Convection is the weather engine responsible for storm clouds, one of meteorology’s most spectacular and potentially dangerous phenomena, both for life at sea and on shore. In this module the basic theory of convection will be explained to you, as well as how cumulonimbus storm clouds form and their structure.

On this page you will find:

A video by Tomàs Molina where he explains convection and the basic dynamics of a storm cloud.
A synopsis of the video with a diagram featuring the structure of a developed cumulonimbus cloud.

1. Convection

2. Synopsis of the video

Convection is the effect of the energy transfer caused by the sun's heat.

It is governed by the Stephan Boltzmann Law: the amount of radiation emitted by a surface is proportional to its temperature elevated to the power of four.

$E = \sigma T^{4}$

Where E is the energy emitted per surface unit (W / m²)

$\sigma$ is the Stefan-Boltzmann constant (W / m². K⁴)

T is the surface temperature

This is the energy that will heat up the air on the surface.

This air heats up and its density decreases and so it rises.

This rising air can be measured using a number of different indexes: The K Index (Li), the Total Totals (TT) Index, etc. Each of these provides a certain characteristic of the atmosphere, according to height, and allows for the calculation of the potential for storms and their intensity, which is the most interesting of the convective phenomena.

The structure of a storm

This diagram shows the basic structure of a developed convective storm cloud (cumulonimbus), viewed laterally.

Upward and downward flows of current

There are two basic air currents: upwards and downwards. Both experience the effects of backing and veering due to the Coriolis effect that causes them to rotate.

The air which rises up from the surface is moist and warm.

Because the wind at surface level and the wind high up do not travel in the same direction, when the air comes into the surface it is moist and warm. When it rises, it begins to cool and the water vapour condenses into droplets and then higher up those droplets freeze. The droplets and stones of ice sink due to the force of gravity, causing a downwards movement.

Rains and electric discharges

The downward flow of current crosses over the upward current, producing an abundance of static electricity, which accumulates in the cloud.

This electricity is discharged in the form of lightning. The descent of drops of water forms rain, and its intensity depends on the development phase the cloud is in. Rain positions itself where the downward current flow exits the cloud here.

The formation of hailstones

Depending on how these two belts rotate, the upward one and the downward one may intertwine, producing a cycle linking the descent with the ascent. If this should take place, it would create a rotor and hailstones would form as part of the following process:

A solid particle is dragged from the surface by upward flows of current. Moisture sticks to the particle and it freezes as it rises. The particle, with frozen water, falls downwards under its own weight.
As it reaches the lower part of the cloud, it is again dragged upwards by upward moist currents, adding even more water to it.
The added water freezes in the upper layers of the cloud and the particle descends with a larger volume of ice.
The cycle may repeat itself a number of times according to the power of the cloud, until the particle weighs enough not to be dragged up by the upward currents and falls to the ground in the form of hailstones.

Wind at the base of a cloud: downburst

The two current flows of air create two zones, known as the in flow and out flow, an area of air which enters and another of air which exits.

The out flow zone, with the exit of downwards air flow generates strong winds at surface level. This zone tends to feature a cloud in the shape of a shelf (shelf cloud) and mostly that cloud signals where the air is coming down with the most force. It is the most dangerous zone of a cumulonimbus for sailing.

This phenomenon is known as a downburst. It is a very sharp downwards flow of air which produces a shock wave and disperses like a fan.