The grésil is a Précipitation made of rain completely frozen after being last in a thick layer of air under zero.

Principal form

In general, the grésil is formed with a weather system synoptic (a depression) where soft air surmounts a thick cold layer close to the ground. Snow which falls into the mass from air above the freezing point will melt. However, while passing by again in the cold layer, the drops freeze again in granules. One then obtains translucent granules which rebound on any surface where they fall. The grésil is generally a transitional period between snow and the rain verglaçante. The code METAR in this case is PL.

Form secondary

They exist one second category of grésil, there is droplets of rain superfused in a convectif cloud like a Cumulus budding. These drops possibly condense when the temperature in the cloud is too low or when they meet a core of congelation. However, the way in the cloud is not long enough so that they reach a large diameter by Accrétion. If the level of condensation is raised, they will melt before falling down on the ground and it is only if this level is enough close to the ground that small granules of ice will fall in downpours. It is thus about a type of precipitations which is seen especially in spring and the autumn.

The formation of this second kind is thus similar to the beginning of the formation of a grêlon. However, there is not the passage in zones of density of different steam which allow the enlargement in onion skin of this last. The code METAR in this case is SHPL.

Use

Code METAR shows that the two phenomena have same name (PL). The first is a continuous precipitation whereas second is in Averse S (HS in METAR). However, the common use of this term varies with the place:

• In Canada, the grésil refers especially to the first form, that is to say a precipitation of granules of ice which occurs between snow and the rain verglaçante. However, it is known like the second also.
• In France and other countries free-Europeans, the grésil term is rather reserved for the second form

See too

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