Born as tropical storms, hurricanes are
	cyclones with a calm central eye and a
	surrounding wall of wind, rain, and
	clouds. When fully developed, they can
	exceed wind speeds of 155 miles per
	hour (250 kph). Hurricanes require an
	entire day or more to pass over a
	given area; they sometimes cause
	great damage.

	Hurricanes (winds of 74 mphll 19 kph or
	more) and tropical storm (winds of 39-
	73 mphl63-117 kph) have distinctive
	features that separate them from most
	other types of storms. Large storms in the
	tropics covering a large area and lasting
	several days without major changes in
	their structure are broadly called tropical
	cyclones, a term that also includes
	tropical depressions (winds under 39
	mph/63 kph). All hurricanes proceed
	through the sequence of tropical
	depression and tropical storm before
	achieving designation as a hurricane.
	Often during a hurricane or tropical
	storm there are mixed layers of cumulus
	and cumulonimbus clouds in the cloud
	shield of the major storm. From the
	ground they sometimes appear as long,
	curved lines or rows of clouds-the rain
	bands-at various atmospheric levels.
	These clouds stretch across the sky.
	All hurricanes have an "eye"-an area
	of nearly calm winds in the middle of
	the eye wall, where'the storm's
	strongest winds blow in a circular band
	that borders the eye. It is near the eye
	that very heavy rainfall often occurs,
	many times exceeding 10" (25 cm)
	or more.
	As a result of the storm's strong effect
	on the surface of the ocean, its bays,
	inlets, canals, bridges, islands, and
	shorelines are vulnerable to catastrophic
	flooding due to the storm surge and
	wave action.
	Because of their sill hurricanes and
	tropical storms take u day or more to
	completely pass over a given place.

	For a tropical cyclone to form, the
	temperature of the ocean should be
	above about 78ºF (25.5ºC). At these
	Temperatures, adequate heat from
	condensing water vapor produces cloud
	growth and leads to the unstable
	conditions that permit the storm to
	become more intense. Also, at these
	temperatures the moisture content of
	the air near the ocean's surface is
	very high.
	A tropical cyclone can form only in
	areas more than about 10º latitude from
	the equator, where the earth's rotation
	is strong enough to translate into a
	rotating storm. The causes may be a
	preexisting traveling disturbance, an
	upper-level trough, or sometimes a very
	complex combination of wind and
	temperature patterns at all levels. These
	specific conditions for storm formation
	can often be identified and forecast
	successfully by highly specialized
	meteorologists using the most recent
	data from the tropics.

	May-November for tropical cyclones
	that form in the Atlantic, Gulf of
 	Mexico, and Gulf of California, and
	that may later affect the United States
	and Canada.

	The strongest tropical storms and
	hurricanes are most likely to occur in
	the late summer along the Atlantic and
	Gulf of Mexico coasts. Within the first
	hundred miles of the coast where the
	tropical cyclone comes ashore, the
	maximum winds, heaviest rainfall,
	and highest storm surge are felt near
	the eye.
	Over the North American land mass,
	the impact of tropical cyclones can
	extend inland for a great distance. In
	the Southern, Eastern, Atlantic, and
	New England states of the United
	States, and the Maritime Provinces of
	Canada, the heavy rain (more than
	5"/13 cm) and strong winds (more than
	50 mph/80 kph) can extend for
	hundreds of miles inland from the
	landfall position. The resulting rainfall
	is often considered beneficial if it is not
	extreme in amount, duration, or
	intensity. Over Texas and the southern
	Plains States, rainfall and clouds from
	storms entering the Texas coast may
	bring substantial to excessive rainfall.
	Southern California and Arizona, and to
	a lesser extent the states to their
	northeast, receive some rain and clouds
	every year from tropical storms or
	hurricanes, or their remnants, during
	the late summer to fall months.

	During a hurricane there
	are often lulls in the wind and rain
	between the very intense rain bands or
	in the eye itself. Since visibility is often
	poor, these bands are hard to see as they
	approach. Wind and wave conditions
	can worsen in a matter of minutes, with
	few visible warning signs in the sky. A
	turbulent sky may be seen just
	before a rain band of a tropical cyclone
	passes overhead. Such a sky can be
	accompanied by strong and very gusty
	winds over wide areas, making aviation
	and boating activities virtually
	impossible. In addition, tornadoes form
	in some hurricanes and tropical storms,
	after they have made landfall.

	The damage from tropical cyclones
	ranges from complete devastation,
	caused by the passage of the eyewall of
	a very intense hurricane along the coast,
	to a minor nuisance, produced by a
	weak tropical storm whose effects
	resemble those of a strong
	thunderstorm. In assessing the threat
	from tropical cyclones, both water and
	wind impacts are important.
	The greatest damage from tropical
	cyclones is from water effects. The
	storm surge is often close to 10' (3 m)
	high or more for a hurricane, and is
	particularly important because it causes
	a general rise in the level of the ocean as
	the storm approaches a bay, inlet,
	island, bridge, or other coastal feature
	or structure. On top of the storm surge
	is the daily tide, which may increase
	the storm surge by several more feet.
	Finally, over this general rise are waves
	ranging from 25' (8 m) or higher
	during the most intense storms.
	Damage from winds increases as the
	square of the speed; that is, the speed
	multiplied by itself. For example, the
	force of the wind in a weak hurricane
	(74 mph/ 119 kph) is doubled when the
	wind increases to about 100 mph (an
	increase of the wind's speed by only
	one-third) . Such information is
	important in order to take precautions
	against the dangerous effects of high
	wind. Precautions against falling trees
	and other windblown objects, power
	lines, and other structures need to be
	anticipated when the forecast projects
	such strong winds. In many locations in
	the United States, winds over 50 mph
	(80 kph) occur almost every year, and
	not too infrequently they reach 75 mph
	(121 kph) without causing much
	damage. However, since a 100-mileper-
	hour wind doubles the damage, it
	is potentially much more significant.

	Most of the United States is affected in
	some way every year by the direct or
	indirect wind and water effects of
	tropical storms and hurricanes. The
	U.S. coastline is becoming increasingly
	populated in urban areas, and a large
	proportion of these people are not
	familiar with the strength and effects
	of tropical cyclones. While the most
	common experience is of the weaker
	tropical storm and its relatively minor
	flooding and winds, the potential exists
	for extreme damage along the coasts,
	and for severe flooding inland under
	specific conditions.

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