|Not our yard, but a random picture of frost on the grass/trees|
Each night the grass or trees radiate infrared energy outward. This is similar to your body radiating infrared energy (what we call body heat). If the grass loses enough energy, water vapor from the air can condense into ice on the grass blades. This is frost. Two things need to occur for frost to appear.
1. The temperature must be below freezing (32 degrees F). If the temperature is above freezing, dew (water) droplets can form. You'll see this many times during the spring/summer/fall.
2. There must be enough water vapor (moisture) in the air. If the air is extremely dry, there will be little moisture to condense as ice on the grass. In this case, frost will not form.
It's funny how science introduces itself to you. I didn't wake up this morning thinking I would be explaining frost to my daughter, but I did. Science is amazing!