How do thunderstorms form? There are 3 conditions:
1) There must be abundant moisture in the lower
2) Some mechanism must lift it so moisture can
condense and release latent heat
NOTE: Latent heat is the heat energy involved in the phase change of
water. The heat then is "hidden" or stored in the water molecule until it is
released during condensation. Latent heat released during condensation is
an important source of energy to drive atmospheric systems like hurricanes
and cumulus clouds.
3) The atmosphere must have a “place” where the
cloud can continue to develop. The atmosphere is
Types (let’s draw…):
1) Air mass thunderstorms
• Orographic lifting
2)Frontal Thunderstorms – due to cold
Stages of a Thunderstorm
1) Cumulus stage – air rises, creates updrafts,
droplets condense, eventually as precipitation.
2) Mature stage – water droplets that form at
high, cool levels of the atmosphere. This cold
air is dense and creates downdrafts/updrafts
that produce the gusty winds.
3) Dissipation stage – the production of
downdrafts. Once moisture runs out, updrafts
slow and eventually stop. Storm loses
What is lightning?
•A visible electrical
discharge produce by
is a giant spark.
•It appears to “flicker”
because it is composed of
a succession of multiple
strokes that usually follow
the exact same path of the
•Several types of lightning are common:
* In-cloud lightning extends from one charged
region of a cloud to another.
* Cloud-to-cloud lightning extends between two
* Cloud-to-air lightning extends from a cloud to
the air, not touching the ground.
* Cloud-to-ground lightning stretches from a
cloud to the ground.
Formation of Lightning:
develop a separation of
electric charge, with the
tops of the clouds
positively charged and
the bottoms negatively
Light weight particles
have a positive
(+)charge and float to
the top of the cloud (like
ice floats to the top of a
(-)charged particles sink
to the bottom.
A “step leader” on
the bottom of the
cloud form as it
resistance to the flow
Rapidly rising air in a
interacts with rapidly
falling air within the
positive and negative
charged areas within
the cloud (friction).
Meanwhile, as the
electrons approach the
ground, the ground is
becoming more and more
positively charged due to
the repulsion of electrons
in the ground. This
positively charged region
moves up through any
conducting objects on the
ground -- houses, trees
and people -- into the air.
This is why it is very
important to stay "low"
during a storm or when
When the downward
moving electrons meet
the upward flowing
positive regions at an
altitude of a hundred
meters or so, they form a
complete circuit (See
Figure) and the lightning
We now see the main
act of lightning, the
This extreme heating
causes the air to
expand at an
explosive rate. The
expansion creates a
shock wave that turns
into a booming sound
wave, better known as
-Seconds after this girl walked
away, lightning struck where
-The positive charge of her
hair is attracted to the negative
charge of a lightning cloud.
Lightning strikes in the Kempsville Lakes area of Virginia
Beach, off Baxter Road, around 3 a.m. July 12, 2005.
August 22, 2007: After a series of thunderstorms passed through
Hampton Roads on Tuesday night, William Coyle (VA Lightning)
photographed these anvil crawlers. In the foreground is the Westin
Hotel & Residences at Virginia Beach's Town Center.
Facts about lightning:
•Lightning discharges can reach up to 200 million
•Can reach temperatures up to 54,000° F. That is
hotter than the surface of the sun!
•Globally, there are about 100 – 125 flashes a second.
That’s about 3 billion a year!
•The chances of being struck by lightning in the
United States is about 1 in 250,000. Approximately
75 – 150 people die each year due to lightning.
Why do we see lightning first, then hear the sound later? Light is faster
•Speed of light: 186,000 miles a second
•Speed of sound in air: At 21 degrees C (70°F): 344 meters per second or
1129 ft per second.
•Speed of sound in water: 1480 m/s or 4856 ft/s. More than 3000 miles per
Foldable: Chapter 13.2, p. 334
Provide characteristics, steps of formation, and
dangers of the following:
1) Severe thunderstorms
3) Fury of Wind
• At Earth’s surface, rising air is associated with
low pressure and sinking air is associated with high
• Rising or sinking air, combined with the Coriolis
effect, results in the formation of rotating lowand high-pressure systems in the atmosphere.
