It is moisture in the car condensing on the cold glass. The hotter the car, the colder the outside - the more likely to get condensation. It is down to humidity in the air in your car, ie. you probably have a little bit of a leak.
However, the sweat off a person is just as likely to cause it. It happens on modern cars, just not as noticeable especially if they have the AC on.
You can get big packs of silica on eBay - when I worked at PCW, I would take it out all the new PC boxes and collect it...I had about 4kgs of the stuff - then my dad binned it
