Fundamentally, a worker who is unable to show up to work due to a cause that was unexpected to both sides loses his salary (Shulchan Aruch, Choshen Mishpat 334:1). However, sometimes the ruling differs, if the preventing factor is of a general nature (i.e., affect many people). The source for this distinction is the mishna in Bava Metzia (105b): "Someone who rents out a field for a set rate of produce and the produce was consumed by locusts or blight, if it was a plague of the region (makkat medina), he reduces from the rental fee." In other words, if the reason that the renter is unable to fulfill his responsibility to provide produce is due to a makkat medina, he is not fully responsible.
The gemara (ibid.) discusses what is broad enough to qualify as a makkat medina? Rav Yehuda says that rubba d’ba’agi (the majority of the valley region) has to be blighted. Ulla says that it suffices for the fields around the one in question to be affected. Rav Yehuda’s opinion was accepted as halacha (Shulchan Aruch, CM 322:2). The Rishonim argued how to define rubba d’ba’agi. Rashi holds that it is within a certain valley region. The Rambam (Sechirut 8:5) says that it refers to most of the fields of the city. In any case, in regard to snow, it rarely makes a difference, as entire regions are generally covered similarly by snow, making it a makkat medina.
The mishna (Bava Metzia 103b) says that if a person rented out a field for produce and its wellspring dried up, the renter cannot reduce the amount he owes even though the field is now more difficult to cultivate. The gemara says that it is talking about a case that only a small artery in the area’s water dried up, and it is not a makkat medina because the renter can still bring water to the field by pail. Based on this, the Rama (CM 321:1) says that if one can fix the problem that arose even with difficulty, he does not reduce his obligations.
The Rama (ibid.) brings two opinions as to how to apportion the loss in the case of a makkat medina. The first is the Maharam, in regard to a teacher, when the government forbade teaching Torah, who says that the employer absorbs the loss. He cites a second opinion that at that time, the employer can back out of his commitment when the matter becomes known, and the employee gets paid only for what was done until then. The S’ma (ad loc.: 6) says that the Rama brought the Maharam incorrectly, as he meant that the two share the loss equally. The Taz agrees with the Rama, as we say that the employer’s "fortune" is that which caused the loss. The Netivot Hamishpat (321:1) accepts the Rama’s second opinion. The Avnei Shayish points out that even according to the Maharam, the worker is paid as a po’el batel (we subtract for the benefit of having vacation).