The majority of research doesn't specify the causes of fingernail growth differences, but rather, just comments on the interesting fact that it happens (unfortunately without explaining why). All blog posts and wiki-type pages I've come across in my search thus far concluded that exposure to sunlight, and therefore possibly more vitamin D, speeds growth. I also read that finger length affects nail growth, as does hand dominance (due to increased blood flow).
However, I found one scientific study that Godwin performed on rats. He experimented on a number of rats (since as we discussed in class this was a practical, economic approach), looking into nail growth based on nutrition and environmental factors. Of particular interest to this blog post are the environmental conditions that Godwin manipulated and measured. As you can read, Godwin notes that cold environments cease nail growth. The article goes on to provide examples of previous researchers commenting that nail growth slowed when studied in cold temperatures. As stated, this may be due to less movement and circulation (125-126).
Another source at Discovery says, "Fingernails also grow more quickly in the summer than in the winter -- scientists aren't sure why, but one theory is that nails that are used more often and exposed to the elements grow faster."
Although not too much research has been done on the environment's influences on nail growth, the speculations are interesting nonetheless.
So, fellow SC200 bloggers, help me out here. Why do our nails grow faster in the summer? The world may never know.
PS - Just for fun, check out this snapshot of the world record
holder for the longest fingernails from Vegas. Photo credit.