CRITERIA RELATED TO PHENOMENA
(a) An area of thunderstorms and cumulonimbus
clouds is considered:
(1) obscured (OBSC) if it is obscured by haze
or smoke or cannot be readily seen due to darkness;
(2) embedded (EMBD) if it is embedded within
cloud layers and cannot be readily recognised;
(3) isolated (ISOL) if it consists of
individual features which affect, or are forecast to affect, an area with a
maximum spatial coverage less than 50 % of the area concerned (at a fixed
time or during the period of validity); and
(4) occasional (OCNL) if it consists of
well-separated features which affect, or are forecast to affect, an area with
a maximum spatial coverage between 50 and 75 % of the area concerned (at
a fixed time or during the period of validity).
(b) An area of thunderstorms is considered
frequent (FRQ) if within that area there is little or no separation between
adjacent thunderstorms with a maximum spatial coverage greater than 75 %
of the area affected, or forecast to be affected, by the phenomenon (at a
fixed time or during the period of validity).
(c) Squall line (SQL) indicates a
thunderstorm along a line with little or no space between individual clouds.
(d) Hail (GR) is used as a further description
of the thunderstorm, as necessary.
(e) Severe and moderate turbulence (TURB)
refers only to: low-level turbulence associated with strong surface winds;
rotor streaming; or turbulence whether in cloud or not (CAT). Turbulence is
not used in connection with convective clouds.
(f) Turbulence is considered:
(1) severe whenever the peak value of the EDR equals or exceeds 0.45; and
(2) moderate
whenever the peak
value of the EDR is equal to or above 0.20 and below 0.45.
(g) Severe and moderate icing (ICE) refers to
icing in other than convective clouds. Freezing rain (FZRA) should refer to
severe icing conditions caused by freezing rain.
(h) A mountain wave (MTW) is considered:
(1) severe whenever an accompanying downdraft
of 600 ft/min (3.0 m/s) or more and/or severe turbulence is observed
or forecast; and
(2) moderate whenever an accompanying
downdraft of 350–600 ft/min (1.75–3.0 m/s) and/or moderate turbulence
is observed or forecast.
EASA aviation regulations define criteria for SIGMET issuance, focusing on hazardous weather phenomena. Thunderstorm area coverage is classified as obscured, embedded, isolated, occasional, or frequent. Turbulence severity is based on EDR values, excluding convective clouds. Icing refers to non-convective clouds, with freezing rain indicating severe conditions. Mountain wave severity depends on downdraft speed and turbulence.
* Summary by Aviation.Bot - Always consult the original document for the most accurate information.
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