The average number of thunderstorm days each year throughout the U.S.

The average number of thunderstorm days each year throughout the U.S. Courtesy of the National Weather Service.

Virtually all summer rainstorms are accompanied by thunder and lightning. No other part of the nation has more thunderstorm activity than Florida. In the western half of the peninsula in a typical year, there are over 80 days with thunder and lightning. Central Florida's frequency of summer thunderstorms equals that of the world's maximum thunderstorm areas: Lake Victoria region of equatorial Africa and the middle of the Amazon basin. The Amazon and east African areas maintain their frequency of thunderstorm activity throughout most of the year, whereas the number of thunderstorms in Florida drops off sharply in the fall and does not pick back up until spring.

The simplest definition of a thunderstorm is a local storm that produces lightning and thunder. The storm itself can either be a single cumulonimbus cloud, a cluster of several thunderstorms, or a line of thunderstorms. In order for thunderstorms to form, there needs to be:

  1. Moisture - to form clouds and rain.
  2. Unstable air - warm air that can rise rapidly.
  3. Lift - cold or warm fronts, sea breezes, mountains or the sun's heat are capable of lifting air to help form thunderstorms.

Once all of these components are brought together, the thunderstorm then goes through a 3-stage life cycle:

Development or Cumulus Stage: A cumulus cloud forms and begins to grow vertical, usually above 20,000 ft. Usually, little, if any, rain occurs during this stage. There might be occasional lightning.

Development or Cumulus Stage

Mature Stage: The cloud has grown in considerable height, now in the range to 40,000 to 60,000 ft. Strong updrafts and downdrafts coexist within the storm. This is the most dangerous stage of the storm and is the most likely time for hail, heavy rain, lightning, strong winds and tornadoes. Storms occasionally have a black or dark green appearance.

Mature Stage

Dissipating Stage: The downdraft cuts off the updraft, which cuts off the supply of warm moist air to the storm and therefore, it dissipates. Rainfall decreases in intensity, along with winds, though strong gusts are still possible. Usually, the anvil top of the cloud is all that remains of the initial cumulus cloud.

Dissipating Stage

Types of Thunderstorms

Thunderstorm Hazards

Lightning develops during the violent circulation of air within the cumulonimbus cloud. The friction causes the positive and negative charges within the storm to separate. In addition, an electrical field develops between the base of the cloud and the ground. However, the electrical field in the cloud is stronger and most of the lightning (~75%) is contained within the cloud.

lightning strike 1 

As the difference in the charge continues to increase, positively charged particles will rise up in taller objects, such as trees, telephone poles, and even buildings. A channel of negative charge, called a stepped ladder, will descend from the bottom of the storm cloud toward the ground. This is invisible to the human eye.

lightning strike 2

The positive charge that has collected in the tall object on the ground 'reaches' out to the approaching negative charge with its own channel, called a streamer. When these channels connect, the resulting electrical transfer is what we see as lightning. If enough of the charger is leftover, additional strokes will use the same channel and give the bolt the appearance of flickering. Lightning heats up the air to 50,000 degrees Fahrenheit, and this rapid heating of the air produces a shockwave that results in thunder.

lightning strike 3

Lightning has both negative and positive polarities. Most lightning forms at the bottom of the cloud, though less than 5% of all lightning occurs from the top of the anvil, making it a positive lightning strike. Positive lightning is very dangerous for several reasons. Since it comes from the top of the anvil cloud, the electric field is much stronger than a negative strike (almost ten times greater!). Some positive strikes can strike the ground beneath the cloud; however, most positive strikes occur near the edge of the cloud or can strike more than 10 miles away. Positive lightning is often responsible for the phenomenon commonly referred to as a "bolt from the blue". Positive strikes are more lethal and cause greater damage than negative lightning.

average annual occurrence of lightning

The global average annual occurrence of lightning, April 1995-February 2003. Courtesy of the National Weather Service.