I've been thinking about this for a while, and to me, the biggest difference between the two is this: We get to watch rain fall, it starts up in the sky in big dark clouds and cascades down on our roofs, our jackets, our faces.
Dew is something completely removed, completely other from rain. Dew appears, as if it were spontaneously created, wherever it will rest. We don't see any intermediary steps, there's often little visible warning, but each morning, there are beads of dew everywhere, covering everything. This is the special nature of dew. It is something that comes straight from HaShem with no intermediaries.
In these months we are praying for dew. Recognise that we are praying for HaShem's direct hand in all of our ventures. We don't focus on seeing HaShem's hand in those endeavours, we just want his hand to be there, guiding everything until suddenly in the light of the morning, the blessing is apparent.
If we stop to think about it, the blessing of children is exactly like that. There are all manner of processes that go on, invisible to the human eye, and even when the swollen belly is visible, we still have no idea what is taking place within. Then at the very end, a baby, a new world, a bright new soul is unleashed on the world, as if from nowhere.
That is the dew we pray for in the summer months. The revellation of blessings so great that we aren't even made privvy to the preparations, only the outcomes.