I've heard rain is considered a symbol of blessing, perhaps because of the flood: that Hashem doesn't repeat the flood so He sends rain in much smaller proportions and to give the land its nourishment. But what's the source for this idea? Also, is there any symbol in fog?
According to Devarim 11:13-14 it is a reward from god:
יג. וְהָיָה אִם שָׁמֹעַ תִּשְׁמְעוּ אֶל מִצְוֹתַי אֲשֶׁר אָנֹכִי מְצַוֶּה אֶתְכֶם הַיּוֹם לְאַהֲבָה אֶת יְהֹוָה אֱלֹהֵיכֶם וּלְעָבְדוֹ בְּכָל לְבַבְכֶם וּבְכָל נַפְשְׁכֶם:
יד. וְנָתַתִּי מְטַר אַרְצְכֶם בְּעִתּוֹ יוֹרֶה וּמַלְקוֹשׁ וְאָסַפְתָּ דְגָנֶךָ וְתִירשְׁךָ וְיִצְהָרֶךָ:
(Artscroll) translation: It will be that if you hearken to My commandments that I command you today, to love Hashem, your God, and to serve Him with all your heart and with all your soul, then I shall provide rain for your Land in its proper time,the early and the late rains, that you may gather in your grain, your wine, and your oil.
One of the sources is Rashi on Bereshis 7:12 in which he comments ויהי הגשם על הארץ. There, he says that initially the rain descended slowly, so if the people repented, the rain would be a blessing and just water the crops. But when the people refused to repent the rain turned into a mabul:
ויהי הגשם על הארץ: ולהלן הוא אומר (פסוק יז) ויהי המבול, אלא כשהורידן, הורידן ברחמים שאם יחזרו יהיו גשמי ברכה, וכשלא חזרו היו למבול
And below (pasuk 17) it is said: “Now the mabul was (upon the earth)” “But when He (Hashem) brought the rains down, He brought them down with mercy, so that if they (the people) repent, the rains would be rains of blessing. When they did not repent, they became a mabul.