Here's why:
* Earth's Temperature: During the day, the Earth absorbs solar radiation (mostly visible light). This warms the Earth's surface. At night, the Earth cools down, and it radiates this absorbed energy back into space as infrared radiation.
* Infrared Spectrum: Infrared radiation is a type of electromagnetic radiation that is invisible to the human eye. It's associated with heat, and all objects with a temperature above absolute zero emit some form of infrared radiation.
While Earth also emits other forms of radiation, like radio waves and some visible light, infrared radiation is the dominant form emitted at night.
