to 65 meters (165-215 feet) to
ensure that colder water won't be drawn to the surface by
the developing storm. Warm surface waters typically
straddle the equator but are absent south of the equator in
the eastern Pacific and Atlantic Oceans due to oceanic
circulation patterns (Fig. 32). The North Atlantic
hurricane season officially lasts from June 1 to November
30, and most U.S. hurricanes strike in August and
September.
•
Earth's rotation as reflected in the Coriolis effect imparts a
clockwise (Southern Hemisphere) or counterclockwise
(Northern Hemisphere) rotation to the growing storm (Fig.
33). The magnitude of the Coriolis effect increases with
increasing latitude and is zero at the equator. Consequently,
the necessary rotation is not imparted on storms within 5
degrees either side of the equator.
•
The inflow of air into the developing low-pressure system
must be matched with an outflow of air in the upper
troposphere to maintain the pressure gradient in the
developing hurricane. If not, the pressure contrast decreases
and wind speed declines.
If all of these conditions are met, a tropical depression (wind
speed <37 km/hr) forms and has the potential of growing to a
tropical storm (wind speed 63-119 km/hr) before developing
into a hurricane with wind speeds of at least 119 km/hr. Wind
speed increases in inverse proportion to the decrease in
pressure in the eye of the hurricane; the lower the pressure the
higher the wind velocity. Rising air in the deepening lowpressure system cools and condenses, releasing latent heat and
generating a dense spiral of cumulonimbus clouds punctuated
25
by a central eye characterized by clear skies. Bands of clouds
spiral outward from the vortex that will continue to grow in
size and intensity as long as the underlying water temperature
remains above 27oC. Precipitation is concentrated within a
radius of approximately 100 km on either side of the eye,
releasing up to 20 billion tons of water per day.
Hurricane Landfall
Atlantic hurricanes, driven westward by prevailing winds at
rates of 10-25 km/hr (6-16 mph), may turn north parallel to the
U.S. east coast or pass south of Florida to strike along the Gulf
Coast or Caribbean islands (Figs. 34, 35). Florida and Texas
experience more hurricane landfalls than any other states (Fig.
35). A hurricane will begin to decay when it passes over land
as it experiences greater frictional drag and a dramatic decrease
in the water supply that is essential for its maintenance.
Although wind speeds will be reduced to the level of a tropical
storm or depression, the storm itself is still capable of dumping
large volumes of rain for some distance inland.
26
Figure 33. Four
hurricanes in the
North Atlantic
Ocean, 1998.
Image courtesy of
NOAA.
Figure 34.
Selected
hurricane tracks
for storms that
originated in the
Atlantic Ocean.
Figure 35.
Number of
hurricane
landfalls by state
1900-1996. Blue
- all hurricanes;
red - major
(category 3, 4,
and 5) storms.
OT (other)
includes
Delaware, Maine,
Maryland,
Massachusetts,
New Hampshire,
and New Jersey.
Much of the destruction associated with hurricanes isn't caused
by high winds or storm surges but is linked to the exceptional
precipitation events that can unload 60 cm (24 inches) of rain
from a single storm system in just a few days. Heavy rains
from Hurricane Mitch (Fig. 36), the most devastating storm to
strike North America and Central America in the last two
centuries, claimed over 10,000 lives from flooding (both inland
and coastal flooding) and landslides. Honduras, one of the
hemisphere's poorest nations, took the brunt of the
storm. Entire villages were demolished, nearly 20% of the
population evacuated their homes, a quarter of the schools were
wrecked, water supplies were cut off, and almost all major
roads were damaged. The nation's economy was devastated and
the loss to sugar, banana, and sugar crops pushed up prices of
those commodities worldwide.
27
Figure 36.
Hurricane Mitch
approaching the
coast of Central
America. Image
courtesy of
NOAA.
As hurricanes approach landfall, winds in the northeast
quadrant of the storm, north of the eye, are blowing onshore,
piling up water in a storm surge (Fig. 37). However, winds in
the southwest quadrant of the storm, south of the eye, are
blowing offshore, and opposite to the direction of the storm's
movement. Offshore winds reduce the local impact of the
storm surge. Emergency workers are especially interested in
identifying where the hurricane eye comes onshore as it allows
them to better gauge the potential for destruction and to
distribute their staff accordingly. The National Hurricane
Center's forecasters add an extra 145 km on either side of the
projected landfall site issued 24 hours ahead of the storm
because of the capricious nature of hurricane motion. Storms
may change speeds, remain stationary, or reverse direction,
making it difficult to predict exactly where and when to
evacuate residents.