• Air in these systems moves in a general
circular motion around either a high- or lowpressure center.
– In a high-pressure system, air sinks, so that when it
reaches Earth’s surface it spreads away
from the center.
– The Coriolis effect causes the
overall circulation around a
high-pressure center to move
in a clockwise direction in
the northern hemisphere.
– High-pressure systems
rotate in a
in the southern
– A wave cyclone, one of the main producers of
inclement weather in the middle latitudes, usually
begins along a stationary front.
– Part of the front moves south as a
cold front and another part of the
front moves north as a warm front.
– This sets up a counterclockwise or
cyclonic circulation that can form
a fully developed
– In a low-pressure systems, air rises, causing an
inward net flow toward the center and then upward.
– In contrast to air in a highpressure system, air in a lowpressure system in the northern
hemisphere moves in a
– This movement is reversed
in the southern
Cyclones are areas of
low pressure. Cyclones
usually exhibit nearly
•As air enters an area of
low pressure from all
directions, the Coriolis
effect bends the direction
of the wind to the right of
•This creates a
rotation around the low.
Figure PC.6 Circulation within a low
pressure system in the Northern
areas of high
pressure that exhibit
•The Coriolis effect
bends the air to the
right of its path
around the high.
Figure PC.7 Circulation within a high
pressure system in the Northern
Because the Coriolis effect works in the opposite direction in the Southern Hemisphere,
circulation around lows are clockwise and inward toward the center at the surface and
highs exhibit a diverging, counterclockwise rotation. You can see this effect in the cloud
pattern created by a cyclone in the Southern Hemisphere.
• A tornado is a violent, whirling column of air in contact
with the ground.
• Before a tornado reaches the ground, it is called
a funnel cloud.
• Tornadoes are often
• The air in a tornado is made
visible by dust and debris
drawn into the swirling
column, or by the
condensation of water
vapor into a visible cloud.
• A tornado forms when wind speed and direction change
suddenly with height, a phenomenon known as wind
1. Wind Shear
forms a horizontal
“barrel” of air.
2. Updrafts in a
the “barrel” of air.
3. Air pressure
and spins faster.
How are the rated?
– The Fujita tornado intensity scale classifies tornadoes
according to their path of destruction, wind speed, and
– The scale ranges from
F0, which is
characterized by winds
of up to 118 km/h, to
the violent F5, which
can pack winds of more
than 500 km/h.
Most tornadoes form in the spring during the late afternoon
and evening, when the seasons begin to change.
– Tornadoes occur most
frequently in a region
Alley,” which extends
from northern Texas
Kansas, and Nebraska.
– In the United States, an average of 80 deaths and
1500 injuries result from tornadoes each year.
– The National Weather Service issues tornado watches
and warnings before a tornado actually strikes.
– The agency stresses that despite advanced tracking
systems, advance warnings may not be possible.
– Signs of an approaching or developing tornado
include the presence of dark, greenish skies, a
towering wall of clouds, large hailstones, and a loud,
roaring noise similar to that of a freight train.
Hurricane Isabel (2003). Viewed from
the International Space Station
What Is a Hurricane?
• A hurricane is an
intense tropical storm
system with powerful
winds that exceed 74
mph. A hurricane is
composed of strong
thunderstorms with well
• Hurricanes have an
average size of 500
miles in diameter.
Hurricane Floyd – September 14, 1999
Hurricane or Typhoon?
• These storm systems
can have two different
names depending on
• The term hurricane is
used when the storm
system is in the
Atlantic or Eastern
• Typhoon is used to
describe a storm
system in the Western
Hurricane Isabel off coast of North Carolina from
oil tanker – 2003
How Do Hurricanes Form?
• First, the water
temperature must be at
least 80 degrees to a
depth of 150 ft.
