The key question is, are the temperatures at the surface and in the atmospheric layers where the precipitation forms about the same (that is, with both the surface and the atmosphere either above freezing or below freezing)? For example, if the temperature at the surface is 25 F (-4 C) and the temperature at one mile (1.6 km) above the surface is 0 F (-18 C), you can count on snow. However, if the surface is 25 F and the atmosphere is 39 F (+4 C), then rain will fall onto a frozen surface. This is freezing rain. In most cases the atmosphere is colder than the surface, but when a warm front approaches, the air aloft warms up more quickly than does the surface air. This is because the air above the surface moves faster than the air near the surface, mainly as a result of greater friction at the ground level.

If the layer of colder air at the surface is fairly thick (about 1000 ft or so), then the rain will have time to freeze as it falls towards the surface. These frozen raindrops are called sleet. Sleet bounces when it hits the ground, while freezing rain and snow do not. Sleet may also be referred to as ice pellets. By the way, snow is not frozen rain. Snow forms when water vapor is deposited onto very small condensation nuclei (usually minute specks of ice or dust).

Graupel is also called snow pellets or soft hail and is composed of loosely compacted ice crystals, which often break when they hit the ground. When snow falls through a layer of frozen air having a particularly high concentration of cloud droplets, a process known as riming may occur. The frozen cloud droplets coat the falling snowflakes, producing particles which are somewhat spherically-shaped. In most places, graupel does not occur as frequently as sleet.

Hail, like sleet, is frozen rain, but hail is larger than sleet and is more common in the spring or summer, when surface temperatures may be quite warm. Hail almost always forms in cumulonimbus clouds (thunderstorms), which can extend eight miles or more above the surface. Within these storm clouds, vertical updrafts and downdrafts occur. When a raindrop is captured in an updraft it moves to higher levels of the cloud where the temperatures, even on the hottest summer day, are well below freezing. The raindrop freezes and falls through the cloud. If the now-frozen pellet (sleet) is again captured by an updraft, it will receive a new coating of ice. This process may be repeated a number of times until the hunk of ice is simply too big to be taken aloft by another updraft, and it finally falls to the ground. If a small ice pellet is not recaptured by an updraft, it will likely melt before it reaches the ground. In the summer, only the largest hail can survive the warm temperatures in the lower atmosphere. If you were to cut through a piece of hail, you would probably see a number of distinct layers or concentric rings of ice. One of the largest chunks of hail ever seen fell in Coffeyville, Kansas on September 3, 1970. It had a diameter of almost six inches, and it weighed about 1.7 pounds!