There are already threads that discuss this issue, and they have links to reports about the study which might be useful to you (use the search function at the top of the WP page, type in rain, choose WP site search when the google search box appears).
In terms of the questions you've asked, the following might help (and is an exert from one of the threads you can find by searching WP as described above)
For the study, Waldman's team looked at the prevalence of autism among children in California, Oregon and Washington. They also used data from the National Climatic Data Center to calculate the average annual rainfall by county in these states.
The researchers found among school-aged children in these states that the prevalence of autism rose as the amount of precipitation increased. In fact, the prevalence in autism increased up to 30 percent in the rainiest counties.