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Chapter 13

Terms in this set (50)

What must exist for a thunderstorm to exist?
1- There must be an abundant source of moisture in the lower levels of the atmosphere
2- Some mechanism must lift the air so that the moisture can condense and release latent heat.
3- The portion of the atmosphere through which the cloud grows must be unstable.
Air in a thunderstorm will keep rising until...
It meets a layer of stable air that it cannot overcome
The rate of condensation, which diminishes with height, is insufficient to generate enough latent heat to keep the cloud warmer than the surrounding air
* usually last 30 min
Air-Mass Thunderstorm
a thunderstorm that results from the air rising because of unequal heating of Earth's surface within one air mass.
Mountain Thunderstorm
occur when an air mass rises as a result of orographic lifting, which involves air moving up the side of a mountain.
Sea-Breeze Thunderstorm
local air-mass thunderstorms caused, in part, by extreme temperature differences between the air over land and the air over water.
Frontal Thunderstorm
thunderstorms that are produced by advancing cold fronts and, more rarely, warm fronts.
Cold Front Thunderstorms
-get their initial lift from the push of the cold air which can produce a line of thunderstorms along the leading edge of the cold front.
-Because they are not dependent on daytime heating for their initial lift, cold-front thunderstorms can persist long into the night.
Stages of Thunderstorm
-cumulus stage
-mature stage
-dissipation stage

classified by the direction in which the air is moving
Cumulus Stage
-In the cumulus stage, air starts to rise nearly vertically upward.
-Transported moisture condenses into a visible cloud and releases latent heat.
-As the cloud droplets coalesce, they form larger droplets, which eventually fall to Earth as precipitation
Mature Stage
-As precipitation falls, it cools the air around it which becomes more dense than the surrounding air, so it sinks creating downdrafts.
-The updrafts and downdrafts form a convection cell.
-In the mature stage, nearly equal amounts of updrafts and downdrafts exist side by side in the cumulonimbus cloud
Dissipation Stage
-The supply of warm, moist air runs out because the cool downdrafts cool the area from which the storm draws energy.
-Without the warm air, the updrafts cease and precipitation can no longer form.
-The dissipation stage is characterized primarily by lingering downdrafts.
Supercells?
self-sustaining, extremely powerful severe thunderstorms, which are characterized by intense, rotating updrafts
-very few thunderstorms become supercells
Lightning
Lightning is an electrical discharge caused by the friction of falling and rising ice crystals within strong drafts of a cumulonimbus cloud
Power of lighting?
A lightning bolt heats the surrounding air to about 30,000°C.
What is thunder?
Thunder is the sound produced as this superheated air rapidly expands and contracts
Downburst
violent downdrafts that are concentrated in a local area and can contain wind speeds of more than 160 km/h
Macroburst
wind speeds of more than 200 km/h, can last up to 30 minutes, and cause a path of destruction up to 5 km wide
Microburst
affect areas of less than 3 km wide but can have winds exceeding 250 km/h
Hail
precipitation in the form of balls or lumps of ice that can do tremendous damage.

-These ice pellets are always encountering super cooled water in the atmosphere making them grow until they are too heavy and fall towards the earth
What causes hail to form?
-Water droplets exist in the liquid state in the parts of a cumulonimbus cloud where the temperature is actually below freezing.
-The abundance of strong updrafts and downdrafts existing side by side within a cloud
Floods
-Flooding can occur when a storm dumps its rain over a limited location
-Floods are the main cause of thunderstorm-related deaths in the United States each year.
Tornado
-violent, whirling column of air in contact with the ground
-often associated with super cells
What is a tornado called before it hits the ground?
funnel cloud
How is a tornado made visible?
dust and debris drawn into the swirling column, or by the condensation of water vapor into a visible cloud
How do tornado form?
wind speed and direction change suddenly with height, a phenomenon known as wind shear

* under right conditions they can produce a horizontal rotation near earth's surface
What forms the violent winds associated with tornado's?
The extreme pressure gradient between the center and the outer portion of the tornado
How does the Fujita tornado intensity scale classify tornados?
classifies tornadoes according to their path of destruction, wind speed, and duration
How does the Fujita scale range?
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 don't exceed the F1 category
When do tornadoes usually form?
Most tornadoes form in the spring during the late afternoon and evening, when the temperature contrasts between polar air and tropical air are the greatest
Where is the tornado valley?
extends from northern Texas through Oklahoma, Kansas, and Missouri
Who issues tornado watches and warnings before a tornado actually strikes?
National Weather Service
What is a Tropical Cyclone?
large, rotating, low-pressure storms that form over water during summer and fall in the tropics

* also known as hurricanes
What do tropical cyclones thrive on?
tremendous amount of energy in warm, tropical oceans
How does a tropical cyclone form?
1. latent heat from ocean is evaporated and released when the air begins to rise and water vapor is condensed
2.rising air creates are low pressure over ocean
3.cyclonic rotation of a tropical cyclone begins as warm air moves toward the low-pressure center to replace the air that has risen
4.as air rises the rotation speeds up as energy is released through condensation
5.the cyclone will continue as long as it has warm air to feed off of
What is required for a tropical cyclone to form?
-An abundant supply of very warm ocean water
-Some sort of disturbance to lift warm air and keep it rising

* usually happen in late summer and early fall b/c oceans contain most amount of stored energy
How do cyclones move?
-according to the wind currents that steer them
-in the deep tropic they are caught up in subtropical high-pressure systems that are usually present
What tropical wave?
weak, low-pressure
* beginning formation of cyclone
* only small amount become hurricanes
What tropical depression?
disturbance over a tropical ocean acquires a cyclonic circulation around a center of low pressure,
What tropical storm?
wind speeds around the low-pressure center of a tropical depression exceed 65 km/h
How do we classify hurricanes?
-Saffir-Simpson hurricane scale classifies hurricanes according to wind speed, air pressure in the center, and potential for property damage.
-The Saffir-Simpson hurricane scale ranges from Category 1 hurricanes to Category 5 storms, which can have winds in excess of 155 mph
When does a hurricane usually run out of energy?
-The storm moves over land and no longer has access to the warm ocean surface from which it draws its energy.
-The storm moves over colder water.
Whats a storm surge?
hurricane-force winds drive a mound of ocean water, sometimes as high as 6 m above normal sea level, toward coastal areas where it washes over the land
What causes a flood?
weather patterns with even mild storms that persist over the same area
What is a drought?
-extended periods of well-below-normal rainfall
What is a heat wave?
extended periods of above-normal temperatures
What causes a heat wave?
formed by the same high-pressure systems that cause droughts

* usually caused when air is humid
What is heat index?
assesses the effect of the body's increasing difficulty in regulating its internal temperature as relative humidity rises
What is a cold wave?
extended period of below-normal temperatures
What causes a cold wave?
large, high-pressure systems of continental polar or arctic origin
What is the wind-chill factor?
-measured by the wind- chill index
-estimates the heat loss from human skin caused by the combination of cold air and wind
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