Solid pool cover
Also called: solid winter cover, solid safety cover.
A solid cover is a pool cover that blocks sunlight, so the pool opens to cleaner water in spring, while rain and snowmelt collect on top of it instead of draining through. On a basic solid cover with no drain, a cover pump is mandatory.
Why it matters
On an in-ground pool, choose solid only if a spotless spring opening is worth managing water on top all winter. Premium solid covers have stopped being solid: GLI's HyPerLite covers ship with a mesh drain panel as standard, and GLI offers a pump "in lieu of" it. On an above-ground pool solid is the default, and the water on top is by design: Blue Wave's instructions say not to pump it off in the fall, because it holds the cover in place against wind. You still want a cover pump for when the load gets heavy and before you uncover in spring.
In numbers
GLI's Invis-A-Drain is quarter-inch holes every six inches down the centre of the cover, over a twelve-inch-wide mesh drain panel that passes water while filtering small debris.