Figure 37.
Hurricane Floyd
(1999)
threatened
much of the
southeastern
U.S. before
making landfall
in North
Carolina. Image
courtesy of
NOAA.
28
The largest peacetime evacuation in U.S. history occurred in
advance of Hurricane Floyd which endangered Florida and
other southeastern states in September 1999 (Fig. 37). Floyd
was unusually large and threatened to affect a substantial
length of highly developed coastline. Authorities err on the side
of caution and evacuate large regions in advance of a hurricane
because of the difficulty in accurately predicting the storms
future path. Florida residents breathed a sigh of relief when
Floyd turned north, eventually coming onshore in North
Carolina. Thousands of people stranded on crowded, slowmoving highways highlighted the difficulty of evacuating large
populations in advance of an impending storm.
Hurricane Measurement
Hurricanes are divided into five categories by wind speed using
the Saffir-Simpson hurricane intensity scale. Destruction
associated with major (category 3, 4 and 5) hurricanes includes
damage to permanent homes, widespread coastal flooding,
uprooting of trees, toppling of power lines. Anticipation of
such damages prompts evacuation of residents from the area of
expected landfall. Hurricane Camille in 1969 was the most
recent category 5 hurricane to make landfall in the U.S.,
coming onshore along the Gulf Coast of Mississippi.
Saffir-Simpson Hurricane Intensity Scale
Category
Wind Speed
km/hr (miles/hr)
Pressure
(millibars)
Storm Surge
meters (feet)
Damage
Description
1
119-154 (74-95)
>980
1.2-1.5 (4-5)
Minimal
2
3
155-178 (96-110)
179-210 (111-130)
965-979
945-964
1.6-2.4 (6-8)
2.5-3.6 (9-12)
Moderate
Extensive
4
211-250 (131-155)
920-944
3.7-5.4 (13-18)
Extreme
5
>250 (>155)
<920
> 5.4 (>18)
Catastrophic
Camille caused 256 deaths and over a $1 billion in damages
(Fig. 38). The cost of the same storm today would be over $11
billion because of the increased development along the
coastline. The most extensive coastal development has taken
place in Florida, greatly increasing the potential damages from
a major storm. A recent study suggests that an unnamed 1926
category 4 hurricane that smashed into southern Florida before
crossing the Gulf of Mexico to Alabama, would generate
damages totaling $77 million (twice the cost of Andrew, Fig.
29
39) if it were to occur today. The convention of naming
hurricanes didn't begin until the 1950's.
Figure 38.
Before and after
pictures of
damage to a
hotel resulting
from Hurricane
Camille. Thirty
people at a
“hurricane
party” on the
site died.
Images courtesy
of NOAA's
photolibrary.
Figure 39.
Destruction of
homes in a culde-sac in
southern Florida,
caused by
Hurricane
Andrew, a
category 4 storm.
Image courtesy of
NOAA.
Think about it . . .
1. Use the Venn diagram located at the end of the chapter
to compare and contrast the characteristics of tornadoes
and hurricanes.
2. You work in a team of disaster specialists for the
Weather Channel. The channel wants to create its own
scoring system that better evaluates the potential
damage from incoming storms. You and your team are
given the assignment to create an evaluation rubric to
assess factors that will influence the risk of potential
damage from a future hurricane. Go to the end of the
chapter to complete the exercise.
30
Summary
1. Where does the daily weather forecast come from?
Despite what they claim about "exclusive" weather
information, the original data for all local weather forecasts
comes from observations made by the National Weather
Service. These observations may be used to generate
impressive graphics by commercial weather companies but the
basic data come from the NWS.
2. What is an air mass?
An air mass is a large region of the lower troposphere with
relatively uniform properties (temperature, moisture content).
An air mass develops when the atmosphere above a land or
water surface adopts the characteristics of the underlying
surface. Air masses are modified as their properties change
when they move over land/water surfaces with contrasting
temperatures and/or moisture content.
3. How are air masses identified?
Air masses are differentiated by their temperatures and
moisture content. The most common types are categorized as
polar (cold) or tropical (warm), continental (dry) or maritime
(humid). The air masses can be represented by symbols such
as cT (continental tropical), cP (continental polar), mT
(maritime tropical), and mP (maritime polar).
4. Which air masses have the greatest impact on U.S.
weather?
