A very short while ago I was climbing out of my home airport (300 feet ASL) on up to my cruising altitude of 5500 feet. It was a very hot day (100 degrees) and I have a low wing, full plexiglass canopy (aka - "oven" on a hot day) on my Zenith 601. I was climbing at max. rate to get up to cooler air quicker and had my overhead shade pulled as far forward as possible. This was in Class E airspace. My route takes me about 15 miles (but no closer) from a Class C airport that has occasional commercial traffic. Usually I monitor the tower frequency until I'm well clear just in case some big iron is in the area. This day, perhaps distracted by the uncomfortable heat, I did not switch to the tower frequency. I just continued my climb. I DO have a habit of lowering the nose and pulling back the sunshade frequently during a climb to look for traffic. A flight instructor, a long time ago, hammered that into my head. So, realizing it had been a few minutes since my last look over the nose, I dropped the nose and pulled pulled back the sunshade. Nothing straight ahead. Then I looked up. It was the closest I have ever come to a commercial aircraft. It was a large two engine turboprop on descent into the Class C airport. He was descending and I was climbing and we were converging on the same point in the sky. I dropped the nose and dove for the ground. I was so close I knew I didn't need to worry about wake turbulence because I would be past him before it descended to my level. I don't know if we would have collided but it was scary-close. No, I don't have ADSB capability now. Yes, I will be adding it as soon as possible. I had recently been practicing emergency descents and this experience avoided the need to "think" about the right response. I knew exactly how to throw the nose over and the approximate extreme pitch-down angle to get max. descent/speed without over-stressing the plane. Another take-away is the need to "look up" when scanning for other aircraft. It's easy to get into the habit of looking straight ahead, and down, for traffic. However, any traffic that is above your level line of sight may not be picked up in your scan. Our vertical peripheral vision is much less that our horizontal. You need to physically look up to see traffic above. I have been flying a lot this year and the more I fly, the more I Iearn, to never be "comfortable" in the plane. There is always something I need to be doing, or thinking about, to ensure a safe flight.