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Thunderstorms
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A mature
thunderstorm, with anvil top.
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How Do Thunderstorms Form?
By Tiffany Means
Thunderstorms
Whether
you happen to be a spectator or a "spook," chances are you've never
mistaken the sight or sounds of an approaching thunderstorm.
And it's no wonder why. Over
40,000 occur worldwide every day.
Of that total, 10,000 occur
daily in the United States alone.
Thunderstorm Climatology
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A map showing the
average number of thunderstorm days each year in the U.S. (2010).
|
In
the spring and summer months, thunderstorms seem to occur like clockwork.
But don't be fooled!
Thunderstorms can occur at all times of the year, and at all hours of the day
(not just afternoons or evenings). The atmospheric conditions only need be
right.
So,
what are these conditions, and how do they lead to storm development?
Thunderstorm Ingredients
In order for a thunderstorm to develop, 3 atmospheric
ingredients must be in place: lift, instability, and moisture.
Lift
Lift is responsible for initiating the updraft--the migration of
air upward into the atmosphere--which is necessary in order to produce a
thunderstorm cloud (cumulonimbus).
Lift
is achieved in a number of ways, the most common being through differential
heating, or convection.
As the Sun heats the ground,
the warmed air at the surface becomes less dense and rises. (Imagine air
bubbles that rise from the bottom of a boiling water pot.)
Other
lifting mechanisms include warm air overriding a cold front, cold air
undercutting a warm front (both of these are known as frontal
lift), air being forced upward along the side of a mountain (known
as orographic
lift), and air that comes together at a central point (known as convergence.
Instability
After air is given an upward nudge, it needs something to help
it continue its rising motion. This "something" is instability.
Atmospheric
stability is a measure of how buoyant air is.
If air is unstable, it means
that it is very buoyant and once set in motion will follow that motion rather
than return to its starting location.
If an unstable air mass is
pushed upward by a force then it will continue upward (or if pushed down, it
will continue downward).
Warm
air is generally considered to be unstable because regardless of force, it has
a tendency to rise (whereas cold air is more dense and sinks).
Moisture
Lift and instability result in rising air, but in order for a
cloud to form, there must be sufficient moisture within the air to
condense into water droplets as it ascends.
Sources of moisture include
large bodies of water, like oceans and lakes.
Just as warm air temperatures
aid lift and instability, warm waters aid the distribution of moisture.
They have a higher
evaporation rate, which means they more readily release moisture into the
atmosphere than cooler waters do.
In
the U.S., the Gulf of Mexico and the Atlantic Ocean are the major sources
of moisture for fueling severe storms.
The Three Stages
 |
Diagram of a
multicell thunderstorm consisting of individual storm cells
- each in a
different development stage.
Arrows represent the strong up-and-down motion
(updrafts and downdrafts) which characterize thunderstorm dynamics.
|
All
thunderstorms, both severe and non-severe, go through 3 stages of
development:
1.
the towering cumulus stage,
2.
the mature stage, and
3.
the dissipating stage.
1.
The Towering Cumulus Stage
 |
The initial stage
of thunderstorm development
is dominated by the presence of updrafts.
These
grow the cloud from a cumulus
to a towering cumulonimbus.
|
Yes, that's cumulus as
in fair
weather cumulus. Thunderstorms actually originate from this
non-threatening cloud type.
While at
first this may seem contradictory, consider this: thermal instability (which
triggers thunderstorm development) is also the very process by which a cumulus
cloud forms.
As the Sun heats the Earth's
surface, some areas warm faster than others.
These warmer pockets of air
become less dense than the surrounding air which causes them to rise, condense,
and form clouds.
However, within minutes of
forming, these clouds evaporate into the drier air in the upper atmosphere.
If this happens for a long
enough period of time, that air eventually moistens and from that point on, continues cloud
growth rather than stifling it.
This vertical cloud growth, referred to as an updraft,
is what characterizes the cumulus stage of development.
It works to build the storm.
(If you've ever watched a cumulus cloud closely, you can actually see this
happen. (The cloud begins burgeoning upward higher and higher into the sky.)
