Rain is a form of precipitation that falls to the earth in liquid form. The drops are greater than 0.5 millimeters in diameter in order to be considered rain. If they are less than that, then the precipitation is considered drizzle. Rain can occur if the temperature is above freezing in all layers of the atmosphere. Rain is ranked light, moderate or heavy depending on the intensity of rainfall.
Snow falls to the earth as a frozen particle, a combination of ice crystals that come together to form a snowflake. The temperature needs to be at or below freezing for snow to occur. Snow is also measured as light, moderate or heavy depending on how much falls. Snow grains are tiny particles of ice and do not accumulate. They are the frozen equivalent of drizzle.
Snow pellets are several millimeters in size. They are formed when snowflakes melt in the atmosphere and refreeze, collecting ice crystals on their way down to earth. They crush easily in the hand and can also bounce off the ground like other forms of precipitation. They usually have air pockets inside of them and are whiter in appearance than plain sleet.
Sleet is frozen raindrops. The drops of precipitation start out as snow in the upper atmosphere and move through a layer that is above freezing. This causes the drop to melt. If it melts completely, the drop will simply fall as rain. If it only partially melts, it will cause other water to ice onto it. It will fall through another subfreezing layer and hit the ground as sleet.
There are two types of hail: hard hail and soft hail. Soft hail, or graupel, has air bubbles and forms when ice crystals and cooled water attach to the hail stone in below-freezing layers of the atmosphere. Hard hail forms when the hail stone endures rapid unfreezing and refreezing with layers of ice forming to make the outer layers hard. Hail is typically 5 millimeters in size, while graupel is less than 5 millimeters.
Freezing drizzle is liquid and less than 5 millimeters in diameter when it hits the ground. However, it quickly turns to ice soon after. Likewise, freezing rain is greater than 5 millimeters in diameter and liquid when it hits the ground. It, too, freezes on contact with the ground. Freezing fog freezes after it has wet the ground.