14. Severe Weather

Photo 25: Large rain cloud above our farm. 2005-09-02

Thunderstorms

Much about Thunderstorms is already in the Unit 12 post, but I will build a bit further on this phenomenon in this post. So, one aid for a thunderstorm is a warm summer season, maybe with a few heat waves (3 consecutive days of 32°C or higher). Another aid is abundant low-level moisture and a strong wind shear.

Thunderstorms come in 3 categories: single-cell, multi-cell, and super-cell thunderstorms. The air mass single-cell thunderstorm is discussed in the post on Unit 12. These storms dissipate quickly. Multi-cell thunderstorms often form in line (squalle-line storms). Super-cells are massive and might form in a large cluster of thunderstorms, but are usually single-celled.

A key factor in thunderstorm formation is lapse-rate. As I said in the post on Unit 5, the average lapse-rate is 6.5°C per 1000 meter; however, this may vary. The greater the lapse-rate is, the higher the thunderstorm will become. The purpose of a thunderstorm is heat transfer, from the surface upward. Thunderstorms often cause severe weather and great dramatic weather effects. Photo 25 shows dramatic weather effects alike; however, the unusual redness in the cloud is actually the setting sun. This picture looks worse than it is, unlike tornadoes.

Tornadoes

Like hurricanes and thunderstorms, tornadoes basically draw hot air from the Earth’s surface. In the case a tornado forms above the ocean, it is called a water spout. The tornado is closely related to a thunderstorm, because it often forms right underneath a thunderstorm. A tornado is a small vortex of air, from 100 m to 500 m in diameter, and can leave a trail of destruction up to 160 km long and 900 m wide. Tornadoes usually spin, but unlike hurricanes who are much larger and strongly affected by the Coriolis force, tornadoes spin due to internal forces, though often in the direction of the Coriolis force. Figure 26 shows the workings of a tornado as part of a thunderstorm. Very similar processes are going on compared to a thunderstorm: cold downdraft in the middle, and very strong hot updrafts reaching far into the Troposphere.

Figure 26: Tornado. Image from Encyclopædia Britannica, Inc.