Spring has a tendency to produce volatile weather across many parts of the world, and none more so than the southern United States, as every year severe thunderstorms leave a trail of damage.

This week is no exception, as a line of severe thunderstorms work their way across Texas, Louisiana and Oklahoma in the Central Plains to Alabama and Georgia in the southeast, producing flooding rains, damaging hail and tornadoes.

Thunderstorms are most likely in the spring and summer months, generally during the afternoon and evening hours with the heating of the day, but they can occur year-round and at all hours.

Worldwide, there are an estimated 16 million thunderstorms each year and at any given moment, there are roughly 2,000 thunderstorms taking place. There are about 100,000 thunderstorms each year in the US alone and about 10 percent of them reach severe levels.

A thunderstorm is classified as "severe" when it contains one or more of the following: hail 25mm or greater, winds gusting in excess of 93 kilometres per hour, or a tornado.

The greatest level of destruction from a severe thunderstorm comes in the form of a tornado.

Tornadoes can occur anywhere in the world and have been documented on every continent, with the exception of Antarctica (it is possible for them to take place there, but none have been recorded).

Tornadoes are dangerous and destructive and are rated after the event, based on the type and level of damage they produce. This measuring system is known as the Enhanced Fujita Scale, with tornadoes rated from EF0, with estimated winds of between 104km/h and 137km/h, to EF5, the most intense of all tornadoes, with winds estimated at more than 322km/h.

The highest number of tornadoes reported takes place in the United States, with more than 1,000 recorded every year. Canada comes a distant second, with just over 100.

Northern Europe, western Asia and Bangladesh also experience frequent tornadoes, with Great Britain seeing more tornadoes relative to its land area than any other country. Fortunately, most UK tornadoes are relatively weak.

So far in 2017 across the United States, preliminary reports total 425 tornadoes, with more than 20 deaths and dozens injured. The most damaging tornado reported so far is an EF4, with winds between 267km/h and 322km/h, termed "devastating" on the EF Scale.

Thankfully, improved forecasting and early-warning systems means that over the years the loss of human life resulting from tornadoes has fallen. But as these pictures show, the widespread damage and destruction remains little changed.