10. Atmospheric Disturbances, Storms and Natural Hazards

Atmospheric Disturbances,
Storms and Natural Hazards
Storms and Atmospheric Disturbances
 Embedded within the wind belts of the general atmospheric
circulation are secondary circulations, which are made
up of storms and other atmospheric disturbances
 Atmospheric disturbance is a broader and more general term
than storms – includes variations in secondary circulation
 Such Disturbances include:
a) Middle Latitude Cyclones
b) Cyclones and Anticyclones, and
c) Weak Tropical Disturbances
(e.g., Easterly Waves, Polar Outbreaks)
 Well-defined Storms include: Hurricanes, Tornadoes,
Thunderstorms, Snowstorms and Blizzards
Middle Latitude Cyclones
Also called Extra-tropical Cyclones or Mid-latitude Cyclones
These migrating “storms” develop at the polar front,
and then travel along it
 With their opposing cold, dry polar air and warm,
humid subtropical air, these can cause significant
variations in day-to-day weather
 They evolve as they move,
and can last a few days or a even a week
 Can travel about 40 mph,
and can be 800 - 1000 miles wide
 Low Pressure at the center of the storm
 Counterclockwise winds
 A warm front
 A cold front
 A pie-shaped
wedge of
warm air
 Surrounded
by cold
air mass
 Lighter, warm, moist air rides up over cold air – Warm Front
 Denser, cold air pushes into warmer air and forces it to rise –
Cold Front
A Model and Cross Sections of a Mid-latitude Cyclone
Stages in the Development of a Mid-latitude Cyclone
Fig. 7-7, p. 183
Tracks of Mid-latitude Storms in the U.S.
– West to East
 A hurricane (tropical cyclone) is a large rotating cyclonic system
of high wind velocity – with a forward motion of 4-5 mph.
[also called typhoons (East Asia), willy-willies (Australia),
bagyos (the Philippines), or simply, cyclones (South Asia)].
 They are the largest and most destructive storms on earth,
often lasting 10-14 days.
 The devastation by these massive storms are not mainly due to the
high winds but because of the associated flooding/storm surges,
tornadoes, and lightning.
 Hurricane Katrina, which struck the Gulf Coast in 2005, wreaking
havoc in Louisiana, Mississippi and Alabama, was among the
most devastating in U.S. history.
Hurricane Formation
 A hurricane develops from a tropical disturbance when sustained
winds exceed 74 mph (Category 1), often going to more than
155 mph (Category 5) – Names are assigned once storms
reach tropical storm status (39-74 mph)
 Among the factors leading to hurricane development are:
 a warm ocean surface of about 77˚F;
 warm, moist overlying air; and
 Coriolis effect must be sufficient to support spiraling
 Therefore, hurricanes do not develop or survive in the equatorial
zone (about 10o N or S latitude) as the Coriolis effect is too
weak closer to the equator.
 The "fuel" for a hurricane comes from the enormous amount of
latent heat released from the warm ocean water.
Hurricane Structure
 In terms of its structure a hurricane is a warm-core low pressure
system with a diameter of 100-400 miles, and extending to
heights of 40,000-45,000 feet.
 The center or eye of the hurricane is an area (12 to 40 miles wide)
of nearly cloudless skies, subsiding air, and light winds.
 At the periphery of the eye is a ring of cumulonimbus clouds that
produce torrential rains and extremely strong winds.
Surrounding the core are the typical spiraling rain bands.
 As a hurricane moves over a colder land surface, it loses its
source of energy and dissipates. But the system remains an
organized storm for several days, flooding the interior with
rainfall. Tornadoes often accompany hurricanes as they move
 Thunderstorms are local storms
in the middle and lower
latitudes that are
accompanied by thunder
and lightning
 Lightning is an intense discharge
of electricity – the charges
are generated by the intense
friction of the air on moving
ice particles within a
Cumulonimbus cloud
 When the difference between the positive and negative charges in
the cloud becomes large enough to overcome the natural
insulating effect of the air, a lightning flash takes place
 These discharges (often over 1 million volts) heat up the air
around to temperatures of over 45,000˚F – the heated air
expands explosively, creating the shock wave – thunder!
 Being an intense form of precipitation, thunderstorms result from
the same uplift mechanisms as in precipitation, especially
convectional, orographic and frontal uplift;
-- cyclonic/convergence uplift is less effective in triggering
severe thunderstorms.
 As in rainfall, hail is often associated with thunderstorms.
 Usually covers a small area of a few miles, but sometimes there
can be a series of thunderstorms covering a larger region,
on or in advance of a cold front, forming a Squall Line.
 Most thunderstorms last about an hour.
• Tornadoes are the most powerful weather phenomenon
known – A violently rotating column of air extending
from a thunderstorm to the ground (See: Fujita Scale).
• Technically, a tornado is an intense, narrow system of low
pressure with violent updrafts and converging winds.
• Though the mechanisms that actually create tornadoes still
elude us, about 80% of all tornadoes are associated
with thunderstorms and mid-latitude cyclones; the other
20% are spawned by hurricanes that make landfall.
• Conditions for Tornado formation: Cool, dry air (e.g., from the
Rockies) colliding with warm, moist air (from the Gulf).
• Tornadoes can be on the ground for an instant to several
hours, but the average time is 5 minutes.
Tornadoes have been documented in most regions of the Earth,
though they are most prevalent in the U.S., particularly along the
Tornado Alley
On average 1000 Tornadoes occur every year in the U.S.
Average Annual Number of Tornadoes per 10,000 sq. mi.
• Tornadoes are common during Spring, when greatly contrasting
air masses collide to produce severe storm systems.
• There is also a distinct Seasonal March of Peak Tornado Activity
Seasonal March of Peak Tornado Activity
Doppler Radar
Image of a
Storm showing
Squall Lines
(in Red)
Additional Slides
for Reference
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