# Quick query

#### Greenhippo

just a quick query, i hope you physics folk can explain

If all of the energy that enters earth comes form EM waves from the sun and once inside the atmosphere the waves change into all different forms of energy (ie heat Ek electrical). Doesnt it make sense to prevent a build up of energy in earth an equal amount of energy must leave earth, and the only way i know of energy escaping earth is via more EM waves.

Does this mean that all energy eventually becomes part of the EM spectrum and leaves, or is there a net build up of total energy or is there simply some physics phenomonia im unaware of

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Originally posted by Greenhippo
just a quick query, i hope you physics folk can explain

If all of the energy that enters earth comes form EM waves from the sun and once inside the atmosphere the waves change into all different forms of energy (ie heat Ek electrical). Doesnt it make sense to prevent a build up of energy in earth an equal amount of energy must leave earth, and the only way i know of energy escaping earth is via more EM waves.

Does this mean that all energy eventually becomes part of the EM spectrum and leaves, or is there a net build up of total energy or is there simply some physics phenomonia im unaware of
Pretty much. The Earth is in rough thermal equilibrium. It has two main sources of heat, sunlight, and internal heating. If its temperature is not to continually increase then it must get rid of heat at the same rate that those two sources provide it.

The only way for a body in empty space (approximately true for the earth) to get rid of heat is by radiation. By a law of physics, the frequency of the radiation, and so the energy density or efficiency of radiation for removing heat, depends on the temperature. So the temperature of the Earth will rise until its own radiation (max in the infra-red) takes away heat at the same rate as the two sources supply it. As soon as this condition is reached, and as long as it is maintained, the overall temperature will stay constant.

As I said, that is roughly the situation we are in.

#### Integral

Staff Emeritus
Gold Member
Essentially, while the sunny side is soaking up energy mainly in the visible spectrum (Is this correct? Perhaps others will correct me if it is not.) The dark side is radiating infra red, according to local surface temperatures. The longer the night, the more heat that is lost. This is why summers are hot and winters are colder. Long days and short nights mean net gain in energy, the temperature increases. In the winter,long nights and short days, a net loss of energy, the temperature decreases.

#### turin

Homework Helper
I think that the light from the sun is peaked in the UV range somewhere; it is at a pretty high temperature.

The summer winter thing is specific to a hemisphere, but the overall Earth doesn't see any net difference from one season to the next. It is summer in Australia when it is winter in Texas.

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