“Did it rain last night?" Have you ever asked that question after waking up to see a wet lawn in the early morning? Maybe it did rain last night, but then again… maybe it didn't!
If a warm, clear day is followed by a cool, clear evening, dew will likely form. On a normal warm day, water evaporates from the warm ground into the air. That means it turns from a liquid into a gas called "water vapor."
When evening comes, the warm ground continues to radiate heat into the air. As the ground begins to cool, the air will not be able to hold all the moisture.
If you're wondering why it's important that the evening is clear for dew formation, it's because cloudy nights hold heat closer to the ground. If the evening is cloudy, the clouds reflect heat back to the ground. When this happens, the ground doesn't cool off enough for dew to form.
If you pay attention, you may notice that on mornings when you find dew on the grass, there's probably not much, if any, dew on the soil of flower beds. Like clouds on a cloudy evening, the plants in a flower bed keep the ground warm enough to prevent dew.