Continental polar and maritime tropical air masses interact over
the central and eastern U.S. Continental polar air forms over
the northernmost portions of North America. High
temperatures and high humidity distinguish the maritime
tropical air masses that move inland from the tropical Pacific
Ocean, Gulf of Mexico, or tropical Atlantic Ocean.
5. What is a frontal system?
Fronts mark the boundaries between air masses of contrasting
properties. Fronts come in two basic varieties. Cold fronts form
where cold polar air masses move over ground previously
occupied by warm air. Warm fronts form where warm air
moves over surfaces previously covered by cold air masses.
One consequence of either situation is that warm air rises
above the underlying colder air, forming clouds and releasing
precipitation.
31
6. What is the difference between weather at cold and warm
fronts?
Heavy rainfall, decreasing temperatures, decreasing humidity
and changing wind directions are associated with passage of a
cold front. Warm air will always rise above cold air so the front
is inclined toward the warmer air mass. Friction at the ground
surface causes the front to steepen, forcing warm air aloft more
rapidly and leading to the development of tall cumulonimbus
clouds. The warm front slopes gently toward the warm air mass
and humid air rises gradually over a distance of hundreds of
kilometers. Warm fronts are characterized by light to moderate
rain, warmer temperatures, increasing humidity, and changing
wind directions.
7. What is an occluded front?
Occluded fronts form when a warm air mass is forced aloft
between two bodies of cooler air. The front marks the location
on the surface where the colder air masses meet.
8. What is a mid-latitude cyclone?
Mid-latitude cyclones are low-pressure systems that may be
over a thousand kilometers wide and migrate from west to east
across North America. The position of the cyclone will change
with seasons but typically marks the boundary between cool
continental polar and warmer maritime tropical air masses.
9. How are frontal systems related to mid-latitude cyclones?
Mid-latitude cyclones develop where cold and warm air masses
collide along the polar front. The warm air pushes northward
and is surrounded by cooler air. The boundaries between air
masses represent fronts. The initial boundary is a stationary
front that becomes distorted to form a pairing of cold and warm
fronts. These fronts travel eastward with the cyclone but the
cold front typically moves more rapidly and overtakes the
warm front to form an occluded front near the east coast.
10. Under what conditions do thunderstorms form?
Thunderstorms are characterized by clouds that span the range
from low- to high-level clouds and form under unstable
atmospheric conditions caused by the rapid rise of warm,
humid air. Warm air rises rapidly at cold fronts (associated
with mid-latitude cyclones) or where differential heating
occurs during hot weather.
11. What are the stages of thunderstorm development?
32
There are three stages in the life of a thunderstorm. A cumulus
cloud grows rapidly during the early (cumulus) stage as
updrafts carry warm, humid air to high elevations.
Condensation produces rain or hail during the second (mature)
stage, and the falling precipitation helps cool the cloud and
stop growth. The third (dissipating) stage is marked by the
decay of the cloud as the supply of moisture is depleted and the
air becomes stable.
12. Is there any similarity between mid-latitude cyclones,
tornadoes, and hurricanes?
All are examples of low-pressure systems where air converges
inward from areas of higher pressure. The speed of the winds is
greatest for tornadoes where the pressure gradient is greatest
(change in pressure over smallest distance) and is least for midlatitude cyclones that have the smallest pressure gradient.
13. How do scientists classify tornadoes?
Tornadoes are classified by their wind speed using the Fujita
Intensity scale. The scale divides tornadoes into six categories
(F0-F5) based upon level of destruction. Destruction is matched
to wind speed that is too high to measure with conventional
instruments. F0 and F1 tornadoes are weak tornadoes with wind
speeds up to 180 km/hr. F2 and F3 are strong tornadoes with
wind speeds from 181 to 332 km/hr. Violent (F4, F5) tornadoes
represent only 2% of all tornadoes and are characterized by
estimated wind speeds of more than 332 km/hr.
14. How do tornadoes form?
Scientists can't get too close to tornadoes because of their
intense speeds so direct observations of tornado formation have
not been made but there are thought to be three stages to
tornado formation. The first stage in tornado formation is the
development of horizontally spinning winds just above the
ground surface. These horizontal spirals form mesocyclones
(small-scale low-pressure systems) that are pulled into
thunderstorm clouds by updrafts of warm air and begin
spinning about a near-vertical axis. Rotation within the
mesocyclone generates smaller intense twisters that grow
downward to the ground surface as narrow funnels.
15. Why does the U.S. have more tornadoes than any place on
Earth?
The location of tornadoes is directly linked to the passage of
the mid-latitude cyclones across the Great Plains and Midwest.
