How can a world have permanent cloud cover?

Question

The inhabitants of my Earth-like planet live high up in the atmosphere at ~1500 meters. When they look down, I don't want them to be able to see the ground. I figure the best way to accomplish this is to have cloud cover below them. However, the cloud cover needs to always be there. No break, no patches of clear sky. The clouds are always there, and they always cover the entire surface of the planet (they can disperse over an ocean if necessary - no one lives above the oceans).

How can I achieve complete and permanent cloud cover over the ground?

Notes:

• The surface is uninhabited, so you can have whatever you want there, as long as it is natural (aka, no pollution).
• This is a fantasy/medieval setting, so modern gases/pollutants from above are not an option.
• Because my people live above the clouds, the lower the clouds the better.

Answer 1

What you want to achieve fits pretty well with the situation on Venus:

Venus is shrouded by an opaque layer of highly reflective clouds of sulfuric acid, preventing its surface from being seen from space in visible light. [...] Venus's surface is a dry desertscape interspersed with slab-like rocks and is periodically resurfaced by volcanism.

Moreover on Venus (terrestrial standard) life is thought to be possible in the layer above the clouds, where temperatures are lower.

You can obtain the effect you want via what is believed has happened on Venus:

billions of years ago Venus's atmosphere was much more like Earth's than it is now, and that there may have been substantial quantities of liquid water on the surface, but after a period of 600 million to several billion years, a runaway greenhouse effect was caused by the evaporation of that original water, which generated a critical level of greenhouse gases in its atmosphere.