What Causes Thunderstorms?

Cumulonimbus clouds often mean severe thunderstorms and other serious weather.
David James/Riser/­Getty Images

What causes thunderstorms? According to the National Oceanic and Atmospheric Administration (NOAA) National Weather Service, approximately 1,800 thunderstorms are occurring at any given time, resulting in about 16-million thunderstorms each year.

Most thunderstorms last about 30 minutes and are typically about 15 miles (24 km) in­ diameter. The biggest threats associated with most thunderstorms are lightning, strong winds, and flash floods.


Where does the thunder (and lightning) come from? The basic idea is that thunder clouds can become giant Van de Graaff generators and create huge charge separations within the cloud. Let's look at how it works.

Clouds 101

Clouds contain millions and millions of water droplets, ice particles, and lighter ice crystals suspended in the air. As the process of evaporation and condensation occurs, these droplets collide with other moisture that is condensing as it rises.

The importance of these collisions is that electrons are knocked off of the rising moisture, creating a charge separation. The newly knocked-off electrons gather at the lower portion of the cloud, giving the surrounding air a negative charge and creating an unstable air mass.


Air Rises, Pressure Builds

The rising moisture that has lost an electron carries a positive charge to the top of the cloud. As this warm air rises, it encounters cold air in the upper cloud regions and begins to freeze. The frozen portion becomes negatively charged and the unfrozen droplets become positively charged.

At this point, rising air currents have the ability to remove the positively charged droplets from the ice and carry them to the top of the cloud. The remaining frozen portion either falls to the lower portion of the cloud or continues on to the ground.


The Anatomy of a Lightning Bolt

The charge separation has an electric field associated with it. Like the cloud, this field is negative in the lower region and positive in the upper region. The strength or intensity of the electric field is directly related to the amount of charge build-up in the cloud.

As the collisions and freezing continue to occur, and the charges at the top and bottom of the cloud increase, the electric field becomes more and more intense -- so intense, in fact, that the electrons at the Earth's surface are repelled deeper into the Earth by the negative charge at the lower portion of the cloud. This repulsion of electrons causes the Earth's surface to acquire a strong positive charge.


All that is needed now is a conductive path so the negative cloud bottom can conduct its electricity to the positive Earth surface. The strong electric field creates this path through the air, resulting in a lightning strike.

Is Thunder More Common in the Summer?

Ever wonder why you hear thunder more often during the summer months? Because moisture and warm air are crucial to thunderstorms, it makes sense that they would occur more often in the spring and summer, particularly in humid areas such as the southeastern United States. The high humidity, in conjunction with warm temperat­ures, creates a warm, moist air mass rising into the atmosphere.


Surrounding Air Instability

Thunderstorms thrive in atmospheres that are unstable. Instability occurs when relatively warm air at the surface rises, and as it ascends, it cools and condenses, forming clouds.

If this rising air is much warmer than the surrounding air, it will continue to rise, leading to stronger updrafts and a more robust thunderstorm. The greater the temperature difference between the surface and the upper atmosphere, the more potential there is for a severe thunderstorm.


The Life Cycle of a Thunderstorm

Every thunderstorm has its life cycle, which comprises three stages: the cumulus stage, the mature stage, and the dissipating stage. In the cumulus stage, towering cumulus clouds form due to the warm, water vapour rich rising. As the cloud grows taller, it enters the mature stage, which is when the heaviest rain, thunder, lightning, hail, and high winds can occur.

After reaching its peak intensity, the storm begins to weaken and enters the dissipating stage. During this phase, the updrafts weaken, and downdrafts take over, signaling the storm's end.


Lightning Strike Safety Tips

Safety is paramount during a thunderstorm. If you hear thunder, you are close enough to be in danger from lightning. Seek shelter immediately inside a sturdy building or a hard-topped vehicle and stay away from windows.

Avoid plumbing, electrical appliances, and landline telephones. If you're caught outdoors, never shelter under an isolated tree and avoid tall objects. Lightning tends to strike the highest objects in an area. Moreover, stay away from water, as it is a good conductor of electricity.


How Thunderstorms Work

Thunderstorms, while a common natural phenomenon, are intricate systems driven by the delicate interplay of atmospheric elements. These powerhouses of the sky not only fascinate meteorologists and weather enthusiasts but also serve as a reminder of the formidable forces of nature.

From the delicate beginnings in the form of charged water droplets to their potential escalation into hazardous events like hailstorms and tornadoes, thunderstorms embody the dynamism of our planet's climate. As we've seen, understanding how thunderstorms form is crucial, not just for science, for the safety too.


Lots More Information

Related HowStuffWorks Articles

More Great Links