How do I prevent fog on the outside of the windscreen?

Question

Every morning when I drive to work, the windscreen is fogging up on the outside. I turn on the heater so it blows warm air on the windscreen until eventually the fog disappears.
When I'm driving (35 mph / 50 km/h) the entire windscreen fogs up in about 2 seconds, so I must use the windscreen wipers so I can see. When standing still (at a traffic light), the windscreen fogs up much slower (about 30 seconds)
There is only a tiny leak in the trunk of the car. The carpet is a tiny bit moist. Can this cause this behaviour? Also I have a moisture absorber in the car.
Why is it fogging up from the outside and how do I prevent this?
Not sure if it's relevant, but the car is a 2004 Toyota Yaris hatchback.

Pᴀᴜʟsᴛᴇʀ2 · Accepted Answer

The easiest way I'm aware of to help your situation (I say "help" because this is nature going on here ... it's not going to fix it entirely), is to use a product like Rain-X on your windshield (NOTE: I have no affiliation with this product). The product is a hydrophobe which basically means it repels water. Due to this, it tends to bead the water rather than let it sit there on your windshield.
In your case, when you first get into your vehicle, you'd most likely see the same thing as you normally would: a fine thin mist on your windshield. After you use the wipers the first time, any other water which settles on there is going to bead and run away without much agitation (if any) from the wipers. This especially applies to when you're moving down the road. Since the water won't want to stick to the glass surface, it will roll right up and off the windshield very easily.
Other than that, using your wipers is about your only choice.

How do I prevent fog on the outside of the windscreen?

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