Most tornadoes grow from thunderstorms formed at cold fronts
33
where cold and warm air masses interact. The North American
continent at relatively high latitudes and the warm tropical
Atlantic Ocean and Gulf of Mexico provide the ideal breeding
grounds for air masses of contrasting properties needed to
generate the necessary atmospheric conditions to form
tornadoes.
16. What are the key conditions needed for hurricane
formation?
Hurricanes form over warm ocean waters with temperatures of
at least 27oC (80oF) extending to depths of ~50 meters. In
addition, the Coriolis effect must be sufficient to impart
rotation on the low-pressure system that will evolve into a
hurricane. Combining these two factors requires that storms
cannot form at the equator (where the Coriolis effect is zero)
and can't form beyond latitudes that are more than 20 degrees
north or south of the equator (where waters are too cool).
17. What hazards are associated with a hurricane?
Hurricanes endanger lives and property because of their high
winds, heavy rainfall (and resulting flooding), and storm surges
that generate waves of more than 7 meters above normal sea
level. The size of a hurricane means that it will affect a large
area if its eye comes within a few hundred kilometers of the
coastline.
18. Why do hurricanes affect the east coast and not the west
coast?
Hurricanes travel in the direction of the prevailing atmospheric
and oceanic circulation systems. Hurricanes move from east to
west across the Atlantic and Pacific Oceans following the trade
winds. The prevailing wind direction therefore carries Atlantic
hurricanes toward a U.S. landfall while transporting Pacific
storms away from the West Coast.
34
Weather Hazards
Examine the maps of extreme weather events for 2000.
1. What patterns can you identify in the weather
characteristics displayed on the maps?
2. Identify three states that are relatively free of weather
hazards.
3. Identify three states that have the highest risk of weather
hazards.
35
Frontal Systems and Mid-latitude Cyclones
Use the map to answer the questions that follow.
1. The map illustrates the relative positions of a warm front
and a cold front associated with a mid-latitude cyclone.
Where is the warm front located?
a) between A and B
b) between C and D
c) at E
2. Where is it raining?
a) A and B
b) B and C
c) C and D
d) B and D
e) A and C
3. Which location is in a maritime tropical air mass?
a) A
b) G
c) E
d) H
4. What direction is the mid-latitude cyclone traveling
toward?
a) F
b) G
c) H
5. Winds at C would be ________________________.
a) southwesterly
b) southeasterly
c) northeasterly
d) northwesterly
6. Which location will become warmer in the next 12 hours?
a) A
b) B
c) C
d) D
e) E
7. Which of the images below best represents conditions
between A and D on the map?
36
Venn Diagram: Tornadoes vs. Hurricanes
Use the Venn diagram, below, to compare and contrast the
similarities and differences between tornadoes and hurricanes.
Print this page and write features unique to either group in the
larger areas of the left and right circles; note features that they
share in the overlap area in the center of the image.
Tornadoes
Hurricanes
37
Hurricane Evaluation Rubric
You work in a team of disaster specialists for the Weather
Channel. During discussions about coverage of the upcoming
hurricane season your boss states that she doesn't believe that
the Saffir-Simpson scale sufficiently reflects the risks
associated with hurricanes because it emphasizes one factor
(wind speed). The channel wants to create its own scoring
system that better evaluates the potential damage from
incoming storms.
You and your team are given the assignment to create a
evaluation rubric to assess factors that will influence the risk of
potential damage from a future hurricane. One factor is
included as an example in the table below, identify five more.
Consider both physical and cultural factors when developing
your rubric.
Factors
Wind speed
38
Low Risk
(1 point)
Moderate Risk
(2 points)
High Risk
(3 points)
Low
(category 1, 2)
Intermediate
(category 3)
High
(category 4, 5)
Reviewing your evaluation rubric you realize that some factors
are more significant than others. Your team decides to double
the score of the most important factor. Which do they choose?
Why?
Read the descriptions of Hurricanes Floyd, Dennis, and Mitch
that follow. Would you change any categories in your scoring
rubric? Rank these storms using your modified rubric.
39
Hurricane Floyd, September 1999
Floyd brought flooding rains, high winds, and rough seas along
a good portion of the Atlantic seaboard from the 14th through
the 18th of September. The greatest damages were along the
eastern Carolinas northeast into New Jersey, and adjacent areas
northeastward along the east coast into Maine. Several states
had numerous counties declared disaster areas. Flooding
caused major problems across the region, and at least 77 deaths
have been reported. Damages are estimated to be $1.6 billion in
Pitt County, North Carolina alone, and total storm damages
may surpass the $6 billion caused by Hurricane Fran in 1996.
