As for fasting on a day when the sky is overcast:

If clouds or haze obscure the sighting of the crescent on the night of the thirtieth of Sha‘bān, some Companions would fast as a precaution, while others would not. We know of none of them who considered fasting that day obligatory. Rather, those who fasted did so in an effort to ascertain the correct course and exercise caution. The reports transmitted from them explicitly establish this, including those related from ‘Umar, ‘Alī, Mu‘āwiyah, ‘Abdullāh ibn ‘Umar, ‘Ā’ishah and others.

The scholars hold differing views on this matter. Some forbid fasting that day or regard it as discouraged, as do some followers of Mālik, al-Shāfi‘ī and Ahmad. Others consider it obligatory, as do a group of Ahmad’s followers. Still others regard either course as permissible, as in the case of abstaining from food and drink when the appearance of dawn is obscured. This is the position of Abū Hanīfah and the explicitly stated position of Ahmad. Ahmad would fast that day as a precaution, following Ibn ‘Umar and others, not because he considered it obligatory. This is like other acts whose obligatory status is uncertain: performing them as a precaution is recommended, without being obligatory.

If someone fasts that day with a conditional intention—intending that, if it is part of Ramadan, his fast should count towards it, and otherwise it should not—and it then becomes clear that the day was part of Ramadan, his fast suffices according to most scholars. This is the position of Abū Hanīfah and the stronger of the two views reported from Ahmad and others. For intention follows knowledge: whoever knows what he wishes to do necessarily intends it, without having to choose to form that intention. But when someone does not know something, he cannot intend it.