Distinguishing between snow and other similar types of precipitation is not always simple, for example it may happen that we can confuse snow with sleet or vice versa, the question seems insignificant but sometimes this distinction is not so trivial, for example if you have to move with a means of transport or even just to cover a stretch of road on foot.
So let’s start, taking into account that in this article we will deal with the typical precipitations, mainly other than snow, of the Mediterranean winter in a simple way.
More technical insights will follow in other articles.
What is a precipitation?
Precipitation is water in solid, liquid or mixed liquid and solid form that falls from the clouds on the surface of the Earth.
Winter precipitation types
–liquid: rain, drizzle;
–mixed: mixed rain and snow;
–solid: snow, snow pellets (also known as graupel or soft hail, round snow in Italy), ice pellet (sleet in the USA), snow grains ;
–freezing: freezing rain, freezing drizzle.
A separate category is that of supercooled drops of water, that is water in the liquid state at temperatures well below zero, down to -40 ° C.
The boundary that divides one type of precipitation from another can generally be identified, altough not completly, with the thickness of a warm layer of air between two layers of cold air.
Let’s start now with the definition of the snow(fig. 1).
By snow we mean a precipitation of ice crystals, singly or stuck together, made of water and air.
Let’s now look at the other types of precipitation, relatively less known.
The graupel (fig. 2) is a precipitation of white, opaque grains of ice that are round or conical with the diameter less than 5 mm, forms in clouds with some vertical development, they have a grainy surface and are relatively light.
The grainy surface is due to the collection of cloud droplets or water droplets called supercooled which then rapidly freeze on contact, their relative lightness is due to the inclusion of air between the ice nucleus and the frozen droplets on the surface.
The graupel is also fragile, when they fall on hard ground, they bounce and often break.
Showers of graupel are possibles and usually occurs when the air temperature is near 0 °C.
Ice pellet (fig. 3)
They are transparent or translucent ice particles, usually spheroidal or irregular, that are less than 5mm in size. It’s a type of precipitation more common than you think, they are formed when a raindrop, and less frequently a snowflake, encounter a layer of warmer air, generally with a temperature above 0 °C, as they descend and then again into a colder layer with temperature below 0 °C where they freeze. The raindrops are more transparent whilst the melted and refrozen snowflakes the less they are melted, the more opaque they are.
The cold layer necessary for their formation is on average more than 1000 meters, based on some research.
When the ice pellet touches the ground often bounces, does not crumble and produces a sound when it hits surfaces such as jackets and windshields, a characteristic ” tap “.
They are more likely to form when a warm air mass is replacing a cold air mass.
Freezing rain (fig. 4) – Freezing drizzle
If the hot layer mentioned above comes closer to the ground or it is more warmer, the raindrops or the now melted snowflakes will no longer have time to refreeze and we will see rain fall, even with sub-zero temperatures, in the latter case the rain that falls tends to solidify once it has landed, producing freezing rain, which can be very dangerous due to the resulting ice sheets. If drizzle falls in the warm layer, then on the ground we will have freezing drizzle.
The intensity with which it falls is usually not high.
Also in this case they are more likely to form when a warm air mass is replacing a cold air mass, even closer to the border of the two air masses, and where there is a stagnation of cold air in the lower layers.
Snow grains (fig. 5)
Very small opaque white particles of ice, which do not bounce or break when reach the ground, they are fairly flat or elongated, generally measure less than 1 mm in diameter and falls in small quantities, except in the mountains, from stratiform clouds or fog at temperatures between 0 ° and -10 ° C.
Water mixed with snow
A semi-liquid precipitation partly rain and partly melted snow. It occurs when the lowest part of the atmosphere has a temperature slightly above 0 ° C.
The following figure (fig. 6) shows the formation of some types of precipitation discussed: if the air layer with temperatures slightly above 0 is practically non-existent, most likely we will see a snowfall, if this layer is thicker then we will have ice pellets, if the same layer is even more thicker and almost approaches the ground we will have freezing rain, finally if that layer reaches the ground we will have rain.
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