Everything is explainable. The seemingly unexplainable is but a result of our insufficient knowledge.- Hans Brewer
Air is a fluid. Just like the ocean, there are more molecules at the bottom since it's under pressure from what is on top. Warm air can hold more actual water molecules as a gas and that is what RH is. If you take warm air with a RH of anything given, say 80%, and cool it, the RH will go up since cool air holds less water vapor than warm air. The same amout of vapor is there, but now it is more concentrated and is at a higher RH%. Dry air (low RH) can cool things off faster through evaporation than cool air by the way. That's how evaporate cooling (swamp coolers, sweating) works. As air is heated, it rises and cools, the RH rises and condenses into clouds once it is high and cool enough, it rains, you know the story.
When you try to visualize this, think of the heat waves generated from a grill or hot asphalt. That is a perfect example of how air is a fluid.
Wow that was pretty hard to articulate. I hope I helped out a bit. By the way, since the air is thinner at high altitudes that's why it's cooler. Fewer molecules to hold the heat.