merklich
noticeable
Clearly perceptible, noticeable, or visible; in such a way that one can notice it.
Nachdem die Heizung angestellt wurde, wurde es im Raum merklich wärmer.
After the heating was turned on, the room became noticeably warmer.

noticeable
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noticeable
Clearly perceptible, noticeable, or visible; in such a way that one can notice it.
Nachdem die Heizung angestellt wurde, wurde es im Raum merklich wärmer.
After the heating was turned on, the room became noticeably warmer.

'Merklich' (noticeable) comes directly from the verb 'merken' (to notice, to perceive). The ending '-lich' means '-able', so something is 'merklich' when it can be noticed.
Hast du gemerkt, dass es kälter geworden ist?
Did you notice that it has gotten colder?
'Merklich' (noticeable/noticeably) can stand before a noun as an adjective or describe a verb or another adjective as an adverb. As an adverb, its form remains unchanged.
Es gab eine merkliche Verbesserung seiner Noten.
There was a noticeable improvement in his grades.
'Merklich' (noticeable/noticeably) is often used to describe a change or a state. Typical combinations are 'ein merklicher Unterschied' (a noticeable difference), 'eine merkliche Besserung' (a noticeable improvement), or 'merklich kälter/leiser/besser werden' (to become noticeably colder/quieter/better).
Der Unterschied zwischen den beiden Versionen ist nur gering, aber merklich.
The difference between the two versions is only small, but noticeable.
clear / clearly
'deutlich' and 'merklich' both describe that something is clearly perceptible. However, 'deutlich' is broader: you can speak clearly, see clear handwriting, or recognize a clear difference. 'merklich' is used more when you notice a change or effect, for example in temperature, mood, or performance. Learners may confuse the words because both often mean 'perceptible' or 'visible'. As a simple guide: 'deutlich' fits clear perception and clear statements, while 'merklich' fits especially noticeable changes.

Nach dem Lüften ist die Luft im Zimmer merklich frischer.
After airing the room, the air is noticeably fresher.
Auf dem neuen Foto sind die Farben der Blätter deutlich zu sehen.
In the new photo, the colors of the leaves are clearly visible.