With sky's rumblings and grumblings of this past weekend, it is clear it is thunderstorm season.
Thunderstorms are transient weather phenomena that evolve with time, growing to full maturity or rage and then fading away. The whole process can take about 30 minutes and typically storms move through a region in cells with one storm following another.
They begin when moist warm air rises into a dry air mass in the cumulus stage. This is where the towering thunderhead clouds emerge.
As the warm air rises, the temperature of the air mass starts to drop. Cooling the air means that some of the water vapour condenses into single or multiple cumulus clouds.
Formation of thunderheads saturates the dry air mass with moisture which, in turn, allows the cloud to continue to grow upwards without evaporating completely into the dry air mass. An important factor in this vertical development of the clouds is that condensation of water vapour generates heat from the latent heat of vaporization. The released energy warms the air in the cloud causing it to rise even further. The result is a very vertical column of warm, moist, rising air.
As the water vapour continues to rise, it passes the freezing point and the water droplets in the cloud start to freeze.
They continue to grow as more moisture from the surrounding cloud condenses on their surface. They become heavier and heavier until, at some point, they are no longer supported by the column of rising air and they start to fall.
At the same time, some of the surrounding dry air is sucked into the cloud through a process called entrainment. This dry air causes some water droplets to evaporate or ice pellets to sublime, cooling the air.
Being now colder and heavier, the air starts to fall more rapidly, descending to the earth below as a freezing downdraft. This cold air has two pronounced effects.
The first is the strong, cold winds associated with a thunderstorm. The second is the speed of descent of the precipitation within the cloud. Rain droplets and hail stones are, literally, driven out of the cloud and thrown at the ground. This is partly why a hail storm can do so much damage and the stones hurt so much when they hit.
At this point, the mature thunderstorm is in full force and the top of the clouds may extend to 12 kilometres high. The storm might be several kilometres in diameter. The updrafts and downdrafts are at their most intense and create heavy turbulence. Rainfall and hail stones are at their maximum.
These short lived storms then enter the dissipation stage which is characterized by the formation of downdrafts through the cloud. This stops the updraft of warm, moist air and shuts down the energy pump that is driving the storm. The lower portion of the cloud drifts away. But the whole process can start over again resulting in a succession of cells and a protracted storm.
Of course, the most exciting part is the thunder and lightning. Everyone knows that lightning is an example of static electricity. It can build up a charge of several million volts and do significant damage when it strikes.
The process for generating lightning in a thunderhead is much the same as the process that we use in winter to shock people. Rub your feet on a dry carpet and touch someone. It is a shocking experience.
However, inside a thunderhead, there are neither wooly socks nor carpets. Instead, there are raindrops and hail stones. The conveyor belt of updrafts and downdrafts mean that these bang into each other as they pass. Hail stones are thrown downwards and upwards within the storm. As they collide, they transfer electrons - from the contents of the cold downdraft to the warm updraft.
The result is a potential energy gradient or a difference in voltage between the top and bottom of the cloud. Essentially, a thunderhead is a giant capacitor, with its positive end pointed out to space and its negative end point towards the earth. It is a capacitor just waiting to discharge.
Opposite charges attract and so the earth below the cloud develops a positive charge in the region under the thunderhead with the charge at its most dense in protruding objects such as trees, poles, buildings, and golfers. However, without a pathway, the electricity has no way to discharge.
A lightning stroke is preceded by a stepped leader - a faint discharge that ionizes a path for the electricity to descend. It is usually met by a similar discharge which reaches up from the earth. When the circuit is complete, lightning strikes, thunder booms, and the storm is at its best.
Thunder and lightning are very frightening but they are just the result of the science of heat and the movement of air.
