Not that severe weather has a brain or a conscience, but it does appear that mother nature takes it a bit easy on the weekends as far as tornadoes go. Various weather studies have have been done recently to try to identify weekly cycles in a variety of weather phenomena (rainfall, lightning, storm heights, etc.). So what did they find? A trend which shows that the number of tornadoes was about 20 percent above average mid-week, and nearly 20 percent below average on the weekend. The hail storm pattern was nearly identical. Why was this happening so frequently? Some people call it "the weekend effect", and it’s thought to be be linked to industrial air pollution associated with the five-day work week, though there has been a lot of discussion about the specifics of that exact connection.
What is causing it? Unlike the fabled relationship between population and global warming, there is a likely candidate for the connection: heat transport. Aerosol particles (dust, smoke, bacteria, pollen, water droplets, etc) are perfect for creating condensation nuclei. For example, when water vapor condenses into a liquid it releases a lot of energy to the surrounding atmosphere/environment. If this energy release occurs higher in the cloud, the aerosols strengthen the upward transport of heat that drives storm clouds—meaning they push the storm clouds closer to their maximum potential for severity. This is all still just speculation based off of studies done, but the results give researchers a new way to look at things.
Below is a map of country for the month of April 2011, one of the most active months for tornadoes in U.S. history. On the chart below the map, notice that for the days that had 50 or more tornado reports (April 4, 15, 16, 19, 25, 26, & 27th) all but one of them were on weekdays. If we go back and look at other past tornado outbreaks of at least 100 tornadoes, many of those also occurred on weekdays. May 25, 2011 had 127 tornadoes (5 total injuries) and happened on a Wednesday. June 17, 2010 had 115 tornadoes (2 fatalities and 10 injuries) and happened on a Thursday. February 5, 2008 had over 130 confirmed tornadoes (28 fatalities and at least 35 injuries) and occurred on a Tuesday. However, the outbreak on May 5, 2007 with 111 confirmed tornadoes (15 injuries) happened on a Saturday. Also, the now infamous Joplin, Missouri tornado which had over 70 confirmed tornadoes (at least 130 killed) occurred on a Sunday. So, while the study does show a decrease on weekends, it obviously shows that threat does not completely go away for weekends.
The one thing to note is that these are not global studies, but many are rather focused on the southeastern states, though the study points out that other similar trends have been identified in other parts of the country as well. For example, they repeated the analysis separately for each month and region of the eastern U.S. to show that the similarity is actually strongest over the summer months in the southeast, and that no other type of significant correlation shows up anywhere. In addition to that, they also confirmed that there is very little difference in the correlation from year to year. Also, that no significant relationship exists for the western United States.
No comments:
Post a Comment