Environmental Science
Frost forms when:

Objects on the ground cool below the dew point temperature.
The dew point is 32oF or below.
Water vapor changes into ice without first becoming a liquid.
All of these

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Environmental Science
Low clouds retard surface cooling at night better than clear skies because:

The clouds absorb and radiate infrared energy back to earth
The water droplets in the clouds reflect infrared energy back to earth
The clouds are better conductors of heat than is the clear night air
The clouds start convection currents between them
The formation of the clouds releases latent heat energy

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