Thunder Storms

• A thunderstorm is a column of moist turbulent air with precipitation, strong wind, lightning and hail.

• A single cell thunderstorm is one that occurs independent of larger storm systems.

  • Single cell thunderstorms can form and grow when cumuliform clouds are forced together by the wind. The collision causes instability and sudden rising moisture which induces a thunderstorm.

  • A thunderstorm that forms beneath strong winds has an increased likelihood for intensification.

    Vertical wind shears can also intensify thunderstorms since the wind at the top of the storm pushes the downdraft away from the updraft, reducing the cooling effect of the surface.

• Single cell thunderstorms form in the following stages.

  • Cumulus Stage - occurs when the surface heats more rapidly than the atmosphere above it. This forms an updraft which causes the formation of a cumulus cloud.
  • Mature Stage - occurs with an unstable atmosphere with sufficient moisture. Here, updrafts are now accompanied by cool downdrafts induced by falling precipitation. The downdrafts move laterally in the direction of the wind. Most lightning occurs here.
  • Dissipating Stage - the falling rain cools the surface and removes the unstable conditions. The cloud disappears and becomes more feathery.

• Clusters of thunderstorms can also form in fronts or as a result of coalescing storms.

  • Multicell thunderstorms form as a result of “chains of thunderstorms”.

    Here the downdraft of one thunderstorm contributes to the updraft of an adjacent thunderstorm.

  • A squall line thunderstorm forms along cold fronts, especially when hot, dry air masses interact with warm, moist air masses.

    These tend to be severed due to abundant moisture.

  • Supercell thunderstorms are tall and powerful storms. Commonly they have anvil shapes as a result of vertical wind shear.

    Vertical wind shear means intense rain, sometimes even with lightning and thunder.

    • A mesocyclone is a rotating vortex in a supercell. It is initially formed horizontally via wind shear before updraft skews it to be vertical.
    • Mesocyclones can become tornadoes due to updrafts strengthening the rotation of the cyclone.
    • The downdrafts from a supercell are strong enough to also cause other storms.

• Lightning storms more commonly form under certain conditions.

  • Lightning strikes are much more common on land than over the oceans. Large landmasses can heat up during the day and cause free convection, resulting in thunderstorms.
  • Lightning also tends to form near coastlines due to being on the border of different air masses.
  • Abundant tropical moisture, strong surface heating, and vigorous trade-wind convergence also contribute to strong thunderstorms,.
  • Polar regions are typically too cold and dry to cause thunderstorms to form
  • They typically occur in summer and in the morning due to the heat.

Lightning §

• Lightning is a sudden and large discharge of electrical energy generated naturally within the atmosphere.

  • Most lightning occurs as cloud-to-cloud lightning or within a single cloud.
  • Sheet Lightning pertains to diffuse sheets of lightning. It occurs within the cloud itself manifesting as brief moments of illumination within the cloud.

• Lightning accumulates within cumulonimbus clouds as a result of a charge differential. Ice particles have a positive charge and there is an imbalance where the upper portions of the cloud have more ice particles.

• Lightning strikes when the electric potential is greater than the insulation capabilities of air.

• Thunder follows from the rapid heating and expansion of air along the path of a lightning bolt. We can think of it as a result of air rapidly trying to fill a vacuum left behind by the air expanding around the lightning bolt.

Hail §

• Hail is a ball of ice that forms under freezing temperatures within a cumulonimbus cloud

• Hail typically forms in supercell thunderstorms as the updraft lifts up water and chunks of ice.

  • Typically, hail forms during summer or spring where there is atmospheric instability due to the surface being warm and the atmosphere cool.
  • Hail tends to form in the afternoon similar to thunderstorms.

• Once the particles are lifted past the subfreezing part of the cumulonimbus cloud, water vapor deposits onto ice particles via the Bergeron Process. This causes the ice particles to grow.

• Once the ice particles are too heavy to be lifted by the updraft, they descend. This can also be influenced by a downdraft.

