8.30 Thunderstorms

8.30.2 State the three conditions required for the development of thunderstorms.

A thunderstorm is a cumulonimbus cloud that produces thunder and lightning.

Thunderstorms require three main conditions to develop:

• Unstable air through a deep layer
• An adequate supply of moisture
• A lifting or trigger action

The trigger action starts the initial upward movement of air. This may be caused by:

• strong surface heating;
• rising terrain;
• convergence of air;
• a cold front;
• another lifting mechanism.

As the moist air rises, it cools and condenses. The release of latent heat helps the air continue rising, allowing the cloud to grow vertically into a thunderstorm.

8.30.4 Describe the three stages in the life-cycle of a thunderstorm.

Thunderstorms normally develop through three stages:

• Growing stage
• Mature stage
• Decaying stage

Growing Stage

The growing stage is dominated by strong updraughts.

During this stage:

• warm, moist air rises rapidly;
• the cloud grows vertically from cumulus into towering cumulus or cumulonimbus;
• condensation releases latent heat, increasing instability;
• precipitation forms within the cloud;
• the precipitation is usually held within the cloud by the strong updraughts;
• little or no rain reaches the ground.