I can take the shapes of many things.
In the forms of birds or ships I'm most commonly clad.
Many people watch me.
I can be either good or bad.
From water, I am bright and happy.
From miasma, I am dark and sad.
One can walk through me when the day is gloomy,
or I can block one's view on a sunny day.
During winter I bring good cheer,
during summer I take it away.
I have been given as a name to humans,
I have been called a bad omen on a traveler's way.

What am I?