A tale of two cities... 

                                                 Chicago, Illinois        Houston, Texas Population                                      2.7 million                 2.15 million
Median Income                                $38,600                      $37,000
% African American                           32.9%                          24%
% Hispanic                                         28.9%                          44%
% Asian                                              5.5%                             6%
% non-Hispanic White                       31.7%                          26%

So far, a reasonably similar match-up, until... 
 
                                                 Chicago, Illinois        Houston, Texas Concealed Carry Gun law                     No                            Yes
 
Number of gun stores                            0                       84 dedicated
                                                                                          gun shops,
                                                                                       1500 places to
                                                                                           buy guns,
                                                                                       (WalMart, etc)
 
Homicides, 2012                                    506                            207
Homicides per 100,000                         18.4                            9.6
Average January temp, farenheit           31                             63

Conclusion: Apparently, cold weather causes murder.


"Chicago is the blueprint for National Gun Laws."  Barak Obama
 
 
An interesting contradictory fact: Chicago is #1 in Gun Violence.