Chapter 16 Notes
Ahrens, C.D., Meteorology Today

    A thunderstorm is a storm that contains thunder and lightning, usually accompanied by gusty surface winds and heavy rain.  The storm may result from single cumulonimbus cloud or a cluster of clouds extending for more than 100 kilometers.  Thunderstorms are born when warm, humid air rises in a conditionally unstable atmosphere.  Thunderstorms develop when converging surface winds are coupled to diverging upper-level winds.

        Ordinary or air-mass thunderstorms are scattered thunderstorms that occur in the summer and are usually associated with weather fronts.  Severe thunderstorms are characterized by high winds, flash floods, damaging hail, and occasionally, tornadoes.

Development of Ordinary (Air-Mass) Thunderstorms

        There are three stages in a thunderstorm's life.  They are the cumulus stage, mature stage, and dissipating stage.

Cumulus Stage

        The cumulus stage is the development stage.  During this stage there is only an updraft and no precipitation reaching the ground. As the storm
develops, precipitation is produced in the upper parts of the cloud.  As the cloud builds well above the freezing level, cloud particles grow larger and become heavier.  Eventually, the rising air can no longer keep the growing droplets suspended and they begin to fall.  Also at this time, drier air from around the cloud is drawn into in a process called entrainment.  This starts the downdraft which marks the beginning of the next stage of thunderstorm development, the mature stage.

Mature Stage

        The mature stage is the height of the storm's strength when it is most intense. During this stage there is both an updraft and downdraft which constitutes a cell, precipitation, and possible severe weather.  The top of the cloud has reached the stable region of the atmosphere and begins to take on an anvil shape.  In some storms, updrafts may become powerful enough to intrude above the stable atmosphere layer in a process called overshooting.  As the precipitation spreads throughout the storm the downdraft grows strongest in the center of cloud and creates severe turbulence.  Within about 15 to 30 minutes of entering the mature stage, the storm begins to dissipate.

Dissipating Stage

        The dissipating stage is dominated by the downdraft. As the downdraft strengthens, it cuts off the updraft and stops the inflow of moist air. Deprived of its rich source of humid air, the storm rains itself out, sometimes leaving behind only the cirrus anvil as a reminder of its once mighty presence.  The three stages of ordinary thunderstorm development may be concluded in an hour or less.

Development of Severe Thunderstorms

        Severe thunderstorms are capable of producing  high winds, flash floods, damaging hail, and tornadoes.  Updrafts in severe thunderstorms are much stronger than in air-mass thunderstorms, and may reach velocities of 100 knots.  These powerful updrafts often extend into the stable atmosphere, sometimes reaching elevations of 60,000 feet.  The violent updrafts can suspend hail stones long enough for them to grow to considerable size.  When the hail becomes to heavy to suspended any longer, they normally fall from the bottom of the cloud, but some may be ejected from the side of cloud. Mammatus clouds may be observed as the form on the underside of the advancing anvil.

Severe Thunderstorms

Downburst, Microburst, and Macroburst

        The intense downdraft may help to maintain the updraft, and vice-versa, enabling a severe thunderstorm to persist for hours.  Intense downdrafts that hit the ground and spread radially along the surface are called downbursts.  Although very intense, a downburst that extends radially for 4 km or less is termed a microburst a downburst with extent greater than 4 km is referred to as a macroburst.  Damaging winds for either may have velocities exceeding 140 knots.  Strong downburst winds associated with a cluster of severe thunderstorms can produce powerful straight-line winds greater than 90 knots that result in damage for at least 400 km along the storm's path are called derecho winds.

The Gust Front

        The boundary that separates cold, downdraft air from warm surface air is a gust front.  As the gust front passes it resembles a passing cold front characterized by gusty winds, dropping temperatures, and increasing surface pressure.  If atmospheric pressure increases by several millibars, a localized area of high pressure called a mesohigh may be created.  Warm, moist air rising along the forward edge of the gust front may form a shelf cloud, particularly if the atmosphere is very stable near the base of the thunderstorm.  Occasionally, a long, ominous looking, slowly spinning, cloud called a roll cloud may form just behind the gust front.

Supercell and Squall-Line Thunderstorms

        A supercell thunderstorm is an enormous rotating thunderstorm with updrafts and downdrafts aligned to that the storm can maintain itself for several hours.  Supercell thunderstorms are characterized by violent updrafts (>90 knots), grapefruit sized hail, damaging surface winds, and large, long-lived tornadoes. HP supercells ( High Precipitation) produce extreme downbursts, flash flooding, very large hail, and heavy precipitation that appears to fall near the center or eastern part of the storm which may conceal tornadoes.  LP supercells ( Low Precipitation) are characterized by minor precipitation, and a vertical, corkscrew shaped tower that  may spawn tornadoes.  A squall line is a line of thunderstorms that forms along a cold front or in the region ahead of the cold front.  Severe thunderstorms may also form in a narrow zone called a dry line where a sharp, horizontal change in moisture exists.

Mesoscale Convective Complexes (MCC's)

        When conditions favor convection, individual thunderstorms can intensify and combine into a single, large convective weather system.  Very large convective systems, often 1000 times larger than an individual thunderstorm that may engulf an entire state (area > 100,000 square kilometers) are called Mesoscale Convective Complexes.  These complexes of thunderstorms may persist for 12 hours or more and produce severe weather including hail, high winds, destructive flash floods, and tornadoes.

Distribution of Thunderstorms

        Thunderstorms are prevalent over equatorial land masses and over water in the intertropical convergence zone.  In polar regions and desert areas with subtropical highs, thunderstorms are rare.  Over the central plains of the United States, intense thunderstorms often form when moist, warm surface air from the Gulf of Mexico pushes northward under dry, cool air aloft that is flowing southwesterly or westerly in direction.

Lightning and Thunder

        The powerful discharge of electricity that occurs during a mature thunderstorm is referred to a lightning.  Lightning may pass from cloud to cloud, cloud to ground, or from cloud into the surrounding air.  Thunder is produced by a shock wave resulting from the explosive heating of the atmosphere as a lightning bolt is generated.  Temperatures of 30,000 degrees Centigrade can occur adjacent to the lightning discharge.  One theory proposes that lightning is created by the net transfer of positive ions from a warmer object (hailstone) to a colder object (ice crystals) within the cloud.  Another idea suggests that during formation of precipitation, regions of electric charge exist within tiny cloud droplets and larger precipitation creating negatively charged particles in the upper part of the cloud and negatively charged particles in the lower part of the cloud.
        Very faint lightning discharges within the cloud that move toward the ground in a series of steps are called stepped leaders.  A luminous lightning stroke that propagates upward from the Earth to the base of a cloud is called a return stroke. A dart leader is the discharge of electrons the proceeds intermittently toward the ground along the same ionized channel taken by the initial lightning stroke.  A luminous sphere that appears to float in the air or slowly dart about for several seconds is an unusual type of lightning known as ball lightning.
        When lightning enters sandy soil, the extreme temperatures may fuse sand grains together forming a rootlike system of tubes called a fulgurite.
When lightning strikes an automobile, the occupants are usually unharmed because the electricity is discharged along the outside of the vehicle and then into the air or into the pavement through the tires.

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