167 Use of the short form to denote temporary state

While the long form implies complete identification of the quality expressed by the adjective with the person or thing it qualifies, the short form indicates a temporary state or condition, cf. Река бурная The river is a turbulent one (an inherent characteristic) and Сегодня река спокойна Today the river is calm (the short form denoting a temporary state)
Similarly Он очень весёлый, бодрый ‘He is very jolly, cheerful’ (i.e.by nature), but Ты был бодр и весел и шутлив всю дорогу (Koluntsev)‘ You were cheerful and jolly and joked the whole way’, where the short forms refer to a person’s mood on a particular occasion.

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