google:
Meringues are basically egg whites and sugar. Anything made with a high percentage of sugar, always attracts moisture because sugar is hygroscopic, that is, it has a great affinity for water (the sugar attracts water from the surroundings and causes small brown beads to form). Thus, it's not the heat that causes the beading on your meringue, but humidity in the warm air, or a steaming kettle, a moist kitchen, an often, a rainy day. This is particularly noticeable on a very humid day. This is a very common problem with meringue, and is perhaps a good reason for eating meringue-topped pies soon after they're prepared!. When the meringue is out of the oven and has cooled, store it in an air-tight container.
Another important factor in making meringues is to add the sugar very slowly to the eggs whites, beating well since undissolved sugar attracts beads of moisture. Consider using superfine sugar which is finer in texture and dissolves more easily than the regular granulated form.
Also, be sure to place the meringue on the filling while it is hot. The residual heat carried by the filling cooks the base of the meringue ever so slightly, making it less prone to leaking.