Geography Picture of the Day - Why has the USA experienced so many tornadoes this year?



An image shows the damage created by a tornado last week in the USA.
The community featured, to the right, is the community of Sturbridge and is
one of many that settlements that was damaged by the tornado.



The brown scar that runs down the middle of this image depicts the path taken, and the devastation left in its path, by a tornado that ripped through south-west and south-central Massachusetts last week. The tornado travelled over 63 kilometres and was 800 metres across at its widest point. Rated 3 out of a potential 5 on the enhanced Fujita, or EF, scale, it left four people dead, overturned cars and reduced buildings to rubble.  The EF scale, similar to the Fujita scale, has the same basic structure of 6 catergories (zero to five) and is used to represent the damage inflicited by tornadoes. The main difference between the Fujita scale and the revised EF is that the EF was designed to reflect better examinations of tornado damage surveys so that winds speeds could be better aligned with the damage caused.

This tornado is one of more than 875 tornadoes to have hit the US since the beginning of the year which, in total, have claimed the lives of more than 363 people, thereby,  making 2011 one of the deadliest years on record for US tornadoes (link to an interactive map that compares the number of deaths, caused by tornadoes, over the last decade)

So, just why have the USA experienced so many tornadoes? Well, for a tornado to form, cold air needs to sit upon warm, moist air whilst the winds need to go in different directions, at different heights.

(Slightly more detailed explanation of tornadoes formation)

 Over the last couple months, the jet stream, which has been stronger than usual, has dipped south over south-west America, therefore bringing them a string of storms. This change in path has been sustained which is why the south-west of the USA has frequently witnessed devastating tornadoes.

 Does it link to global climate change? It is impossible to unambiguously link this to climate change, due to a lack of older records, but many say that the frequency of such tornadoes has increased over the past decade. So, could this become a common occurence.............. It is hard to say as the effect of global climate change on tornadoes is unknown as, although rising global average temperatures will mean that their is more warm air around, it could also reduce the temperatre gradient between the poles and equator - the crucial factor that generates the jet streams in the first place. One thing is for sure though, after the destruction caused, so far, this year, the people of America will be hoping that this doesn't become an annual event.