Which conditions result in the formation of frost?

Frost formation occurs when the temperature of a collecting surface is at or below the dew point of the adjacent air, and the dew point is itself below freezing. This means that water vapor in the air condenses into ice crystals rather than water droplets, which is fundamental for frost to form. When these conditions are met, moisture in the air transitions directly from its gaseous state to a solid state without becoming liquid first, a process known as deposition.

To clarify, while the other choices mention conditions related to temperature and humidity, they do not sufficiently encompass the specific conditions necessary for frost formation as described. The first option refers to small moisture droplets, which would not lead to frost unless the conditions involve freezing temperatures. The third statement discusses air temperature without considering the dew point, and the fourth suggests reaching saturation without addressing how the temperature factors into frost formation specifically. Each of these elements may relate to moisture in the air, but only the correct choice captures the critical two-part relationship between temperature and dew point that is essential for frost formation.