One of the most confused weather phenomena is sleet and hail. While sleet and hail may look the same, since they’re both ice pellets, they are very different. The words sleet and hail cannot be used interchangeably. So this brings about the question, how do they differ and how do each form?
Sleet is a winter phenomenon, caused by varying temperatures in the atmosphere. Because cold air is more dense than warm air, sometimes cold air gets forced underneath warm air. This creates a situation where the air higher in the atmosphere is above freezing and the air lower atmosphere (where we are) is below freezing. When the snow falls, it encounters the layer of warm air, causing it to melt into raindrops. Eventually, the raindrops will reach the layer of below-freezing air near the ground. When this happens, there are two possible results: 1) the rain can freeze before it hits the ground, producing sleet, or 2) the rain doesn’t freeze until it reaches the ground, creating freezing rain. To learn more about freezing rain, visit one of our previous blogposts found at http://cardinalwxservice.com/2021/02/02/freezing-rain/. The exact temperature of the air near the ground will determine whether or not it will fall as sleet or freezing rain. So this means that sleet is simply frozen raindrops.
The formation of hail is more complicated. Hail is a thunderstorm phenomenon. There are a couple of important factors that need to be in place for hail to form. Firstly, it’s important to note that even during the summer, the air high in the atmosphere is below freezing and that strong thunderstorms can grow high enough to encounter this cold air. It’s also important that a strong thunderstorm can have strong enough upward winds to blow raindrops upward. When hail forms, the raindrops are blown from the bottom of the storm to the top where they can freeze, they then turn into ice pellets. From there, the ice pellets then fall back into the lower part of the storm where raindrops collide into the small hail stone before getting blown back up and refreezing, making the hailstone a little larger. This process can repeat over and over again until they become so large that the storm isn’t powerful enough to suspend it and it falls to the surface. Because hail forms in a repetitive process, hail grows in layers, and looks much like the rings of an onion inside. Here is a diagram of how this process plays out:
Unlike sleet, hail can continue to grow in size, sometimes large enough to do major damage to roofs, crops, and cars. According to Weather Underground, the largest hailstone ever recorded in the U.S. was a massive 8 inches in diameter. When hail becomes large, it falls to the ground faster, sometimes the largest hailstones can reach speeds of over 100 mph. Here is a picture of what large hail looks like:
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