Cumulonimbus cloud formation

In the life of a Cumulonimbus formed by convection from an air mass, there are usually 3 phases (lasting on average 15 to 30 minutes each):

Birth: Rising air currents lead to the formation of a Cumulonimbus cloud. The first charges of water arise, but no lightning occurs yet. At the top of the cloud the ice crystal growth process begins to produce large precipitation particles.

Maturity: Vertical growth reaches its maximum and the top of the cloud is flattened into the characteristic shape of an anvil. Usually this occurs when rising air encounters a stable temperature inversion (eg warmer tropopause air - boundary between troposphere and stratosphere). The prevailing winds at altitude begin to spread Cirrus from the top of the cloud. The front bases get lower and lightning begins to occur across the cloud.

Within the cloud the turbulence is intense and irregular, with a balance between upward and downward currents. The weight of the precipitation particles is already sufficient to counteract the upward currents and begin to fall, dragging the air around you. As precipitation particles fall into the warmer regions of the cloud, the dry air present in the environment enters the cloud and may cause these particles to evaporate. Evaporation cools the air, making it denser and heavier.

It is all this cold air that falls through the cloud with precipitation that forms the downward stream of air that, when it hits the surface, can spread sideways, forming a gusting front that displaces and replaces the warmer surface air. . At this stage the thunderstorm produces strong winds, lightning and heavy precipitation.

Dissipation: The cloud begins to spread sideways, in layers, and the descending cold currents become predominant. Cold air replaces the warmer surface air, turning off the upward movement within the thunderstorm. At this stage there are only weak down currents and light precipitation. There are only many Cirrus, Altostratus and Cirrostratus left that can even contribute, with their shadow, to decrease the surface warming.

Thunderstorms are most common in the afternoon, when the daytime warming of the earth by the sun leaves the underside of the troposphere unstable. Sometimes some thunderstorms can form when the upper atmosphere becomes cold due to the approach of a higher air disturbance. In this case, thunderstorms can form at any time of the day, even when there has been no daytime heating of the earth.

An absolute requirement, however, is that there must be enough water vapor to power the storm. This is the fuel for her. While the storm uses this fuel, it is converted to precipitation.

Storm Types

Storm clouds can be presented in two ways: isolated (also known as isolated storms or local storms), or in groups, forming organized storms. The latter are usually more severe and have more intense rains and winds, as well as hail.

Thunderstorms occur more on the continents than on the oceans, and are usually more frequent in the afternoon (maximum occur between 16 and 18 local hours), although they occur at all hours of the day. Over the mountains, the maximum occurrence tends to happen earlier, around one o'clock in the afternoon.

The frequency of storms at a given location depends on a number of factors, including topography, latitude, proximity to water bodies, continentality, and the presence of different weather systems. A small percentage of thunderstorms that occur every year are considered severe storms.

In general, severe thunderstorms are associated with organized storms and have one or more of the following characteristics: hail (a few inches round ice), tornado and high winds. Although lightning is not considered to be one of the defining characteristics of a severe thunderstorm, most severe storms are associated with a large number of lightning strikes. Storms accompanied by hail in the soil are often referred to as hail storms.

Storms accompanied by tornadoes are often referred to as tornadic storms. Tornadoes form in regions of the storm several kilometers long, where there are strong rotational movements, called mesocyclonic regions. Severe thunderstorms also often produce high intensity descending air currents (in some cases speeds greater than 100 km / h) known as bursts and microbursts.

Gusts are generally up to ten kilometers long and last from a few minutes to a few dozen minutes. Micro bursts are short bursts (between 5 and 15 minutes) and affect regions of a few kilometers in length (typically 1 to 3 km). There are currently no statistics on the frequency of occurrence of micro bursts in the world.

An important aspect associated with multicellular, supercellular storms and organized storms is the existence, in the region of the atmosphere where they form, of a vertical wind gradient, the wind shear. The presence of this gradient causes downward drafts to tend to occur in a region distinct from upward drafts, thereby allowing the storm to persist for a longer period of time than a unicellular storm.

