In the winter months, but especially in the transition phase from the cold to the warm season, you can always read about sleet showers in the weather reports. But what exactly is sleet and how does it differ from the better-known hail or snow?
Sleet – different forms
Sleet is a solid form of precipitation consisting of small, round or sometimes conical grains of ice. The grains typically have a diameter of between 1 and 5 mm. However, not all sleet is the same – A distinction is made between two types of sleet.
Frost sleet has a soft, opaque core which is encased in a transparent layer of ice. The grain is therefore semi-transparent. Due to the surrounding layer of ice, the grain usually bounces up on contact with the earth's surface without disintegrating. This type of sleet is particularly common in spring when temperatures are slightly positive.
Fig. 1: Frost sleet; Source: Pixabay
The second type of sleet is called rime sleet. These grains are opaque, brittle and have a snow-like texture. Due to the lack of an ice layer, they disintegrate relatively easily. This type of sleet mainly occurs in winter and is formed when snow thaws in high clouds and then freezes again. The temperatures are therefore slightly negative.
Fig. 2: Hoarfrost; Source: Wikipedia
Formation of sleet
Sleet belongs to the convective form of precipitation, on the precipitation radar it normally resembles a patchwork quilt (see figure below). The lability of the atmosphere plays a crucial role in the formation of sleet. Lability refers to the ability of the air to rise or fall vertically due to temperature changes. When the atmosphere is unstable, air masses can rise or fall more easily. This often occurs in spring when cold polar air flows in at high altitude. Typical clouds are cumulonimbus clouds (thunderstorm clouds). These clouds contain water in various states of aggregation, i.e. liquid, solid and supercooled (water below 0 degrees, but in liquid form). If supercooled water droplets come into contact with snowflakes or ice crystals, these serve as nucleation nuclei, allowing the supercooled water to freeze.
Fig. 3: Precipitation radar with typical patchwork; Source: MeteoNews
Difference to hail and snow
Frost graupel is often equated with hail, but by definition it is not the same thing. A classic hailstone must have a diameter of at least 5 mm and mainly occurs during thunderstorms in the summer months. Due to the greater temperature differences between the individual layers of air, convection is more pronounced (stronger updrafts and downdrafts within the cloud). This allows the later hailstone to go through the growth process more often by freezing supercooled water droplets and to grow layer by layer (comparable to the growth rings in plants or the skin of an onion). This process is visualized in the following diagram.
Fig. 4: Origin of hail; Source: MeteoNews
Hoarfrost, on the other hand, is often confused with snow. However, these "mini snowballs" differ from conventional snow in several ways. Rime graupel are round grains, whereas snow is irregularly shaped, hexagonal ice crystals (more on this in this blog). Another distinguishing feature can be seen by looking at the sky. Sleet in general (including hail) always originates from convective clouds, whereas snow can come from various cloud types (nimbostratus, altostratus, cumulonimbus or, in rare cases, stratocumulus).
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The content of this article has been at least partially computer translated from another language. Therefore, grammatical errors or inaccuracies are possible. Please note that the original language version of the article should be considered authoritative.