• Ice particles that descend will melt, but also acquire more water vapor on the descent. These particles, if light enough, can be carried again by an updraft.

• The combination of accretion via the Bergeron process and melting of water vapor produces spherical masses consisting of several layers of ice. These finally fall as precipitate when they are too heavy to be lifted.

• Hail formation is most likely at the time of year when the cloud extends substantially both above and below freezing.

  • If a cloud is entirely warmer than freezing, it will only produce rain.
  • If the entire cloud is below freezing temperatures, it will produce snow.

• Other factors that influence hail formation include

  • Amount of moisture
  • The vertical extent of the cloud.
  • Strength of the updrafts and downdrafts.
  • The temperature across the front.

Tornados §

• A tornado is a rapidly spinning vortex of air and debris that extends from the base of a cumulonimbus cloud to the ground.

• Tornados become visible due to the condensation of moisture and the debris flying within the vortex

• Tornado formation generally begins with horizontal winds moving across a surface. A vertical wind shear within the wind column causes an instability that causes the initial vortex.

  • The updraft within a thunderstorm then lifts the turbulent vortex and makes it more vertical.
  • In most cases, tornadoes spin in the same direction as their parent cyclone.
  • In rare cases, a tornado can tighten its rotation resulting in more angular momentum and more destructive power.

• Tornados are more common in flat areas because rough topography can disrupt the vertical wind shear required for its formation.

• A storm can form tornadoes if it has rotational motion coupled with strong updrafts. Storms that likely spawn tornadoes have a hook echo pattern in radar.

• Most tornadoes are associated with multi-cell or supercell thunderstorms

• Tropical cyclones can also spawn tornadoes but this tends to be rare because the tropical cyclone’s rotation negates the effects of the updraft needed for tornado formation.

• Single-cell thunderstorms can spawn tornadoes but these tend to be smaller and weaker because the storm itself tends to be less powerful.

• Tornadoes formed in any setting have a sharp low forming cloud called a wall cloud.

• Few tornadoes are known to occur over open seas because surface heating is far less intense over oceans than over land

• Tornadoes tend to strike in the afternoon and evening.

Other Storms §

• A waterspout is a rotating columnar vortex of air and water droplets over a body of water..

  • Tornadic Waterspouts are tornadoes that form over water or over land and moved over the water. They form similarly to tornadoes but are less powerful because of the cooler surface on water.
  • Fair Weather Waterspouts are associated with normal cumulus clouds formed due to wind shear and mild updrafts near the water. The water in the waterspout is from water vapor. This tends to be weaker than tornadic waterspouts.

• A derecho is a windstorm characterized by strong, nonrotating winds. The winds travel at a high velocity.

  • Derechos on radar appear in the shape of a bow as the winds dissipate.
  • They are caused by downdrafts. In particular, a downdraft from one storm strengthens an adjacent thunderstorm.
  • They tend to occur in summer at any time of day.
  • They tend to be accompanied with a distinctive shelf cloud.

• A microburst is a brief but strong downward moving wind. The wind moves in a straight line downwards.

  • The strength of the microburst can be as severe as a tornado.
  • They pose an aviation hazard due to the wind shear.
  • Microbursts can either be wet (associated with rain) or dry (not associated with rain)
  • A virga is a phenomenon where the downdraft carries a diffuse column of raindrops that evaporate before they reach the ground.
  • Microbursts tend to form late in, or at the end of, the life cycle of a thunderstorm.

• A haboob pertains to severe downdraft—related nonrotating dust storms.

  • They form at the end of a thunderstorm’s life cycle.
  • They can pick up dust from desert areas thus forming a thick dust cloud.
  • They form in arid areas.
  • They tend to be weaker than actual storms but they can also cause erosion in the landscape.

• A dust devil or dust storm is a storm characterized by the presence of dust particles.

  • They form in arid areas.
  • They tend to be weaker than actual storms but they can also cause erosion in the landscape.

Links §

• Exploring Physical Geography by Reynolds