How Do Thunderstorms Form?

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,and not that many here in Cornwall.

 

Thunderstorm Climatology

 

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?

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.

The Three Stages

 

 

 

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.

The Towering Cumulus Stage

 

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.

The Dissipating Stage

 

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 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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