hailstones are really strange and form the same way (in a manner of speaking) as pearls .little ones are just frozen rain but they can be dragged up and down in the clouds and get more and more layers of ice and can get very large.
ice starts to form at 4c (the water "thickens" and the suspended particles of dirt start to get a minute layer of ice) and form a slush if the temperature drops really slowly untill it freezes solid ,a rapid fall of temperature freezes downwards and gives a clear ice ,the warmer temperature below it stops a deep lake freezing completely ,if nature didnt work this way lakes would freeze to the bottom and never thaw out in the summer ,high pressure stops water freezing as well so the ice due to its weight slows freezing beneath it and forms a insulator from the colder temp above it ,an ploy to stop semi tender plants getting burnt by the frost is to wet the leaves as the temp falls thus stopping the drying effect of the frost burning them ,it works but is time consuming.
black ice is really clear ice (its black because its on tarmac) and is freezing rain which comes about because the air temperature is warmer than the ground temperature and rain falling on it immediatly freezes (not nice for fat people on honda 90's but still passable) i,v seen this coat everything cars included !.
hoar frost i think is frozen mist ,its certainly strange how frost grows even on a dry windscreen ,as ice is water then it must be very minute particles of water that are freezing indeed so it may even be the water vapour thats the humidity bit in everyday life freezing out ,air is far drier in the winter in frosty conditions so i guess its taking the moisture out of the air.
i like cold its far more interesting than hot