The white frost seen on grass at dawn is not necessarily dew that formed first and then froze. On a clear, calm night, the ground and grass blades lose heat by radiation to the sky, and their surfaces may fall below freezing. If the nearby air also reaches saturation below freezing, water vapour can deposit directly onto the grass as interlocking ice crystals, without passing through a visible liquid stage.
This differs from frozen dew. Frozen dew begins while the surface is still above freezing, so water vapour first condenses into liquid droplets. If the temperature later falls below zero, those droplets freeze. Their appearance can differ as well: frost formed by direct deposition often has feathery or branching crystals, whereas frozen dew looks more like small beads of ice.
It is therefore possible to see frost on a lawn even when a weather report gives a minimum air temperature of 2°C. Official air temperature is measured some distance above the ground, while thin grass blades can cool below the air above them and cold air can collect near the surface. Frost risk depends not only on one air-temperature reading but also on surface temperature, humidity, wind and cloud cover. The key distinction is simple: frost can pass directly from gas to solid, while frozen dew passes through liquid water first.
https://www.weather.gov/source/zhu/ZHU_Training_Page/fog_stuff/Dew_Frost/Dew_Frost.htm
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