Sleet Versus Hail
(Above Left) Truck covered in a mixture of sleet & freezing rain, 2007 Syracuse, IN. Courtesy of the NWS Northern Indiana office. (Above Right) Quarter sized hail from Nazareth, PA.
We've all heard it before, a friend states "wow, look at that hail falling outside" during the middle of a winter storm or "this thunderstorm is so strong, it seems to be sleeting out there...it's even covering the roads!" Are these statements scientifically correct? Hail and sleet are different, and cannot be used interchangeably. You might be thinking "but hail and sleet are both just 'balls of ice' they must be the same"...the fact remains that hail and sleet are formed in very different ways. Below we will go through their differences in more detail.
First, we'll discuss how hail is created. The above diagram is a good visual aid for how hail develops in thunderstorms. Hail falls mainly during the warmer months of the year, and is generally associated with strong to even severe thunderstorms. Hail can vary in size from pea size to softball size (or in rare instances even larger). Severe hail is defined by the National Weather Service as being larger than 1.0" in diameter (the size of a quarter) and can cause damage to cars, vinyl siding, windows and other objects. Large hail forms when severe thunderstorms, with powerful updrafts collect rain drops and push them higher into the atmosphere. As the drops are forced higher and higher by these powerful updrafts they encounter below freezing air temperatures where they develop into pieces of ice. The ice particles then ride wind currents up and down within the thunderstorm where they collect additional rain drops, which freeze into new layers of ice on each ascent. Eventually, the large ice particles become too heavy for the updrafts to keep them suspended and they fall to the Earth as hail.
Winter precipitation type graphic, courtesy of the NWS/NOAA.
Sleet forms in a much different way than hail. During a winter storm, ice crystals form in the cloud layer and become heavy enough to start falling toward the ground as snowflakes. For sleet to form, these snowflakes encounter a shallow warm layer usually between 5,000 to 10,000 feet above ground level and melt (or partially melt) into liquid rain drops. These raindrops then fall into a shallow below freezing layer closer to the ground and refreeze into small, round, ice particles called sleet. This scenario is nicely depicted in the diagram above. Occasionally, in very dynamic winter storms heavy sleet can be accompanied by thunder! (Click this link for a video of heavy sleet & thunder in Norman, OK on March 2nd, 2014)