During the
cumulus stage, a normal cumulus cloud can grow into a cumulonimbus having a
height nearly 20,000 feet (6km).
At this height, the cloud
passes the 0°C (32°F) freezing level and precipitation begins to form.
As precipitation accumulates
within the cloud, it becomes too heavy for updrafts to support.
It falls inside of the cloud,
causing drag on the air. This in turn creates a region of downward directed air
referred to as a downdraft.
2. The Mature Stage
 |
In a
"mature" thunderstorm,
n updraft and downdraft co-exist.
|
Everyone who has experienced
a thunderstorm is familiar with its mature stage--the period when gusty winds
and heavy precipitation are felt at the surface.
What may be unfamiliar, however, is the fact
that a storm's downdraft is the underlying cause of these two classic
thunderstorm weather conditions.
Recall that
as precipitation builds within a cumulonimbus cloud, it eventually generates a
downdraft.
Well, as the downdraft
travels downward and exits the base of the cloud, the precipitation is
released.
A rush of rain-cooled dry air
accompanies it. When this air reaches the Earth's surface, it spreads out ahead
of the thunderstorm cloud--an event known as the gust
front.
The gust front is the reason
why cool, breezy conditions are often felt at the onset of a downpour.
With the
storm's updraft occurring side-by-side with its downdraft, the storm cloud
continues to enlarge.
Sometimes the unstable region
reaches as far up as the bottom of the stratosphere.
When the updrafts rise to
that height, they begin to spread sideways. This action creates the
characteristic anvil top. (Because the anvil is located very high up in the
atmosphere, it is comprised of cirrus/ice crystals.)
All the
while, cooler, drier (and therefore heavier) air from outside of the cloud is
introduced into the cloud environment simply by the act of its growth.
3. The Dissipating Stage
 |
| Diagram of a
dissipating thunderstorm - its third and final stage. |
In time, as the cooler air
outside of the cloud environment increasingly infiltrates the growing storm
cloud, the storm's downdraft eventually overtakes its updraft.
With no supply of warm, moist
air to maintain its structure, the storm begins to weaken.
The cloud begins to lose its
bright, crisp outlines and instead appears more ragged and smudged--a sign that
it is aging.
The full life cycle process takes about 30 minutes to complete.
Depending on thunderstorm type, a storm may go through it only once (single cell), or up to multiple times (multi-cell). (The gust front often triggers the growth of new thunderstorms by acting as a source of lift for neighboring moist, unstable air.)
Tiffany
Means is a meteorologist, science writer, and avid
cloud watcher/photographer.
Experience
Tiffany
has been finding beauty skyward and sharing it with others since the age of 5.
By twelve, she knew she wanted to pursue weather professionally—thanks in part
to the release of the blockbuster film Twister. Since
those days, Tiffany has interned with the domestic and international weather
departments at CNN, written monthly climate reports for NOAA’s National Centers for Environmental Prediction, and
participated in a number of science outreach events (such as the Science
Olympiad Competition). She has personally experienced such weather greats as
the Blizzard of 1993, and
the floods of Hurricane Francis (2004) and Ivan (2004).
Education
Tiffany
holds a bachelor’s degree in Atmospheric Science with a concentration in
weather forecasting from the University of North Carolina at Asheville.
Tiffany
is a proud member of the American Meteorological Society (AMS).
Tiffany
Means
"Weather affects us all. We check it on a daily basis, and
talk about it with complete strangers...but it is so much more
than 5-day forecasts and small talk! Through my enthusiasm for and expertise in
the weather sciences, I hope to spark your curiosity about our atmosphere,
create an awareness that will keep you weather ready and safe, and
strengthen your environmental responsibility to our atmosphere, water, and
earth."
Contact
Tiffany: Tiffany can be reached at aboutweatherexpert@gmail.com with
questions, comments, reprint requests, or suggestions. You can also connect
with her via the social links below.
https://www.thoughtco.com/how-thunderstorms-form-3444271
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