What defines if moisture falls as snow or hail?

Question

They are both frozen rain right? So what makes hail come rather than snow or vice versa?

Answer 1

Snow is not frozen rain.  Hail or sleet is water that was frozen.

Snow occurs when water vapor in cloud temperatures that are colder than about 10 degrees below zero sublimates directly from gas to solid, forming a crystalline shape.  

Hail occurs when water drops in a storm freeze.  These drops can alternately rise and fall in the storm, coating more water on the initial stone and freezing, making the hailstone larger and larger.  Eventually the hailstone becomes too heavy to be supported by the updraft in the storm, and it falls to the ground.

Answer 2

precipitation

Answer 3

If they are not solid pieces of ice, they could be snow pellets. These are formed by snow falling through a layer of air that is just above freezing where the snow only partially melts and then refreezes as it falls back into cold air next to the earths surface. It differs from sleet in that with sleet, the snow is totally melted and then refreezes into a small piece of ice.

It could be graupel, they are normally formed where small supercooled rain drops form directly from water vapor and collide to form ice pellets that are solid. The only difference from graupel to snow pellets is that the center of the ball is still soft and break apart easily and graupel does not.

As for hail, it is entirely different and found in thunderstorms.  What happens is the updraft/downdraft strength in a storm can cause a very small ice pellet to keep getting pushed up in the cloud over and over.  As it does, a process of coalescence is realized as other water droplets freeze together alllowing it to grow until it reaches the necessary weight to fall out of the cloud.