Let us first define the ways in which rain differs from dew.
Rain implies temporality.
For example, there is a rainy season.
Furthermore, I only want rain at a specific time.
I certainly do not want rain when I am harvesting or when I am sitting in my sukkah.
The Sages say, "Sometimes you want it, sometimes you do not.” (Babylonian Talmud, Rosh Hashanah 27b, and elsewhere.)
Dew is different.
The Prophet Hosea was able to say, "I will be to Israel like dew,”(Hosea 14:6) because the dew is always desirable.
That is the first feature.
Second, rain is part of a cycle.
Waters rise from the earth, evaporate, and then fall again on the earth.
In contrast, dew is only dependent on the atmosphere.
It is not connected in any way to a give-and-take cycle as rain is.
In addition, rain is perceived as a response to men's needs and activities.
Man does what he has to do, and then the rain makes his work flourish, whereas dew is a spontaneous gift of God.
In fact, during Sukkot we give thanks for dew.
We only start to pray for rain after the festival ends.
Summer is the time of interaction between God and man.
When it is over, and when Sukkot comes, we start to ask for rain.
Sukkot belongs to the world of dew.
–Rabbi Adin Steinsaltz
From The Seven Lights by Rabbi Adin Steinsaltz