Have you ever been outside on a summer night and wondered why it is so humid? Why glasses will “fog” up? Why you feel like you just got out of a shower at times? It is really a very simple reason that you do not need a Ph.D. to understand.
During the day, while temperatures are hot and the sun is shining through, much of the water in the air is undergoing evaporation. That is, it is being heated by the Sun’s light to the point that it turns into a gaseous state. This is not to be confused with boiling, although it is basically the same concept but with much lower quantities of water and far less heat.
Now at night time, when the sun is well below the horizon, the sky is dark and the heat of the sun is no longer directed at us, all of that water vapor in the air begins to condense due to the lower temperatures we see at night. Once it condenses it will lay closer to the ground, which gives us the wonderful sights of dew and morning fog. Since the temperatures are cooler and evaporation is no longer happening, it gives the water a chance to regroup. All of the molecules that have been getting spread out during the day are now being attracted back to one another, sinking lower in the air to ground level and forming drops. It is exactly the same principal as what gives us rain.
Yet the temperatures must remain in a fairly confined zone for the humidity to stay as vapor and small drops on the leaves. If it gets too cold it will simply become snow and as such will take more humidity out of the air and actually form ice crystals which lay on the ground. Which is why in the winter the night air is so much dryer than in the summer.
So this is basically it. The cooler night time temperatures trap the air and moisture closer to the ground, creating optimal conditions for fog, dew, and that annoying “steam” on your glasses. . Thus giving us those balmy summer nights and incredibly dry winter nights. This is why the winter skies are usually clearer to look up at the stars in. Instead of being trapped in the air as fog (summer) in the winter all of that moisture is trapped in ice crystals which are heavier than water so they sink to the ground itself.