Most severe storms form in an atmosphere with a strong vertical wind gradient. Tornado-associated multicellular and supercellular storms often form in an atmosphere where the vertical wind gradient has a strong vorticity component (spinning motion).

The height reached by the top of storm clouds in their various stages depends mainly on geographical latitude. In medium to high latitude regions, the top of storm clouds rarely exceeds 8 km in height, while in medium to low latitude regions (latitudes below 45 degrees), the top usually exceeds 10 km and can reach altitudes. over 20 km.

The highest incidence of storm clouds with peaks around 20 km seems to occur in northern Australia, Indonesia and New Guinea. The height of the base of storm clouds tends to follow the height of the top, ranging from 1 to 4 km.

Isolated multicellular storms often present cells at different stages. Convective complexes, in their lifetime, seem to go through stages similar to those of an isolated storm cloud. During their development phase, they can trigger tornadoes. In their mature stage, they usually produce intense precipitation.

Organized storms, also called mesoscale convective systems, are a very common phenomenon. In general, they tend to be larger than isolated storms and last longer. Some particular types of these systems are storm lines, instability lines, and mesoscale convective complexes. The rest of the systems are referred to as generic storm clusters.

Organized storms usually have two distinct regions: a convective region and a stratiform region. The convective region is characterized by strong convection and high cloud top height, while the stratiform region lies at the back of the cloud in relation to its movement, and is characterized by a large horizontal cloud layer (hundreds kilometers) and lower top height (similar to an extensive anvil).

Storm lines are formed by individual storms that move close together without interacting, arranged in a line.

Unlike a storm line, storm clouds on an unstable line interact with each other, being connected by the stratiform region. Unstable lines can extend for hundreds of kilometers. Typically these lines produce very strong and sometimes weak tornado winds, and are usually formed near the interface between a hot and humid air mass and a cold air mass.

Unlike isolated storms, they rarely remain stationary. Due to the displacement of the system, as the clouds dissipate, new clouds are formed so that the storm can last several hours.

Mesoscale convective complexes are the largest members of the mesoscale convective systems. They are almost circular systems with typical diameters of 300 to 400 km, containing hundreds of interconnected storms inside. They last from 10 to 12 hours and occur mainly at night, although on occasion they may regenerate, lasting several days.

Because they generally move slowly (typical speeds of 20 to 40 km / h) they can affect a region over a long period of time. Evidence indicates that a single convective complex may account for up to 50% of the annual lightning density of a given region.

Throughout your life, one type of storm can evolve into another type. For example, storm lines may evolve to unstable lines. These in turn can be divided into supercellular storms.

Finally, storms can be grouped into synoptic scale dimension systems. These are tropical storms (or cyclones) and extratropical storms. These systems range in size from hundreds to thousands of kilometers, typically have winds in excess of 300 km / h, can last several days, and have a structure that is characterized by storm bands, with widths of a few tens of kilometers, that move around of a central region of almost circular shape, called "system eye".

Due to their size, they are affected by the rotation of the earth, such that they tend to rotate clockwise in the southern hemisphere, and counterclockwise in the northern hemisphere. Due to their high degree of organization, such storms are associated with much higher rainfall levels than any other storms. Tropical storms with central winds greater than 100 km / h are also known as hurricanes.

Hurricanes can reach up to 2000 km in diameter and usually form in the oceans and migrate to continents. Its eye has an almost circular shape, with a diameter of 10 to 30 km. The smaller the eye of the hurricane, the greater its intensity. When they reach continents, they usually provoke tornadoes.

About 50 hurricanes occur per year. About 70% of them form in the oceans, between 10 and 20 degrees north and south of Ecuador, in regions where the surface water temperature exceeds 27 ° C. Unlike tropical storms, extratropical storms are formed from atmospheric temperature gradients in mid-latitude regions and have an average diameter of around 3000 km.

Isolated Cell Multicell Supercell