We are now
beginning to prepare for our July chase season. In these past few days, our
local Seattle area has seen many thunderstorms pass of to the east, mainly
centered around the Cascade Mountains. In turn, I was forced to watch the
devolving cells from my window, being that I am separated from the eastern part
of the sound by large quantities of deep, murky water. It is because of the
past days storms that I figured it to be time to give a lesson on the
development of thunderstorms, severe and non- severe.
As most spotters, such as myself,
will tell you, there are just three ingredients you need to come together for a
thunderstorm:
>Moisture
> Instability
> Lift
MOISTURE
In the U.S.,
Sources of moisture typically originate from large bodies of water (Gulf of
Mexico, Atlantic, and Pacific). The water temperature also plays a large part
in the development of thunderstorms. Warmer waters such as those found in the
Gulf tend to evaporate faster compared to the cooler temperatures found in the
Atlantic and Pacific oceans. This is why we typically see the southern regions
of America receive larger frequencies of thunderstorms.
INSTABILITY
Textbook
instability occurs when you have a pocket of warm air near the ground and a
pocket of cooler air on top. Instability is defined when air continues to rise
when given a slight push upwards (or downwards).
LIFT
In order to
achieve that little “nudge” to initiate the instability in the air, a lifting
mechanism must be in place. Lifting can take place in several different forms:
>Differential heating
Differential
heating takes place when the sun warms to surface below at different rates.
Example: Pavement will heat faster than
surrounding fields. This will causes to different air densities to form. The
cooler air will lower, thus forcing the warmer air further up into the
atmosphere.
>Fronts
Fronts are defined
as the boundary between two air masses of different temperature, thus creating
different air densities. If moisture is present, there is a chance that
thunderstorms will form.
>Dry Lines
Dry lines are the boundary between two air masses of
different moisture content and separate warm, moist air from hot, dry air. The
following air mass is typically more dry and of higher temperature. Because the
air ahead is cooler and more moist, buoyancy occurs. All in all, the air along
the dry line lifts and forms a thunderstorm.
>Terrain
As air encounters a mountain it
is forced up because of the terrain. Up-slope thunderstorms are common in the
Rocky Mountain west during the summer.
LIFE CYCLE
All
thunderstorms begin with what is known as a thunderstorm cell. These typically
have a hardy life span of 30 minutes.
Stage #1: Towering Cumulus
Cumulus clouds
will begin to grow vertically (occasionally reaching 20,000 feet). At this
early stage, the cell is made up solely of updrafts.
Stage #2: Mature Cumulus
A mature cumulus
will typically have strong depth, which will occasionally reach to 40,000 feet
into the atmosphere. Updrafts and down draft coexist. This is considered the
thunderstorms most dangerous stage because it is at this time that the threat
of large hail, strong winds, and flash flooding occurs.
Stage #3: Dissipating Stage
As the cold airs
being brought in by the down drafts undercut and suffocate the cell from its
source of warm air being brought in from the updrafts, the thunderstorm begins
to die. Light rain and outflow winds are all that may remain besides the top of
the thunderstorm’s anvil.
