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

A reasonably similar matchup -until:

  Chicago, IL  Houston, TX 
Concealed carry
gun law 		 no   yes
 # of gun stores  0  84 dedicated gun shops,
1500 places to buy guns
(Walmart, etc.)
Homicides, 2012  506  207
Homicides per 100k  18.4 9.6 
Average January
high temp, F		  31  63

Conclusion: cold causes murder.