The Nature of Storms

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The Nature of Storms

• Thunderstorms 13.1

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Thunderstorms

• A thunderstorm is a storm that generates
  lightning and thunder.
• Thunderstorms frequently produce gusty winds,
  heavy rain, and hail.
• At any given moment, nearly 2000 thunderstorms
  are occurring around the world.

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Thunderstorms

• About 45,000 thunderstorms take place each day.
• More than 16 million occur annually around the
  world.
• The US experiences about 100,000 thunderstorms
  each year, most frequently in Florida and the
  eastern Gulf Coast region.

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Thunderstorms

• Thunderstorm Time Lapse

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How Thunderstorms Form

• For a thunderstorm to form, three conditions must
  exist
• There must be an abundant source of moisture in
  the lower levels of 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.

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Limits to Growth

• Air will keep rising until
• it meets a layer of stable air it cannot
  overcome
• The rate of condensation is insufficient to
  generate latent heat to keep the cloud warmer
  than the surrounding air

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Air-Mass Thunderstorms

• Thunderstorms are often classified according to
  the mechanism that caused the air to rise.
• If the air rose because of unequal heating of
  Earths surface within one air mass, the
  thunderstorm is called an air-mass thunderstorm.

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Air-Mass Thunderstorms

• The unequal heating reaches its max during
  mid-afternoon. Thus, air-mass thunderstorms are
  common then.
• There are two common types of air-mass
  thunderstorms
• Mountain thunderstorms
• Sea-breeze thunderstorms

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Air-Mass Thunderstorms

• Mountain thunderstorms occur when an air mass
  rises as a result of orographic lifting.
• Sea-breeze thunderstorms are local air-mass
  thunderstorms caused in part by extreme
  temperature difference between the air over land
  and the air over water.

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Mountain Thunderstorm Due to Orographic Lifting
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Frontal Thunderstorms

• Frontal thunderstorms are produced by advancing
  cold fronts and warm fronts.
• In a cold front, cold air pushes warm air rapidly
  up the steep cold-front boundary. This can
  produce a line of thunderstorms along the leading
  edge of the cold front.

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Frontal Thunderstorm

• Less frequently, thunderstorms can develop along
  the advancing edge of a warm front.
• In a warm front, a warm air mass slides up and
  over a cold air mass.
• If the warm air behind the warm front is unstable
  and moisture levels are high, a mild thunderstorm
  can develop.

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Stages of Development

• A thunderstorm has 3 stages
• Cumulus stage
• Mature stage
• Dissipation stage
• The stages are classified according to the
  direction in which the air is moving.

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Cumulus Stage

• Air starts to rise upward.
• This creates updrafts, which transport moisture
  to the upper reaches of the cloud.
• The moisture condenses into visible cloud
  droplets and releases latent heat.
• The droplets coalesce and fall to Earth as
  precipitation.
• This begins the mature stage of a thunderstorm.

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Mature Stage

• As precipitation falls, it cools the air around
  it.
• The newly cooled air is more dense than
  surrounding air and sinks rapidly to the ground.
• This creates downdrafts.
• The updrafts and downdrafts form a convection
  cell that produces the gusty surface winds
  associated with thunderstorms.

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Dissipation Stage

• The production of downdrafts is ultimately the
  thunderstorms undoing.
• The convection cell can exist only if there is a
  steady supply of warm, moist air at Earths
  surface. Once that supply runs out, the updrafts
  slow and eventually stop, and precipitation can
  no longer form.
• The storm is then in the dissipation stage.

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