What are "snail-trails"

Snail trails can simply be described as shiny vertical run-marks seen on dry paint films. They are often seen soon after water-based paints have been applied, particularly if it has rained or after a heavy dew-fall.
Dry paint films are made up of pigment and binder, but to present the paint in a usable form, many additives are used in the formulation. These additives are mostly water-soluble and remain in the dry paint film until they are effectively washed out by water - be it via rain, dew or by irrigation sprinklers. If the amount of water falling on the surface is low, then only part of the water-solubles are washed out, and they can dry on the surface. This leaves what we call "snail-trails". They tend to be sticky, and can act as a dirt trap as seen in the photo below. Generally these additives are washed out of the paint after the dry film has been wet a couple of times.
Matt paints tend to show snail-trails more readily than sheen paints due to the fact that there is less binder in the dry film. Snail trails can also be seen in kitchens and bath-rooms due to the high humidity. Wiping with a damp cloth often can remove these snail trails.