Although Hurricane Floyd reached category 4 intensity in the
Bahamas, it weakened to category 2 intensity at landfall in
North Carolina. Floyd's large size was a greater problem than
its winds, because the heavy rainfall covered a larger area and
lasted longer than with a typical category 2 hurricane.
Approximately 2.6 million people evacuated their homes in
Florida, Georgia, and the Carolinas—the largest peacetime
evacuation in U.S. history. Ten states were declared major
disaster areas as a result of Floyd, including Connecticut,
Delaware, Florida, Maryland, New Jersey, New York, North
Carolina, Pennsylvania, South Carolina and Virginia. There
were several reports from the Bahamas area northward of wave
heights exceeding 50 feet. The maximum storm surge was
estimated to be 10.3 feet on Masonborough Island in New
Hanover County, NC.
Hurricane Dennis, August 1999
The coastal areas of North Carolina had their fourth tropical
storm scare in as many years during August 29th and 30th.
Hurricane Dennis developed over the eastern Bahamas on the
26th and drifted northward parallel to the southeast U.S. coast
from the 26th through the 30th. Dennis became an immediate
threat to southeastern North Carolina on the 29th. The center
approached to within 60 miles of the coast early on the 30th as
a strong category 2 hurricane with highest sustained winds of
105 miles per hour. Due to the fact that the hurricane never
made landfall, damage was only moderate. Rainfall amounts
approached 10 inches in coastal southeastern North Carolina
and beach erosion was substantial. This area is no stranger to
hurricane activity. Category 2 hurricane Bertha and category 3
hurricane Fran hit Brunswick County in 1996 and Hurricane
Bonnie (category 2) followed nearly the same path in 1998.
Prior to 1996, the area had been spared from the direct impact
40
of a hurricane since Charlie (category 1) hit Carteret County in
1986.
Dennis made a return visit in September as a tropical storm,
moving west-northwest through eastern and central North
Carolina. The main impact this time was flooding due to heavy
rains, with the maximum preliminary report being 13.82 inches
in Allisonia, VA. However, due to the storm lingering off the
coast for several days, beach erosion and damage to coastal
highways was significant. Residents of Hatteras and Ocracoke
Islands were stranded for several days because of severe
damage to Highway 12.
Hurricane Mitch, October/November, 1998
Hurricane Mitch will be remembered as the most deadly
hurricane to strike the Western Hemisphere in the last two
centuries! The death toll currently is reported as 11,000 with
thousands of others missing. More than three million people
were either homeless or severely affected. In this extremely
poor third world region of the globe, estimates of the total
damage from the storm are at $5 billion and rising. The
President of Honduras, Carlos Flores Facusse, claimed the
storm destroyed 50 years of progress.
Within four days of its birth as a tropical depression on
October 22, Mitch had grown into a category 5 storm on the
Saffir-Simpson hurricane scale. On October 26, the monster
storm had deepened to a pressure of 905 millibars with
sustained winds of 155 knots (180 mph) and gusts well over
200 mph!
Mitch moved westward and on October 27, the category 5
storm was about 60 miles north of Honduras. Preliminary
wave-height estimates north of Honduras during this time at
the height of the hurricane are as high as 44 feet, according to
one wave model. Although its ferocious winds began to abate
slowly, it took Mitch two days to drift southward to make
landfall. Coastal regions and the offshore Honduran island of
Guanaja were devastated. Mitch then began a slow westward
drift through the mountainous interior of Honduras, finally
reaching the border with Guatemala two days later on October
31. Although the ferocity of the winds decreased during the
westward drift, the storm produced enormous amounts of
precipitation caused in part by the mountains of Central
America. As Mitch's feeder bands swirled into its center from
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both the Caribbean and the Pacific Ocean to its south, the stage
was set for a disaster of epic proportions. Taking into account
the orographic effects by the volcanic peaks of Central
America and Mitch's slow movement, rain fell at the rate of a
foot or two per day in many of the mountainous regions. Total
rainfall has been reported as high as 75 inches for the entire
storm. The resulting floods and mud slides virtually destroyed
the entire infrastructure of Honduras and devastated parts of
Nicaragua, Guatemala, Belize, and El Salvador. Whole villages
and their inhabitants were swept away in the torrents of flood
waters and deep mud that came rushing down the
mountainsides. Hundreds of thousands of homes were
destroyed.
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