Download The Nature of Storms

The Nature of Storms
Chapter 13
Earth Science
2013-2014
Thunderstorms
• At any given moment, there are nearly 2,000
thunderstorms occurring around the world.
• Some are capable of producing hail, swirling
tornadoes, and high surface winds.
• Can also provide energy for tropical storms
and hurricanes.
How Thunderstorms Form
• Must have abundant moisture in the
atmosphere.
• Some mechanism must lift the air so that the
moisture can condense and release latent
heat.
• The portion of the atmosphere through which
the cloud grows must be unstable.
Air-Mass Thunderstorms
• A thunderstorm that occurs if the air rose
because of unequal heating of Earth’s surface.
• The unequal heating of Earth’s surface reaches
its maximum during mid-afternoon.
• Two main types:
– Mountain thunderstorms
– Sea-breeze thunderstorms
Air-Mass Thunderstorms
• Mountain thunderstorms
– Occur when an air mass rises as a result of orographic
lifting (air moving up the side of a mountain).
• Sea-breeze thunderstorms
– Local air-mass thunderstorms.
– Caused in part by extreme temperature differences
between the air over land and the air over water.
Frontal Thunderstorms
• Produced by advancing cold fronts (and rarely
warm fronts).
• Produces a line of thunderstorms along the
leading edge of a cold front (and rarely warm
front).
• Can be hundreds of kilometers long.
Stages of Development
• A thunderstorm usually has three stages:
– Cumulus
– Mature
– Dissipation
• Classified according to the direction the air is
moving.
Stages of Development
• Cumulus stage
– Air starts to rise nearly vertically upward.
– Creates updrafts that transport moisture to the
upper reaches of the cloud.
– Moisture condenses and releases latent heat.
– Larger droplets form and fall to Earth.
– Leads to mature stage.
Stages of Development
• Mature stage
– As the precipitation falls, it cools the air around it.
– Newly cooled air is more dense, so it sinks.
– Creates downdrafts.
– Updrafts and downdrafts form a convection cell
that produces the gusty surface winds.
– Nearly equal amounts of updrafts and downdrafts
in the mature stage.
Stages of Development
• Dissipation stage
– Production of downdrafts is what ends
thunderstorms.
– Convection cell can exist only if there is a steady
supply of warm, moist air at Earth’s surface.
– When the supply runs out, the updrafts slow and
then stop.
– This stage lasts until the cloud runs out of
previously formed raindrops.
Severe Weather
• Storms vary in severity.
– Severe thunderstorms
– Lightning
– Wind
– Hail
– Floods
– tornadoes
Severe Thunderstorms
• With an increase in temperature difference, the instability of air
increases.
• This increases the strength of the storm’s updrafts and downdrafts.
• Of all annual thunderstorms, less than 10% are considered severe
and even fewer are supercells.
• Supercells
– Self-sustaining, extremely powerful storms.
– Characterized by intense, rotating updrafts.
– Can last for several hours and have updrafts as strong as 240 km/hr.
Lightning
• Electricity caused by the rapid rush of air in a
cumulonimbus cloud.
• Lightning bolt forms when friction between the updrafts
and downdrafts separate electrons from some of their
atoms.
• Some become positively charged and others become
negatively charged.
• Return stroke from the ground up to the cloud illuminates
the channel.
Lightning
• Heats the surrounding air to 30,000°C.
– About five times the temperature of the Sun.
• Thunder is the sound heard from lightning.
• May see lightning much sooner than you hear thunder,
even though they begin at the same time.
• Accounts for about 7500 forest fires every year.
• Cause a yearly average of 300 injuries and 93 deaths.
Wind
• Sometimes the downward energy is not dispered,
but becomes concentrated.
• Downbursts
– Violent downdrafts.
– Resulting winds have speed of more than 160 km/hr.
– Macrobursts – path of destruction up to 5 km wide
with wind speeds of 200+ km/hr and can last up to 30
minutes.
– Microbursts – less than 3 km wide with wind speeds
over 250 km/hr lasting less than 10 minutes.
Hail
• Precipitation in the form of balls or lumps of
ice.
• Most common during the spring growing
season and can do significant damage to
crops.
Hail
• Two characteristics help hail form.
– Water droplets exists in liquid state where the
temperatures are below freezing. Come in contact
with ice pellets and help the ice pellet grow.
– Abundance of strong updrafts and downdrafts.
Helps them come into contact with more water
and grow larger.
Floods
• Occur when weather systems and resulting
storms move slowly.
• Storm dumps rain on a limited location.
• Rain can fall faster than the ground can absorb
it or rivers and streams can transport it out of
the area.
Tornadoes
• A violent whirling column of air in contact with the ground.
• Called a funnel cloud before it hits the ground.
• Forms when wind speed and direction change suddenly
with height and can go from a horizontal rotation to
vertical.
• Rarely exceed 200 m in diameter.
• Usually on last a few minutes, but can be extremely
destructive.
