At least nine natural catastrophes resulted in insured losses of more than USD1 billion during the first half of 2011. Five were related to tornadoes and severe weather in the United States, causing a combined insured loss of more than USD14 billion. The La Niña climate phenomenon helped create the necessary conditions for tornado formation (warm/humid air and strong south winds near the surface, with colder air and strong westerly winds in the upper atmosphere). A very strong jet stream also contributed to the favorable conditions. If considered a single event, the tornado losses in the second quarter would have ranked as the fifth most expensive disaster in U.S. history, according to the Insurance Information Institute.