Which condition facilitates the development of frost according to the Frost Line?

The development of frost is primarily facilitated by ground temperatures falling below freezing. This is because frost forms when the temperature of the surface, such as grass or soil, drops to 32°F (0°C) or lower, causing moisture in the air to condense and freeze. The Frost Line concept refers to the depth at which the ground is expected to freeze, which is influenced by the combination of air temperature and ground temperature.

When ground temperatures fall below freezing, it creates conditions ideal for frost to form overnight, especially when there are clear skies and calm winds that promote radiational cooling. The presence of moisture in the air also plays a crucial role, as frost is a result of condensation and freezing of that moisture.

In contrast, soil exposed to sunlight during the day typically warms up, which would not support frost formation. Air temperatures rising above freezing would raise the surface temperature as well, preventing frost. Rainfall before sunrise can temporarily warm the surface, which also wouldn't facilitate frost formation immediately after as it may keep temperatures above freezing temporarily. Thus, falling ground temperatures are the key factor in the formation of frost.