Life Cycle of a Cumulus Cloud/Thunderstorm

2017-04-15 08:36:18.000 – Taylor Regan, Weather Observer

 

With the recent spell of warm weather (the summit made it to 50 degrees Fahrenheit this past week); I have been reminded that we are well on our way out of the winter months, and moving full speed ahead towards spring and summer. While winter on the Rockpile brings its challenges: transportation, higher winds (on average), epic snow and riming events; the spring and summer months bring their own unique challenges to collecting weather data up here on the summit. One challenge in particular is a thunderstorm.

Just for everyone’s peace of mind, when there’s an active thunderstorm impacting the summit, which can occur nearly two dozen times over the season, the observers do not go outside. Most of our hourly observation can be done in this case from the safety of indoors, and what we can’t observe by being outdoors (such as collecting the precipitation can) can be deferred from a 6 hour to 12 hour observation. It’s not ideal, but it is an option. Being so remote up here, safety is paramount, and not something we take lightly.

 

Figure 1. Cumulonimbus overhead.

This Sunday, as a cold front moves through the region, the summit is beginning to see indications that there is the potential for severe weather, particularly, thunderstorms, looming on the horizon. What is a thunderstorm? And how does a small puffy cumulus cloud turn into something so violent?

Figure 2. Cumulus clouds of little vertical development. Photo by Thierry Lombry.

The cycle of a thunderstorm begins with a humble cumulus cloud. The word cumulus comes from the Latin cumulo, meaning “heap” and reflects the nature that cumulus clouds build from the bottom up. Typically these clouds begin at relatively low altitudes (around 3,000 ft) but given the right conditions they can grow upwards in excess of 60,000 ft.

A cumulus cloud is fed by a warm updraft that forces increasing amounts of water vapor to cool and condense. As the cumulus cloud grows, it becomes a cumulus mediocris cloud, which is to say, a fluffy cumulus cloud that is beginning to look like it is growing upward. At this stage, the cloud is not yet producing precipitation, but it is well on its way to becoming a more developed cloud.

The primary stage of the thunderstorm is a cumulus congestus cloud, or towering cumulus. At this stage the cloud is rapidly growing, and beginning to sport the iconic cauliflower like bubbles. The cloud is now taller than it is wide. At this point, the cloud is capable of producing showers and the air within the cloud is dominated by updrafts, rapidly feeding warm moist air into the cloud.

 

Figure 3. Towering cumulus schematic. Photo from.srh.noaa.gov

In the second phase of the development of a thunderstorm, the towering cumulus continues to build, rising to heights of 40,000 to 60,000 ft. This is now a mature cumulonimbus, and the cloud often possesses the characteristic anvil or flattened top. At this stage, both updrafts and downdrafts persist, and the cloud is capable of producing extremely dangerous weather, such as frequent lightning, tornadic activity, large hail, heavy precipitation, and high wind gusts. This is typically the most dangerous stage.

 

Figure 4. Mature cumulonimbus schematic. Photo from srh.noaa.gov

The third and final stage of a thunderstorm is the dissipating stage. At this point, the downdrafts have cut off the upward flow of warm, moist air, and the storm loses its fuel. As a result, it continues to expend what energy it has left until only the remnants of the anvil top remain. Light rain and weak winds are typical at this stage before the storm is just a memory.

 

Figure 5. Dissipating cumulonimbus schematic. Photo from srh.noaa.gov

 

Taylor Regan, Weather Observer

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