In-Depth Notes on Thunderstorms and Lightning

THUNDERSTORM BASICS

  • Definition: A thunderstorm is characterized by the presence of lightning and thunder.
  • Associated Weather Conditions: Often accompanied by gusty winds, heavy rainfall, and hail.
  • Formation: Can form in a single cumulonimbus cloud or in clusters that may extend several hundred kilometers.

CAUSES OF THUNDERSTORM FORMATION

  • Convection: Warm, saturated air rises in a conditionally unstable environment.
    • When air parcels reach the Level of Free Convection (LFC), they become unconditionally unstable and rise naturally.
    • Thunderstorms develop in warm, unstable air.
  • Surface Heating: Strong surface heating can lead to convection.
    • Also, thunderstorms can derive from colder surface air but may contain very cold air aloft, leading to conditional instability.

SEVERITY OF THUNDERSTORMS

  • Most thunderstorms are not severe. For a thunderstorm to be classified as severe, it must meet at least one of the following criteria:
    • Presence of a tornado.
    • Hail diameter of 1 inch or greater.
    • Trivia: The original severe hail criterion was 3/4 inch diameter, which was the size needed to crack a WW-II era DC-3/C-47 aircraft windshield in flight.
    • Wind gusts of 50 knots (58 mph) or greater.

VERTICAL WIND SHEAR (VWS)

  • Definition: VWS refers to abrupt changes in wind speed and/or direction with height.
    • Strong jet streams can create significant VWS.
    • Ordinary (pulse) thunderstorms typically occur when VWS is weak or absent (e.g., in Florida during summer months).

THE LIFE CYCLE OF AN ORDINARY THUNDERSTORM

  1. Cumulus (Growth) Stage:

    • Warm, moist air rises, cools, and condenses into a cumulus cloud.
    • The initial cloud is shallow and may grow upward if the surrounding air remains sufficiently saturated.
    • Precipitation, lightning, and thunder are usually rare in this stage.

  2. Mature Stage:

    • The cloud grows larger with larger droplets being formed; some droplets descend, creating a downdraft.
    • A mature thunderstorm displays both an updraft and a downdraft, resulting in severe turbulence.
    • When the updraft reaches the stratosphere, it cannot rise any further, leading to outward airflow and the formation of an anvil.
    • Gust Front: Upon hitting the surface, the downdraft spreads out, causing gusty winds and a drop in temperature.
    • Facilitates further updraft enhancement.

  3. Dissipating Stage:

    • The downdraft dominates, cutting off the warm moist air supply for the updraft.
    • Often results in virga, where precipitation evaporates before reaching the ground.
    • Only remnants of the storm can remain, such as the anvil.

MULTICELL THUNDERSTORMS

  • Definition: These thunderstorms contain multiple cells, each at different developmental stages.
    • Unlike ordinary thunderstorms, multicell thunderstorms develop in regions with moderate to strong VWS.
  • Characteristics:
    • Tilted updrafts help to prevent rain from falling back into the updraft, thus enhancing thunderstorm longevity.
    • Different stages of cells contribute to continuous severe weather conditions.

MICROBURSTS

  • Definition: A microburst is an intense, localized downdraft that spreads rapidly once it hits the ground.
    • Can be wet (where precipitation is associated) or dry (no significant moisture).
    • Responsible for some severe weather incidents previously misidentified as tornado damage.

THUNDERSTORM CLIMATOLOGY

  • Thunderstorm Days: The average number of thunderstorm days per year varies significantly across regions in the U.S.
  • Severe Thunderstorm Days: Reports of severe storms including tornadoes, hail, and strong winds.
  • Regions of Interest:
    • Florida is known for a high frequency of ordinary thunderstorms.
    • The central and southern plains are recognized for severe thunderstorms.
    • Colorado and Wyoming are noted hail capitals.

LIGHTNING BASICS

  • Definition: Lightning is an electrical discharge that occurs within or between clouds, or between a cloud and the ground.
    • Major types include intra-cloud (IC), cloud-to-cloud (CC), and cloud-to-ground (CG) lightning.
    • Majority (80%) of lightning strikes are intra-cloud.
  • Characteristics:
    • Heats the surrounding air to approximately 30,000 °C (54,000°F), which causes thunder.
  • Thunder: A shock wave produced by the rapid heating of air caused by lightning.
    • The 30/30 rule for safety in lightning scenarios: If you hear thunder within 30 seconds after seeing lightning, you are in danger, and seek shelter immediately if the time between lightning and thunder is less than 30 seconds.

SAFETY AND PREVENTION

  • Lightning Safety Tips:
    • Avoid open fields, isolated trees, and water. Seek shelter in buildings or vehicles.
    • Utilize lightning rods and stay indoors during a storm to minimize risk.
  • Detection Technologies:
    • Real-time detection of lightning through satellite technology and ground detectors.

CONCLUSION

  • Understanding the basic structure and dynamics of thunderstorms, as well as the associated phenomenon like lightning, is crucial for safety and preparedness against severe weather. This includes recognizing the stages of thunderstorms and employing safety protocols during lightning and storm events.