I'm going to guess that the grout in between the tiles was holding dampness before you placed the glass on the top. It's really the only way moisture is getting under the glass.

I don't know how big this table might be, but before the glass and the rubber bumpers are installed, the top has to be thoroughly dry by placing it in the direct sun to dry out and or using fans or a hair dryer to make sure the grout is not holding any moisture.

48" round table

So the sun beats down on the glass top of the table and as it heats up, all the moisture from the grout forms condensation on the underside of the glass. You have to figure out a way to thoroughly dry the table out.

I have a mini heater fan - will set that up to blow on the table top for a few hours to dry the table and then replace the glass and see what happens.

I doubt that taller bumpers will make any difference at all. As long as there is humidity and a temperature difference between the air and a surface, there will be condensation. The grout is but one source of moisture, acting as a reservoir. Water vapor in the air (humidity) is the other source. The glass cover is probably doing more harm than good by impeding the evaporation of moisture in the grout as well as being a moisture source via condensation. Freeze/ thaw cycles will hasten the failure of the grout.

And honestly the trapped moisture will do more damage than good. Humidity will put more there ever after its all dried out. And taller bumpers won't help.

AI says: In the case of your outdoor table, the cool surface is the stone, and the warm, moist air comes from the atmosphere. High humidity and a significant temperature drop (often overnight) cause the stone's surface temperature to fall below the dew point, the temperature at which condensation occurs.

So your idea of raising up the glass is a good one. @justmyopinion's suggestion sounds the most professional solution. (I was going to say cut a clear shower curtain sized to fit...lol).

At this point I did raise the glass and used a blow dryer to dry up the condensation. We will see what tomorrow brings, but it is dry for tonight when we expect to fall below freezing. Might take the glass off for the winter and try it during the summer to protect it from the hail storms. Other winters I have just covered with ball in the center so the snow would slide off.

I don't know why I didn't think about condensation - glass seemed like the right answer and when asking the local glass shop about the solution they agreed it was a good answer. Didn't buy from them because their prices were 3 times what I could get for the same thing online.

Hate when a solution becomes yet another problem. . .

First if you are able to store it inside a garage or shed for the winter so you do not get the exposure to snow would be my first choice. Even if you remove the glass and are able to lean it against the house and tarp it so it is not exposed to the freeze and thaw of snow on it. If those are not possible build a lean-to over it with plywood and a tarp for the winter so it never gets snow on it. The freeze and thaw of snow will eventually deteriorate the grout. Was the grout sealed? Resin is not UV rated and it is a not easy to pour if new with resin. Consider plexiglass or acrylic instead as it does not have the same thermal conductivity as glass.

The table and base are too heavy to move and I don't have a place to store them in the winter. My original thought was to get plexiglass but was talked out of it by the glass cutter (they sell both) because the plexiglass would discolor in the sun.

Glad I got it dried out yesterday - getting our first snow today - threw a tarp over it - not much condensation was on the glass before the snow started - just a fog in the center.