On rainy or humid days, meringue will “weep” in a pie by breaking down and puddling on the bottom or beading on top.
We consulted food scientist Shirley Corriher, who told us that the puddling underneath the meringue is from undercooking. The beading on top of the pie is from overcooking. We discovered that if the filling is piping hot when the meringue is applied, the underside of the meringue will not undercook; if the oven temperature is relatively low, the top of the meringue won’t overcook. Baking the pie in a relatively cool oven also produces the best-looking, most evenly baked meringue.
To further stabilize the meringue, we like to beat in a tiny amount of cornstarch; if you do this, the meringue will not weep, even on hot, humid days.