Download 13-1 Thunderstorms

13-1 Thunderstorms
Learning Target
I can explain how local conditions
impact the formation of
thunderstorms.
Average annual occurrences of thunderstorms
in the United States
• What region of the U.S. shows the lowest average of thunderstorms
per year?
• What regions show the second lowest average per year?
• What region shows the highest average per year?
• Hypothesize why these regions rank in this order?
How do Thunderstorms Form?
Thunderstorms requires 3 ingredients
to form:
–Source of moisture
–A lifting mechanism
–Unstable atmosphere
Sources of Moisture
• Typical sources of moisture are large bodies of
water such as the __________
and
Pacific
Gulf of Mexico
_________oceans
as well as the ____________
Atlantic
Water temperature affects how much
moisture is in the atmosphere
• Warm water puts more moisture into the
atmosphere than with cold water b/c…?
• Warm water’s higher thermal energy
makes evaporation from liquid water to
vapor easier
• Warm ocean currents occur along east
coasts of continents (including the Gulf).
• Cool ocean currents occur along west
coasts.
Water Sources for thunderstorms and their relative temperatures
Cool ocean
currents in
Pacific
Warm ocean currents
from Gulf and Atlantic
• What factors contribute to the southeastern U.S.
having a higher number of thunderstorms?
• Increased moisture in atmosphere from the
warmer waters of the gulf and Atlantic moisture
sources.
How do Thunderstorms Form?
Remember…Thunderstorms requires 3
ingredients to form:
– Source of moisture
– Lifting mechanisms
– Unstable atmosphere
Lifting Mechanisms
• Basic premise – moist air must rise and
condense for thunderstorms to develop.
• Mechanisms that cause air to rise?
• Uneven heating of Earth’s surface creates
warmer air that is less dense than surrounding
air, and thus it rises, cools, and may condense
• Orographic lifting – air forced up over landforms
• Fronts – particularly cold fronts for
thunderstorms
How do Thunderstorms Form?
Remember…Thunderstorms requires 3
ingredients to form:
– Source of moisture
– Lifting mechanisms
– Unstable atmosphere
Unstable Atmosphere
Simply put, “unstable”
air is air that is moving
vertically.
• An unstable air mass is characterized by warm moist air near the
surface and cold dry air aloft.
• The warm moist air is less dense and rises. As it rises it cools and
some of the water vapor condenses, forming a cumulus cloud.
• Condensation releases latent heat and allows continued lifting which
can result in tall cumulonimbus cloud formation.
13-1 Thunderstorms
Learning Targets
•I can identify the processes that
form thunderstorms
•I can describe the life cycle of a
thunderstorm - (a.k.a. three stages)
Life Cycle of a Thunderstorm
• Three stages:
– Cumulus Stage
– Mature Stage
– Dissipation Stage
Cumulus Stage
• Characterized by updrafts
• Water vapor is transported to upper, cooler
regions.
• The water vapor condenses forming a cumulus
cloud.
• Condensation releases latent heat.
• Latent heat fosters continued lifting, more
condensation, more latent heat, and so on.
Cumulus Stage
Air within the cloud is
dominated by updrafts
Mature Stage
• Strong updrafts and downdrafts side by side in
the cumulonimbus
• Precipitation cools the air as it falls.
• Newly cooled air sinks with precipitation
creating downdrafts.
• The convection of updrafts and downdrafts
create strong surface winds.
• The friction between the up and downdrafts
create separate areas of charge (lightning
potential)
Mature Stage
• Strong updrafts and
downdrafts coexist.
• Most dangerous
stage- wind-lightening
Dissipation Stage
• Cool downdrafts spread in all directions
when they reach Earth’s surface.
• This cools the areas the storm draws its
thermal energy from.
• The updrafts cease and clouds can no
longer form.
Dissipation Stage
Cooling rain and downdrafts
cools the surface area that was
providing the thermal energy for
updrafts.
This stage ends when the last
precipitation reaches Earth.