The Thunderstorm Life Cycle

      As promised here is the 2nd installment. We have all seen thunderstorms. But how many of you have actually wondered what makes them tick. This blog post will try to cast some light on this mystifying and sometimes petrifying beast that prowls the skies overhead.  Before I get  into the lifecycle of a thunderstorm, a little attention should be spent on what a thunderstorm is and how it forms.

What is a Thunderstorm:

     Simply put a thunderstorm is just a rainstorm during which you hear thunder. Since thunder comes from lightning, all thunderstorms have lightning. The average thunderstorm is around 10-15 miles in diameter and last 20-30 minutes. In order for a thunderstorm to form, a few basic ingredients must be in place. These are moisture, unstable air, and something to give a nudge, the term for this is a lifting mechanism.  First, as air rises in what is called an updraft, moisture is squeezed out of the air. This moisture forms into small water drops which form clouds. As the air is squeezed it gives off heat, making the air warmer. If you have ever seen a hot air balloon you know warm air rises. The air will continue to rise as long as it's warmer than the surrounding air. Air can be stable, neutral, or unstable. 

Which brings us to our second condition. Instability is when atmospheric conditions  allow air to  rise freely on its own. The more unstable the air the faster the air will rise. Our last condition is lift, Lift is the machinery that starts an updraft in a moist, unstable air mass.  The lifting mechanism can take on several forms. However, the most common is simply the sun heating the ground which warms up the air above it causing it to rise.

The Thunderstorm Life Cycle:

     Most of us have seen how it can go from a clear blue sky to a thunderstorm in less than 30 minutes. But, it has to begin somewhere. All thunderstorms, whether or not they ever became severe, go through a life cycle, this life cycle can be divided into three stages: developing (sometimes known as the cumulus stage), mature stage, and dissipating stage.

The developing stage:

     There is little to no rain during this stage but occasional lightning. The developing stage lasts about 10 minutes. The developing stage  is marked by a cumulus cloud. During the development stage, only the updraft is very dominant. Sometimes the speed at which a cloud builds in height can be shocking (pun intended).  The updraft pushes the the warm moist air within the cloud higher and higher. This process continues and works to form a towering cumulus cloud. After awhile, the cloud gets so high that the water droplets in the cloud coexist with ice crystals in the upper atmosphere.  At this point precipitation starts to fall within the cloud.  Falling precipitation and cool air from the environment start the initiation of cool downdrafts. These downdrafts are the beginning of the next stage of the life cycle.

                  Diagram of how the updraft builds a Cumulus cloud  (Image credit NWS)

                                                              
                                                                Cumulus cloud

The Mature stage:

     The mature stage lasts on average for 10 to 20 minutes, but may persist for longer periods of time if the storm is severe. The thunderstorm at this stage of the cycle has both updrafts and downdrafts within the cloud.  At this stage the cumulus is still growing as the thunderstorm continues to  develop the top of cloud starts to flatten out and forms an anvil shape. At this point the cloud has become a cumulonimbus cloud.   Cloud to ground lightning generally (but not always) begins when the precipitation first falls from the cloud base.  When the downdraft hits the ground, it spreads out in all directions. When this happens, a gust front can form.  A gust front is from the cool air rushing down and out from a thunderstorm. The mature stage is the most likely time for hail, heavy rain, frequent lightning, strong winds, and tornadoes. The storm sometimes has a dark green appearance. Eventually, the downdraft will become the overriding feature in the storm, which will cause the storm to weaken and enter the final stage.

                                                       

                                                           Image credit NWS

                                                       A mature cumulonimbus

The dissipating Stage:

    This stage lasts about 10 minutes. During this stage the rain is pulling air downward, this strengthens the downdraft . During the dissipating stage the downdraft is king. A thunderstorm is a creature of warm moist inflow. As the downdraft hits the ground and pushes away from the storm, it cuts off the updrafts which are feeding it. Since warm moist air can no longer rise, cloud droplets can no longer form. The storm dies out with light rain as the cumulonimbus cloud disappears from bottom to top. Sometimes, the anvil can be left behind and we call this an “orphan” anvil.

                                                           Image credit NWS

                                                    A dissipating cumulonimbus

I feel thunderstorms are one of nature’s most beautiful events. They can come out of nowhere and disappear again within minutes. I hope you found this informative.

Rebecca Ladd