Thunderstorms

Thunderstorms are a weather phenomenon that develops from well developed cumulonimbus clouds. It is estimated that every day there are about 44,000 thunderstorms across the planet. Most student pilots will only observe thunderstorms from a safe distance in the air and will never experience flying under one for the purpose of landing. Over 60% of aviation accidents related to meteorological conditions were caused by the existence of thunderstorms. Most flight academies have very strict operation procedures that protect the pilot and his aircraft from the hazards of thunderstorms since training is usually conducted in light aircrafts.

For a thunderstorm to form, the following conditions need to exist:

1. An environmental lapse rate greater than the standard atmospheric lapse rate (2 degrees Celsius increase every 1,000 ft) through a depth of at least 10,000 ft
2. Sufficient water vapor to provide early saturation and to form and maintain the cloud
3. A trigger action to start the lifting process. This can take several forms

A trigger action is always another weather phenomenon that provides a sort of kick start to the thunderstorm formation process.

A thunderstorm cloud usually consists of several self contained cumulonimbus cells, each in a different state of development. New and growing cells can be recognized by their cumuliform shape with a clear-cut outline and cauliflower top. The tops of more mature cells appear less clear-cut and are frequently surrounded by fibrous cloud.

Development of cells is not always seen since other clouds may obscure the view.  In most common conditions, extensive layer cloud structures may obscure a view of the development of cumulonimbus thunderstorm cells.

Each thunderstorm goes through 3 stages of development that differ in the conditions they offer and the dangers a pilot could face while flying at or near them.

The growth stage is the first stage at which several small cumulus clouds combine together to form a large cumulus clouds of about 5 miles across. Strong updraughts are present, typical on the order of 1000 to 4000 foot per minute which can easily cause structure damage to an aircraft. Air is then drawn in from the sides and underneath the cloud, replacing the lifting air within the cloud. This stage lasts approximately 15 to 20 minutes.

The mature stage is characterized by the onset of precipitation. This precipitation is produced by the combination of ice crystals and water droplets. The precipitation causes downdraughts of 2000 to 3000 foot per minute. This is the stage at which the most precipitation happens and the stage where the most hazardous conditions for pilots and aircrafts happen. The updraughts are still present, increasing to as much as 10,000 feet per minute and the cloud top can reach the tropopause which makes aircrafts unable to clear the tops of some thunderstorms. The recommended practice in avoiding a mature thunderstorm is to have a horizontal separation distance of at least 20 miles to avoid encountering hail from the thunderstorm. The mature stage lasts approximately 20-30 minutes.

The dissipating stage begins when the local supply of moisture is no longer sufficient to maintain the mature stage. This stage is characterized by the appearance of an anvil which happens when the cloud top reaches the tropopause and is spread out by strong upper winds to form a flat-topped anvil shape which later transforms to a cirrus cloud.  The updraughts cease and the cloud starts to dissipate as the downdraughts remove the moisture from the cloud. The precipitation diminishes and the downdraughts are too strong to support roll clouds. Lightning might still occur.

The dissipating Stage lasts about 30 minutes but the clouds lasts for 2 to 3 hours.

Thunderstorms are important in balancing out and recycling the moisture in the air to keep our atmosphere stable enough to be used in flying. The hazards of thunderstorms are very well known and can easily be avoided if we abide the rules or operational procedures written by the civil aviation authorities and flight academies. These rules instates the thunderstorms separation minimums that provide a safe flying environment to inexperienced student pilots