What do the Boston Marathon bombing, the Oklahoma City bombing, the seige in Waco, the Columbine shooting, and the Virginia Tech massacre have in common? Aside from being devastating deadly events that have captured public attention, all of these incidents occurred between April 15 and April 20.

Is it a coincidence or could there be an environmental infuence? After all, there are allergy and flu seasons. Could there be a season for violence?

Some of the perpetrators chose the date with the other events in mind. The date of the Waco, TX seige was determined by the Bureau of Alcohol, Tobacco and Firearms (ATF) when they decided to raid David Koresh's compound on April 19, 1993. Years later, Timothy McVeigh cited the Waco incident as one of his primary motivations for the Oklahoma City bombing in 1995. After Dylan Harris and Eric Klebold opened fire in Columbine High School in 1999, the media quickly proposed a connection between the three events and their dates.

After the Virgina Tech massacre in 2007 and this week's bombing at the Boston Marathon, many are convinced it is more than coincidence.

Criminal psychologists Ellen G. Cohn and James Rotton of Florida International University study the relationship between weather and crime. In a study published in 2000, they found that disorderly conduct and assault calls peaked significantly between 70 and 80 degrees.

At extreme temperatures, people generally stay at home or work, and avoid venturing out into public spaces. The surge in violence may be explained simply by an increase in more interactions of any kind between human beings. It's not just violent criminals out and about, but also sun-starved students, runners, and ATF officers.

One cannot help but wonder, however, if perpetrators spend the winter planning attention-gathering tragedies and schedule them for maximum impact--when lots of people are out in public.

Then again, it might not be the weather. Mother Jones magazine recently released a guide to 63 mass shootings in America since 1982. There was no correlation between the timing and events. There were just as many shootings in April as July, and December had the most events with 12 mass shootings.

The sad reality of American life is that violence occurs every day, 365 days a year, no matter the weather.