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Meteorology
Unit 5 Section 1
Thunderstorms
Thunderstorms
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Thunderstorms are convective storms
Form with rising warm air
Need unstable environmental conditions
As long as rising air parcel is warmer than the
air surrounding it, it will rise
– Warm air is less dense than cold air
Thunderstorms
• Each air parcel is called a “cell”
• Involve wind shear – the change in wind speed
and/or direction over a distance
Unstable Environmental Conditions
• Unequal heating at the surface
• Effect of terrain
• Converging surface winds = contrasting
temperatures and humidities
• Diverging upper-level winds + converging
surface winds + rising air
• Usually many factors working together
• Can form in warm or cold air temperatures
Severe Thunderstorms
To be considered severe, a storm must have at
least one of the following:
• Large hail >0.75 inches in diameter
• Surface wind gusts of 58mph or more
• Produces a tornado
Simple Thunderstorm
Many names:
• “Scattered thunderstorms”
• “Pop up storms”
• “Ordinary cell thunderstorms”
• “Air-mass
thunderstorms”
Simple Storm
• Rarely become severe
• Less than a kilometer wide
• Predictable life cycle, complete in less than an
hour
• Form in regions with limited wind shear –
wind speed and direction do NOT abruptly
change
Simple Storm Life Cycle
1. Cumulus stage
2. Mature stage
3. Dissipating stage
Cumulus Stage
• Growth stage
• Warm air rises, cools, and condenses into a cumulus
cloud or cluster of clouds
• Vertical Growth – Warm, humid air at the top of the
cloud dissipates, which adds more moisture into the
air, which cools and condenses, which adds more cloud
to the top
• Appears as a dome or tower
• Can form a Towering Cumulus cloud in a matter of
minutes
• Not enough time for precipitation to form, no thunder
and lightning
Cumulus Stage
Mature Stage
• Most intense stage
• As the cloud builds above freezing level, cloud
particles get larger and heavier and begin to
fall
• Drier air is drawn into the cloud, causing some
raindrops to evaporate, which cools the air
• Air is colder and heavier, forms a downdraft
• Downdraft gets stronger if falling precipitation
pulls air with it
Mature Stage
• Mature storms have the original convective
updrafts and downdrafts occurring at the
same time
• Top of cloud, containing ice crystals, reaches
stable air and flattens out into an anvil shape
• Severe turbulence in the middle of the cloud
from updrafts and downdrafts
• Lightning and thunder are present
• Heavy rain and occasionally small hail
Too much talking. Not enough pictures.
Mature Stage
Mature Stage
• Feel a downrush of cold air on the surface before
precipitation
• When downdraft of cold air reaches the surface,
it spreads out horizontally
• Gust front – surface boundary that separates the
advancing cold air from the warm surface air
– High wind shear, wind is rapidly changing speed and
direction
– Forces warm air back up into storm, which adds to
updraft
Gust Front
Dissipating Stage
• Usually 15-30 minutes after start of mature
stage
• Updrafts weaken as gust front moves away
• Downdrafts dominate, destroy humid updrafts
= no fuel for storm
• Light precipitation
• Cloud disperses from ground up, the cirrus
anvil is often the last thing to be seen
Life Cycle
Complex Thunderstorm
• “Multi-cell thunderstorm”
• “Supercell thunderstorm”
Multi-cell Thunderstorm
• Contain a number of cells, can be in different
stages of development
• Form in regions with moderate to strong
vertical wind shear
Multi-cell Thunderstorm
• Wind on one side of the storm is stronger,
causing it to tilt
• When tilted, downdrafts will not cut off
updrafts = storm doesn’t lose fuel
• Storm will survive longer
• Can become intense and produce severe
weather
Multi-cell Thunderstorm
• Air at the gust front may form a shelf cloud
that is attached to the base of the storm
• Roll clouds may form just behind the gust
front
Microbursts
• Beneath an intense thunderstorm, downdraft
may be very localized, forming a microburst
• Can produce intense winds up to 170mph in a
small area
• Heavy damage to trees and structures
• Can capsize boats and crash planes
Microbursts
Mesoscale Convective Complexes
(MCCs)
• Large, circular convective weather system
made of individual multi-cell thunderstorms
• Can be 1,000 larger than a simple
thunderstorm
• Can often cover an entire state
• Move slowly and can exist for 12 hours or
more
MCCs
MCCs
• Pros – provide significant portion of seasonal
rainfall over grain belt of USA
• Cons – can produce severe weather, like hail,
high winds, flash floods, and tornadoes
Supercell Thunderstorms
• Supercell- A large, long-lasting thunderstorm
with a single, violently rotating updraft
• Form in regions with strong vertical wind
shear
• Tilted so storm doesn’t lose fuel
• May exist for hours
Supercell
Supercell Thunderstorms
• Wind shear may be strong enough to create
rotation
• Can produce updrafts of 100mph or more,
damaging surface winds, and tornadoes
• Strong updrafts suspend hailstones for long
periods of time, allowing them to grow
• Updraft can be so strong that precipitation
can’t fall through it, creating a rain-free base
Supercell
Done.