Isolated Cell

An isolated storm (cell) is a single Cumulonimbus, or an isolated group of Cumulonimbus, whose life span is typically less than one to a few hours (about 80% lasts less than three hours).

A cell has an average diameter of 10 to 20 km, reaches altitudes of 6 to 20 km (about 50% exceed 15 km in height) and moves at speeds of a few kilometers per hour to 50 km / h. It can usually be identified by its broad, bright whitish top, the anvil, which projects toward the winds.

While a single cell storm (called a single cell storm) can last less than an hour, multicell or supercell storms, as well as organized storms, can affect a region for several hours.

Isolated storms can produce up to a few hundred lightning strikes over their lifetime. They typically produce up to four cloud-to-ground lightning bolts per minute. The average distance between two consecutive lightning strikes on the same cloud is 3 km.

Lightning produced by isolated storms tend to occur predominantly in the late afternoon. Unstable lines or convective mesoscale complexes, in turn, can produce hundreds of lightning bolts per minute. In this case, there is no preferred time of occurrence, and maximum lightning can occur throughout the day or even at night.

Evolution

During its lifetime, a unicellular storm goes through three different stages: development stage, mature stage, and dissipative stage, just like the other types of storms. In the case of a unicellular storm, each stage lasts about 20 to 40 minutes.

The formation of an isolated storm begins from the gathering of small Cumulus clouds. If the weather conditions are favorable, these clouds may be formed into larger clouds and become like a small cauliflower (Cumulus mediocris). These in turn can converge to form an even larger, cauliflower-shaped cloud called Cumulus congestus.

At this point, although the cloud base is still about 1 km, its top reaches heights of between 3 and 5 km and its horizontal extent reaches a few kilometers. The cloud is then made up of hundreds of Cumulus clouds.

In some cases, the cloud ceases its development at this point, not evolving into a Cumulonimbus, dissipating without lightning. Otherwise, the cloud continues its upward movement, surpassing the freezing level. We then have a unicellular storm in its developmental stage. The cloud diameter at this stage varies between 3 and 8 km, the top is between 5 and 8 km and has irregularities due to ice particles.

Air movement within the cloud is predominantly upward, dragging water droplets and ice particles upward. In general little rain and few or no lightning occur at this stage. As the speed of the ever-growing water and ice particles becomes greater than the rate of ascension, the particles begin to fall generating downward currents. At this moment, the cloud reaches the mature stage.

In the mature stage, the unicellular storm has at its bottom both upward and downward movements. It is at this stage that most rain, lightning, hail, high winds and tornadoes occur. Downward movements occur due to non-support of water droplets and ice particles that have grown in size.

Rain and hailstorm drag air down with it, intensifying downward air currents and producing horizontal air currents, the fronts of which are called gust fronts, as air spreads as it reaches the ground.

The gust fronts can act as forcing, starting another storm cloud. Downward drafts may also be intensified by entrainment. At this stage upward and downward movements can reach speeds as high as 100 km / h.

The diameter of the cloud is typically 10 km, although in some cases it may reach tens of kilometers.

The height of the cloud base can range from just under 1 km to about 4 km, depending on the humidity, and is usually relatively flat. The summit reaches heights ranging from 8 to 20 km, reaching and even surpassing, in some cases, the tropopause. The shape of the top also tends to widen relative to the cloud diameter, caused by the horizontal scattering of ice particles when they reach equilibrium.

Due to the influence of winds, this region can extend horizontally causing the cloud to resemble an anvil, pointing downwind at high altitudes. The anvil is basically ice crystals. Cloud-to-ground lightning is usually preceded within a few minutes by intra-cloud lightning and can occur both before and after the onset of rain. Measurements have also shown that the higher the top of the cloud, the greater the frequency of lightning.

From a given moment downward air currents act to inhibit new upward currents within the cloud, and this tends to cause the cloud to begin to dissipate. This process initiates the dissipative stage, where air movement is almost exclusively descending, causing the cloud to cool down in its vicinity.

Rain intensity and lightning activity decrease, although they remain significant. The height of the top of the storm cloud at this stage tends to decrease until the cloud is completely dissipated. The winds at higher levels scatter the ice crystals, so the anvil (anvil) is the last remaining part of the cloud, taking a form similar to Cirrostratus and Altostratus clouds.

Cumulonimbus cloud formation Multicell Supercell