From AT&T Southeast FAQ - http://www.dslreports.com/faq/1331
Can weather affect my DSL connection?
Answer: Yes, it can. The primary cause of poor connections due to
weather are moisture related. Since ADSL technology is electrical in
nature, all of the exposed equipment can be affected by moisture which
is an excellent conductor of electrical current. This, combined with
the fact that the properties of water allow for "seepage", gives
greater exposure to equipment that would, otherwise, be impervious to
other weather conditions.
While it isn't a research article, it is an answer provide by a company who provides DSL service on their FAQ page, so I would think that there is some credibility behind the answer.
Additionally, I have programmed ADSL modem device drivers in the past and I do know that ADSL does self adjust its transmission rate in order to maintain reliable communications. So if the weather is causing communication errors, then the ADSL modem will try slower data rates until it reaches a rate where reliable communications can be maintained.