As you cross your lawn, you might notice that your shoes are getting wet. This moisture condensed onto the grass and other objects near the ground is called dew. Dew is most likely to occur when several factors are present. These include warm, moist soils; clear still nights, and high relative humidity. If the temperatures are below freezing, you’ll get frost instead of dew. If the temperature falls below freezing AFTER the dew forms, the frozen dew is known as white dew. And while the dew forms a pretty picture, particularly when it is highlighting a spider web or a pretty plant, it can be a problem. Dew can be an important factor in the spread of some plant diseases, such as late potato blight.
Mary Knapp, Weather Data Library