Quahog, razor, surf, or soft shell clams. I was under the impression during low tide, I could see the small hole they breath through and dig down a few feet...or just dig randomly during low tide.
When I was a child, my family used to spend much of the summer along Westbrook Harbor. One of the favorite pastimes for all the kids involved digging for clams on the sandbars during low tide. Truth be told, we got surprisingly good at it (not to toot my own horn or anything). Anyhow, I still try my hand at it from time to time just to see if I've still got it. It never fails that I manage to pull at least a handful of razor clams and steamers out of the mud by hand in all of 5 or 10 minutes. Quahogs tend to be a bit more rare in the intertidal zone in my experience -maybe 1 out of 20 catches- though the distribution may differ along other stretches of the Connecticut coast.
But to get to the point, your comment that a person can "see the small hole they breath through and dig down a few feet" is dead on in my experience. That's always how I've targeted them and it very rarely fails. Randomly digging a hole is not a tactic that I've ever tried, but I'd imagine it might work. However, I feel like you could catch more clams in less time if you target them based upon the holes they leave in the sandbars.
I can tell you with certainty that I have never noticed any correlation between rain and the presence of clams. That's not to say that there is definitely no connection there, though. It's just that I've probably caught a thousand assorted clams throughout my life and I've never noticed any such pattern of behavior.
Perhaps certain areas of the coastline are affected differently by certain weather patterns. I'm not necessarily ruling anything out, but I am very skeptical that rain really has that profound an affect.
Thank you very much for the info Red. I appreciate the detail and the time you took to write it. I know clam beds are typically closed during periods of rain because of all the "run off." Fertilizers and chemicals are filtered through the clam and apparently this does not make for a very seafood treat.