Final answer:
Surface hoar forms under cold temperatures and clear skies, which facilitate rapid cooling of the snow surface, allowing moisture in the air to deposit as frost.
Step-by-step explanation:
Surface hoar is a type of frost that forms on snow surfaces when specific weather conditions are met. The correct conditions needed for surface hoar to form include cold temperatures and clear skies, which align with option 1) Cold temperatures and clear skies.
These conditions are necessary because they allow the snow surface to cool rapidly, causing the moisture in the air to deposit as ice crystals directly onto the snow surface. This process is similar to how condensation forms dew, but when temperatures are below freezing, the moisture forms as frost instead. Clear skies play a crucial role because they increase the loss of heat into the atmosphere through radiation, allowing the surface temperature to become colder than the air above it, which is conducive to the formation of surface hoar.
It is also helpful to have calm weather conditions, as too much wind can disrupt the delicate ice crystals before they have a chance to fully develop. By contrast, high humidity aids in providing sufficient moisture in the air for the ice crystals to form.