When I was young, I would insert a piece of paper into a kite's thread. The paper would travel upward till it reaches the head of that kite. What makes it to go up against the gravity even if the kite's head is about hundreds of meters away from the ground?
Because the kite thread is never vertial, it is inclined in the direction of the wind:
So the wind will push your paper in the same direction, and the thread will force it to go upwards.
For more detail, in the paper there will be 3 forces:
- Gravity. It pulls down.
- Wind. It pushes to the right.
- Reaction of the thread. It will force the paper to move in line with the thread.
If the force of the wind times the cosine of the angle is greater than the weight of the paper, then it will go up. Else, it will go down.
Kite fliers call it a "line climber" and here's a video of one called a "Popper." It opens like an umbrella and it will climb the line much faster in heavier wind!