Tornadoes
• Tornado Classification
– Fujita tornado intensity scale – ranks tornadoes
according to their path of destruction, wind
speed, and duration.
– Scale ranges from F0 (winds up to 118 km/hr) to
F5 (winds of more than 500 km/hr).
– Most do not exceed F1.
– Only about one percent reach F4 or F5, but can lift
buildings from their foundations.
Tornadoes
• Tornado Distribution
– Can occur at any time and at any place.
– Some times and locations are more conducive:
• Most form in spring.
• Late afternoon/evening.
• Central United States – “Tornado Alley” extends from
northern Texas through Oklahoma, Kansas, and
Missouri.
Tornadoes
• Tornado Safety
– Average of 80 deaths and 1500 injuries from
tornadoes each year.
– NWS issues tornado watches and warnings.
– Signs of an approaching or developing tornado:
•
•
•
•
•
Severe thunderstorm
Dark, greenish skies
Towering wall of clouds
Large hailstones
Roaring noise similar to freight train
Tropical Storms
• Tropical cyclones
– Large, rotating, low-pressure storms.
– During summer and fall.
– Known in the United States and Atlantic Ocean as
hurricanes.
Tropical Cyclones
• Air rises, usually due to some existing weather
disturbance.
• As they produce more precipitation, more
energy is released.
• Coriolis effect causes the moving air to turn
counterclockwise in the northern hemisphere.
Tropical Cyclones
• Formation of Tropical Cyclones
– Require two basic conditions:
• An abundant supply of very warm ocean water.
• Some sort of disturbance to lift warm air and keep it
rising.
– Conditions exists in all tropical oceans except the
South Atlantic Ocean and the Pacific west of the
South American Coast.
Tropical Cyclones
• Movement of Tropical Cyclones
– Move according to the wind currents that steer them.
– Often caught up in the circulation of tropical high-pressure
systems.
• Carries them west until they reach the far west end of the system,
then turn towards the pole.
• Eye – the development of a calm center of the storm.
• Eyewall – the strongest winds that are concentrated in
a band immediately surrounding the eye.
Tropical Cyclones
• Stages of Tropical Cyclones
– Tropical disturbance
• Common during the summer and early fall.
• Only a small percentage ever become hurricanes.
– Tropical depression
• When a disturbance acquires a cyclonic circulation around a
center of low pressure.
• Wind speeds around the center up to 65 km/hr
– Tropical Storm
• Winds of 65 km/hr to 120 km/hr.
– Hurricane
• Winds above 120 km/hr.
Classifying Hurricanes
• Saffir-Simpson hurricane scale
– Classifies according to wind speed, air pressure in
the center, and potential for property damage.
– Category 3 and above are considered major
hurricanes.
Classifying Hurricanes
Saffir-Simpson Hurricane Scale
Scale Number (Category)
Sustained Winds (mph)
Damage
1
74-95
Minimal
2
96-110
Moderate
3
111-130
Extensive
4
131-155
Extreme
5
>156
catastrophic
Classifying Hurricanes
• Running Out of Energy
– A hurricane will last until it can no longer produce
enough energy to sustain itself.
– Usually when it makes landfall or cold water.
– No longer has warm water to draw energy from.
Hurricane Hazards
• Storm Surges
– Occurs when hurricane-force winds drive a mound
of ocean water toward coastal areas, where it
washes over land.
– Can sometimes reach 6m above normal sea level.
– Causes major damage along with high tide.
– Also leads to flooding.
Hurricane Hazards
• Hurricane Advisories
– NHC (www.nhc.noaa.gov) issues a hurricane
warning at least 24 hours before a hurricane
strikes.
– Also issues regular advisories with a storm’s
position, strength, and movement.
– Tracking greatly reduces death tolls over the past
several years.
Recurring Weather
• Floods and droughts
• Heat waves
• Cold waves
Floods and Droughts
• Floods can occur with large storms,
hurricanes, and when weather patterns cause
storms to stall over the same area.
• Small amounts of rain per hour are not an
issue, but can be if it rains for 18-24+ hours.
Floods and Droughts
• Droughts
– Extended periods of well-below-normal rainfall.
– Usually the result of shifts in global weather
patterns that allow large high-pressure systems to
persist for weeks or months over continental
areas.
Heat Waves
• Extended periods of above-normal temperatures.
• Can be formed by the same high-pressure systems that
cause droughts.
• Blocks cooler air masses from moving into the area.
• Humidity reduces the body’s ability to evaporate
perspiration, and can lead to serious health problems such
as heatstroke, sunstroke, and even death.
• See table 13-6 on page 349 for the heat index.
Cold Waves
• Extended period of below-normal temperatures.
• Also brought on by large, high-pressure systems.
• Caused by systems of continental polar or arctic origin.
• Wind-chill factor
– Cold air is worsened by wind.
– Measured by the wind-chill index, which estimates the
heat loss from human skin caused by the combination of
cold air and wind.