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Observation of a thunderstorm must as a rule be indicated in the synoptic weather report using the relevant codes. With regard to thunderstorm observations, the emphasis here is still placed on the difference in meaning of the terms thunderstorm and sheet lightning. A thunderstorm is when both the lightning and the accompanying thunder are observed. Sheet lightning is when the reflection of the lightning is visible in or against the clouds, without hearing the thunder. Sheet lightning is usually caused by far distant thunderstorms; silent discharges are also possible however. Altocumulus castellanus and altocumulus floccus often herald a thunderstorm.