What is dew?
Dew is just atmospheric moisture that’s been warmed by solar radiation throughout the day. As night comes, surfaces cool and the moisture begin to accumulate on those surfaces near the ground including grasses, metal, glass, tree leaves, plastic sheeting, rain ponchos and so on.
Usually, dew won’t settle on the ground itself or on boulders and rocks since they’re still conducting heat, which does not allow the moisture to condensate. The change in temperature is what makes the moisture to accumulate. This is just like filling a glass with cold water. Drops of moisture will quickly form on the outside of the glass.
If there is a heavy cloud cover during the night, dew will not collect on surfaces. Clouds reduce radiant heat loss, which will limit moisture condensation on surfaces because they will not cool quickly enough to create condensation.
To gather dew you can put a sheet of plastic or a poncho over some vegetation. Leave small depressions for the moisture to collect. You may also absorb the moisture from grasses, bushes or any other surfaces employing a clean cotton cloth, and when the cloth has absorbed as much as it can squeeze the moisture into a container or straight into the mouth.
The water obtained is generally safe to drink if the surfaces are not contaminated. Be careful when collecting dew from vegetation. You don’t wish to get dew from poison ivy or poison oak for self-evident reasons.