How Do Thunderstorms Form?

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Thunderstorms

A mature thunderstorm, with anvil top. NOAA National Weather Service

Whether you happen to be a spectator or a "spook," chances are you've never mistaken the sight or sounds of an approaching thunderstorm. And it's no wonder why. Over 40,000 occur worldwide every day. Of that total, 10,000 occur daily in the United States alone.

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Thunderstorm Climatology

A map showing the average number of thunderstorm days each year in the U.S. (2010). NOAA National Weather Service

In the spring and summer months, thunderstorms seem to occur like clockwork. But don't be fooled! Thunderstorms can occur at all times of the year, and at all hours of the day (not just afternoons or evenings). The atmospheric conditions only need be right.

So, what are these conditions, and how do they lead to storm development?

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Thunderstorm Ingredients

In order for a thunderstorm to develop, 3 atmospheric ingredients must be in place: lift, instability, and moisture.

Lift

Lift is responsible for initiating the updraft--the migration of air upward into the atmosphere--which is necessary in order to produce a thunderstorm cloud (cumulonimbus).

Lift is achieved in a number of ways, the most common being through differential heating, or convection. As the Sun heats the ground, the warmed air at the surface becomes less dense and rises. (Imagine air bubbles that rise from the bottom of a boiling water pot.)

Other lifting mechanisms include warm air overriding a cold front, cold air undercutting a warm front (both of these are known as frontal lift), air being forced upward along the side of a mountain (known as orographic lift), and air that comes together at a central point (known as convergence.

Instability

After air is given an upward nudge, it needs something to help it continue its rising motion. This "something" is instability.

Atmospheric stability is a measure of how buoyant air is. If air is unstable, it means that it is very buoyant and once set in motion will will follow that motion rather than return to its starting location. If an unstable air mass is pushed upward by a force then it will continue upward (or if pushed down, it will continue downward).

Warm air is generally considered to be unstable because regardless of force, it has a tendency to rise (whereas cold air is more dense, and sinks).

Moisture

Lift and instability result in rising air, but in order for a cloud to form, there must be sufficient moisture within the air to condense into water droplets as it ascends. Sources of moisture include large bodies of water, like oceans and lakes. Just as warm air temperatures aid lift and instability, warm waters aid the distribution of moisture. They have a higher evaporation rate, which means they more readily release moisture into the atmosphere than cooler waters do.

In the U.S., the Gulf of Mexico and the Atlantic Ocean are the major sources of moisture for fueling severe storms.

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The Three Stages

Diagram of a multicell thunderstorm consisting of individual storm cells - each in a different development stage. Arrows represent the strong up-and-down motion (updrafts and downdrafts) which characterize thunderstorm dynamics. NOAA National Weather Service

All thunderstorms, both severe and non-severe, go through 3 stages of development:

  1. the towering cumulus stage,
  2. the mature stage, and
  3. the dissipating stage.
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1. The Towering Cumulus Stage

The initial stage of thunderstorm development is dominated by the presence of updrafts. These grow the cloud from a cumulus to a towering cumulonimbus. NOAA National Weather Service

Yes, that's cumulus as in fair weather cumulus. Thunderstorms actually originate from this non-threatening cloud type.

While at first this may seem contradictory, consider this: thermal instability (which triggers thunderstorm development) is also the very process by which a cumulus cloud forms. As the Sun heats the Earth's surface, some areas warm faster than others. These warmer pockets of air become less dense than the surrounding air which causes them to rise, condense, and form clouds. However, within minutes of forming, these clouds evaporate into the drier air in the upper atmosphere. If this happens for a long enough period of time, that air eventually moistens and from that point on, continues cloud growth rather than stifling it.

This vertical cloud growth, referred to as an updraft, is what characterizes the cumulus stage of development. It works to build the storm. (If you've ever watched a cumulus cloud closely, you can actually see this happen. (The cloud begins burgeoning upward higher and higher into the sky.)

During the cumulus stage, a normal cumulus cloud can grow into a cumulonimbus having a height nearly 20,000 feet (6km). At this height, the cloud passes the 0°C (32°F) freezing level and precipitation begins to form. As precipitation accumulates within the cloud, it becomes too heavy for updrafts to support. It falls inside of the cloud, causing drag on the air. This in turn creates a region of downward directed air referred to as a downdraft.

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2. The Mature Stage

In a "mature" thunderstorm, an updraft and downdraft co-exist. NOAA National Weather Service

Everyone who has experienced a thunderstorm is familiar with its mature stage--the period when gusty winds and heavy precipitation are felt at the surface. What may be unfamiliar, however, is the fact that a storm's downdraft is the underlying cause of these two classic thunderstorm weather conditions.

Recall that as precipitation builds within a cumulonimbus cloud, it eventually generates a downdraft. Well, as the downdraft travels downward and exits the base of the cloud, the precipitation is released. A rush of rain-cooled dry air accompanies it. When this air reaches the Earth's surface, it spreads out ahead of the thunderstorm cloud--an event known as the gust front. The gust front is the reason why cool, breezy conditions are often felt at the onset of a downpour.

With the storm's updraft occurring side-by-side with its downdraft, the storm cloud continues to enlarge. Sometimes the unstable region reaches as far up as the bottom of the stratosphere. When the updrafts rise to that height, they begin to spread sideways. This action creates the characteristic anvil top. (Because the anvil is located very high up in the atmosphere, it is comprised of cirrus/ice crystals.)

All the while, cooler, drier (and therefore heavier) air from outside of the cloud is introduced into the cloud environment simply by the act of its growth.

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3. The Dissipating Stage

Diagram of a dissipating thunderstorm - its third and final stage. NOAA National Weather Service

In time, as the cooler air outside of the cloud environment increasingly infiltrates the growing storm cloud, the storm's downdraft eventually overtakes its updraft. With no supply of warm, moist air to maintain its structure, the storm begins to weaken. The cloud begins to lose its bright, crisp outlines and instead appears more ragged and smudged--a sign that it is aging.

The full life cycle process takes about 30 minutes to complete. Depending on thunderstorm type, a storm may go through it only once (single cell), or up to multiple times (multi-cell). (The gust front often triggers the growth of new thunderstorms by acting as a source of lift for neighboring moist, unstable air.)

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Means, Tiffany. "How Do Thunderstorms Form?" ThoughtCo, Mar. 1, 2017, thoughtco.com/how-thunderstorms-form-3444271. Means, Tiffany. (2017, March 1). How Do Thunderstorms Form? Retrieved from https://www.thoughtco.com/how-thunderstorms-form-3444271 Means, Tiffany. "How Do Thunderstorms Form?" ThoughtCo. https://www.thoughtco.com/how-thunderstorms-form-3444271 (accessed April 19, 2018).