• Next, the evaporation of
water into the air forms
clouds or thunderstorms.
• If the atmosphere has no
disturbance, but has light
winds and low pressure,
the clouds that form will
start to slowly rotate
Growth of a Hurricane: Storm Systems
1) Tropical Depression
•At first the storm system
will appear as an
unorganized system of
have wind speeds less
than 38 mph.
2. Tropical Storm
- The tropical depression then becomes more organized.
- A tropical storm has wind speeds from 39-73 mph.
• The tropical storm then gains strength and energy until it is highly
organized. Normally at this point the eye of the hurricane forms.
• Once again the winds of a hurricane are a minimum of 74 mph.
Hurricane Katrina - 2005
Structure of a Hurricane
• The eye is described
as the relatively calm
center of a hurricane
that is more than half
surrounded by a wall
• The eye wall is an
organized band of
surrounding the center
of the hurricane. The
eye wall holds the
highest wind speeds.
Movement of a Hurricane
• Hurricanes move across
the ocean slowly at about
• The direction a hurricane
travels is determined by
global wind patterns and
ocean currents, high and
low pressure systems, and
Effects of a Hurricane
• Storm surge is the
abnormal rise in sea
level that accompanies
• The low pressure of
the eye causes the sea
to be pulled into a
“bulge.” Storm surges
can reach 18-20 ft in
• Storm surge is the
most destructive effect
of a hurricane and
responsible for the
National Park Service Ranger Jeff Goad looks at the destruction to N.C. 12 on the north edge of
Rodanthe on Hatteras Island. Storm surge from Hurricane Irene destroyed a 900-foot section of
the road, stranding residents and workers and cutting off tourist access to the area.
How Are Hurricanes Rated?
• The Saffir-Simpson scale is a 1-5 rating of hurricane strength.
• In this scale 1 is the weakest and 5 is the strongest.
• Also, a “major” hurricane is classified as a category 3 or higher.
How Do Hurricanes Compare on
the Saffir-Simpson Scale?
• Category1: no real damage to building structures. Damage
primarily to unanchored mobile homes, shrubbery, and
trees. Also, some coastal flooding and minor pier damage.
• Category 2:some roofing material, door, and window
damage. Considerable damage to vegetation, mobile
homes, etc. Flooding damages piers and small crafts.
How Do Hurricanes Compare on
the Saffir-Simpson Scale?
Category 3:some structural damage to small residences and utility.
Mobile homes are destroyed. Flooding near the coast destroys smaller
structures with larger structures damaged by floating debris. Terrain
may be flooded well inland.
Category 4:some complete roof structure failure on small residences.
Major erosion of beach areas.Terrain may be flooded well inland.
• Category 5:complete roof failure on many residences and industrial
buildings. Some complete building failures with small
buildings blown over or away. Flooding
causes major damage to lower floors of all structures near the
shoreline. Massive evacuation of residential areas may be required.
• The official season for
the Atlantic Basin
Caribbean sea, and
gulf of Mexico) is
from June 1st to
1. Hurricane Watch
• A hurricane watch is an announcement
given to specific coastal areas that a
hurricane poses a possible threat, generally
within 36 hours.
2. Hurricane Warning
• This is a warning that sustained winds of 74
mph or higher are expected in a specific
coastal area within 24 hours or less.
• A hurricane warnings are usually in effect
even after the strong winds have decreased
due to the dangerously high water and
According to the NHC, hurricanes were previously named after saints, like Hurricane Santa Ana,
which struck Puerto Rico in 1825. Women's names are thought to have emerged before the end of the
19th century. Tropical storms in the Pacific were named for wives and girlfriends by Army and Navy
meteorologists throughout the 1940's, as reported by the Wall Street Journal.
The WMO also handles retiring names and
altering the lists for reasons of sensitivity or
inappropriateness during committee meetings.
Devastating hurricanes like Igor in 2010,
2008's Ike and Katrina in 2005, have all been
retired as hurricane names due to the extreme
damage caused in those years.