It is called a hoar-frost.
When do we have a hoar-frost?
We have heavy frosts when the air is full of moisture.
Why does a heavy frost indicate rain or snow?
Because it shows that the air contains much moisture.
Are heavy frosts usually followed by rain or snow?
Yes; a heavy frost, on two successive mornings, is mostly followed by rain or snow within a few hours.
Rain, Snow, and Hail.
What is a mist?
A mist is very small drops of water falling through the air.