Thunderstorm

Thunderstorm

Thunderstorms can be relatively small- scale events that vary from short- lived single cells to destructive super cells. A thunderstorm or electrical storm is usually made up of one or more convective clouds with electrical discharge (lightening) that is heard as thunder. Cumulus and cumulonimbus are convective clouds form when strong warmer air currents more upward (as updrafts), carrying moisture high into colder layers of the atmosphere. Their bases can range in height from 1000 to 10000 feet (300- 3000 m) and the cloud “tower” can rise as high as 59000 feet (18000m). These thunderheads are often indicators of unstable air.

Nearly 2000 thunderstorms producing some 100 lightening flashes per second are occurring worldwide at any one time, most frequently in rain forested areas. In temperature regions either side of the equator they are more common in spring and summer.

 

There are three main types of Thunderstorms:

1. Single-cell thunderstorm,

2. Multi-cell thunderstorm, and

3. Super-cell thunderstorm.

 

 

Single-Cell Thunderstorm

A single cell thunderstorm forms when the wind speed and direction of the updraft (the wind shear) have relatively little effect, so the storm remains poorly organized and short – lived. The downdraft of the cool air, which brings some rain, cuts off the storm growth cycle, so the storm are typically for no more than an hour.

 

Multi-Cell Thunderstorm

Multi cell thunderstorm form when the wind shear is of sufficient strength and persistence to increase the storm`s growth cycle, giving it a more organized structure and a longer lifetime. They are made up of several cells one after another, leading 30 to 40 minutes each. Severe multi cell storms might have several cells operating at any one time. Squall lines form sometimes through convective updrafts from an orderly line up of multi cell that can have gust fronts. Associated with squall lines are heavy rain and hail and damaging winds, particularly the derechos of North America. The term “derecho”, from the Spanish word for “direct” is used to describe straight – line winds, in comparison to the turning or rotating winds of a tornado. Derechos are very widespread, occurring over areas hundreds of miles long and more than 100 miles (160 km) across.

 

Super-Cell Thunderstorm

Super cells, although rare, are the most severe of all thunderstorms and are responsible for a remarkably high percentage of severe weather events, especially tornadoes, extremely large hail, and derechos. A super cell is caused by a persistent rotating updraft or meso cyclone. It can form from a multi cell storm when there is sufficient directional wind shear to skew the updraft and so allow the cool downdraft to flow free of it. As a result, the main updraft can strengthen and start rotating. Super cell fronts can take the form of a line of cumulus clouds that usually step up toward the tallest clouds near the main storm or that produce gust fronts that form from the winds associated with downdrafts. Super cell storms can last several hours and spawn tornadoes, create wall clouds and drop very large hail (in rare cases up to 4 inches/ 10 cm across). The meso cyclone, the storm scale region of rotation, usually measures around 2 to 6 miles (3 – 10 km) in diameter.

 

Thunderstorm severity

A thunderstorm is said to be severe when it produces either hail with a diameter of 8/10 inch (2 cm) or more when it hits the ground, wind gusts at the ground of 56 miles per hour heavy rain with possible flash flooding. The size of a hailstone depends on the strength of the updraft. The smallest hailstone requires an up-draft of at least 22 mph (36 km/h), those of golf ball size (1 ¾ inch / 44 mm diameter) require updrafts of around 55 mph (88 km/h) and the larger, and softball size hail needs updrafts of over 100 mph (160 km/h)

 

In the United States, when weather warnings are issued, familiar objects are used for comparison:-

1. Pea: 6mm

2. Marble or Mothball: 13mm

3. Penny or Dime: 19mm

4. Nickel: 22mm

5. Quarter: 25mm

6. Half Dollar: 32mm

7. Walnut or Ping-pong ball: 38mm

8. Golf ball: 44mm

9. Hen`s egg: 51mm

10. Tennis ball: 64mm

11. Baseball: 70mm

12. Tea cup: 76mm

13. Grape fruit: 102mm

14. Soft ball: 114mm.

 

Precautions during Thunderstorm

1. Listen to weather updates for warnings about the severity of the storm and whether hail is forecast.

2. If possible, remain indoors and keep away from windows, they can be shattered by large hail.

3. In the case of hail, close any window covers to reduce the risk of broken glass.

4. Avoid contact with telephones and other electrical appliances (unplug them), metal pipes, and electrical wires- lightening can and does travel along these.

5. If you are in your car, shelter in it with the windows up and unplug any chargers.

6. If hail is forecast, park your vehicle in a garage or under shelter. Hail damage can be costly.

7. If no shelter is nearby, stay away from trees, poles and metal objects such as fences, sports equipment or bikes. Away from open ground, but if it is unavoidable, then try to minimize body contact with the ground by crouching with your arms together. Stay away from bodies of water, like lakes or the ocean, which can conduct the electricity.