I heard that 'depressed' is used for serious conditions such as melancholiacs. Then what word could be used for daily lives?
'Depressed' is used in a multitude of ways by English speakers, and its meaning may even be evolving at the present time. It is used in a strict technical sense, but also in a more casual and lazy way by speakers to mean anything between 'a minor case of the blues' to 'severe mental health challenges'.
As Virolino said, if you say:
I feel depressed on rainy days.
Most people will immediately understand that you mean
I often feel a bit sad on rainy days.
Technically, that person is not depressed at all (does not have clinical depression), and some would probably suggest that we should be phasing out this casual usage as it can both stereotype and belittle genuine sufferers. If society is trying to reduce the stigma associated with depression and mental illness, it does not help to have people suggesting it is the same thing as a minor case of the glums.
So, I would suggest you are right to try to find more accurate descriptions for cases of ephemeral sadness to cases of serious clinical depression.
I feel a bit sad.
I sometimes feel blue.
He's not on top form.
Her mood is affected by the weather.
I feel really sad.
They are suffering from (clinical) depression.
It may even be helpful to avoid the verb 'to be depressed' entirely (if talking of mental illness, and not 'I depressed the button with my finger'), because it is used so carelessly to mean many different things. 'Depression' is the illness, and is less likely to be underestimated or misinterpreted.
I have depression.
Is clearly about someone with clinical depression.
I am depressed.
Can mean just about anything on the scale of sadness.