According to local weather experts, the day that this review runs will be the hottest in Chicago in seven years. Every single time the mercury crests 100, I think of the summer of 1995, when a heat wave killed over 700 people in the city of Chicago, a vast majority of them on the South Side. In a timely coincidence, a film about that heat wave and the underlying social issues that impacted the death toll is starting its second week at the Siskel Film Center. Judith Helfand’s “Cooked: Survival by Zip Code” shines a light on the issues of poverty, race, class, and education that underly how natural disasters take lives. It’s a well-meaning but scattershot piece of work that loses focus as often as it drives a point home. Still, there’s value in Helfand’s main thesis, which is that we should reconsider how we define the word “disaster” to encompass the issues that face a community before a heat wave or hurricane hits it.