Hi Sutherland,
I'm not a guitar technician, but those lines appear to be surface cracks, though I could be wrong. If you are concerned, and it seems that you are, perhaps taking it to a trusted technician for evaluation would be a good idea.
You could purchase a mirror
http://www.stewmac.com/Shopping?actn...tool&x=44&y=11 that can be placed through the sound hole. With the use of a small flashlight, you could like at the underside of the top to see if the cracks go all the way through the wood. That would tell you how series the cracks are.
Do you live in a very dry climate? Summertime is usually not a time of low humidity unless you live in a semi-arid region. Is the relative humidity well below 40% where you live?
Best of luck on this. I hope it's not a serious problem for you.
- Glenn