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THUNDERSTORMS
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TOPICS:
• What are Thunderstorms?
• Ingredients
• Stages
• Types
• Lightning
• Downburst and Microburst
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What are Thunderstorms?
A storm produced by a cumulonimbus cloud that
contains lightning and thunder
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Convective Ingredients
• Instability
• Moisture
• Lift
• Exhaust
– All of these must be present for thunderstorms
to occur
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Instability
• A condition of the atmosphere where by a
parcel of air, given an initial start vertically,
then continues to move upward without any
outside force.
• Instability can be caused by:
– Surface heating
– Cooling aloft
– Evaporational cooling
• Dry air is advected into a moist area
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Moisture
• Necessary for cloud formation
• Increasing moisture will enhance instability
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Lift (trigger)
• Force needed to get a parcel of air to it’s
Level of Free Convection
• The three main triggers
– Boundaries
• Fronts, troughs, outflow boundaries and moist
discontinuities (dry-lines)
– Topography
• Mountains and coastlines
– Differential heating
• Strong temperature gradient over a short distance
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Exhaust
• Upper level
divergence is
required
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Evolution of Thunderstorm
Development
• Cumulus Stage
• Mature Stage
• Dissipating Stage
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Cumulus Stage
• Begins with the formation of the first
convective cloud
• Dominated by updrafts
• Updrafts hold the prospective precipitation
aloft
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Mature Stage
• Cloud particles grow too large for the rising air to keep them suspended
• The particles then fall against the updraft and create a downdraft
• The cloud reaches the stratosphere and the top begins to spread out
• Most violent and active stage
• Thunder and lightning begin in this stage
• The heavier the precipitation the stronger the downdraft
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Mature cont.
• Updrafts and downdrafts reach their greatest strength in the middle of the cloud, creating severe turbulence
• Ice crystals suspended near the melting level alternate between freezing and melting, producing hail
• Hailstones continue to grow until it is too heavy to be supported by the updraft core
• Aircraft have encountered hail in clear air several kilometers away from the storm
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Mature cont.
• The leading edge of the downdraft at the
surface is known as the “gust front” or
“outflow boundary”
• The gust front produces “Low-Level Wind
Shear”
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Dissipating Stage
• Begins when the updraft collapses and the
downdraft dominates
• The downdraft cuts off the inflow of warm,
moist air into the storm
• Cloud droplets stop forming
• The cloud begins to stratify into layered
clouds
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Types of Thunderstorms
• Single cell
• Multi-cell
• Supercell Thunderstorms
• Squall line
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Single Cell Thunderstorms
• “Air mass thunderstorms”
• Short lived (30 to 60 minutes)
• Severe weather is rare
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Multi-cell Thunderstorms
• Requires a greater degree of instability
• Strong mid level winds carry the
precipitation downstream so it doesn’t fall
into the updraft
• New cells continue to form on the right
flank and develop into mature storms
• Storms last longer due to their ability to
renew themselves
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Supercell Thunderstorms
• Updrafts and downdrafts are so sufficiently
in balance that it can maintain itself for
hours on end
• Producers of tornadoes and destructive hail
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Squall Line
• A line of thunderstorms that is not readily
circumnavigable
• Normally occur 50 to 300 miles ahead of a
cold front
• Produce high winds
• Hail and tornadoes are possible
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Lightning
• A discharge of electricity which occurs
during the mature stage of thunderstorms
• Types of lightning
– In cloud
– Cloud to cloud
– Cloud to air
– Cloud to ground
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• Lightning
hazards
– Flash
blindness
– Puncture
aircraft skin
– Shock and
burns
– Fuel ignition
– Damage to
equipment
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Downburst
• Strong concentrated downdraft with a great
potential for damaging surface winds
• Usually located behind the gust front
• Wind speeds are greater than 40 knots at the
surface
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Microburst
• Concentrated downburst
• 4 kilometers or less
• Change in velocity of 50 knots or greater
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Questions?