What determines the emissivity of a material?

What determines the emissivity of a material?

Emissivity is a surface property. One might think the emissivity of a material is determined by its properties at the surface. However, this doesn’t seem to be the case.

Metals usually have very low emissivity. The emissivities of aluminum, iron, copper and silver are all below 0.1. The emissivities of on-conductors, such as water, are usually close to one. Why is that?

Metals have high conductivity. When light shines on the surface of a metal, electrons quickly absorb the energy of the photons. Heat quickly transfers into the interior of the metal. The whole volume of the metal stores heat. Only the surface emits heat. That is why the temperature of a metal can be very high. Equivalently, the emissivities of the metals are very low.

Non-conductors have low conductivity. Heat stays near the surface and emits back into the air easily. That is why emissivities are high for non-conductors.

I think emissivities may also be related to the temperature of the light source. If the temperature is low, or the frequency of light is low, emissivities of the metals may be higher. This is because light photons carry lower levels of energy. Energy input and energy output on the same surface may differ less. The amount of energy emitted are relatively high and emissivities are high. This is my guess.

When the concentration of carbon dioxide in the atmosphere increases, vegetation on the surface increases. The emissivities of vegetations are higher than rocks and sands. This helps lower the temperature of the earth. Another reason is that vegetations evaporate water more. This also lowers the temperature.