# Why is solar radiation equal to long wavelength emission?

• aldo sebastian
In summary, in heat transfer analysis, solar radiation is often taken to be equal to long wave radiation emission when the system reaches thermal equilibrium. This means that the power in from solar gain is equal to the power out from radiated loss. However, if the solar gain increases, the temperature will rise and the emission will also increase. Similarly, if the temperature is stable, the power going into the system will be equal to the power lost, but the loss will be greater due to a higher temperature gradient.
aldo sebastian
In my heat transfer course, I always had to do analysis of long-wavelength radiation between surfaces first, and find the heat radiated from one of the surface. Then, solar radiation is added to the problem, and suddenly it was said that solar radiation=heat radiated from this surface! Does anyone know why we take solar radiation to be equal to long wave radiation emission?

Perhaps because it's reached thermal equilibrium eg temperature has stopped rising. Under that condition power in = power out.

CWatters said:
Perhaps because it's reached thermal equilibrium eg temperature has stopped rising. Under that condition power in = power out.
But then now the solar radiation adds energy to the system; shouldn’t the emission increase because now it receives more radiation than before? i.e. the solar radiation adds more energy to the surface than before, and thus should increase its temperature more, and thus its emission?

Yes, if solar gain increases then the temperature will rise and it will radiate more.

While temperature is rising solar gain > radiated loss.

When temperature is stable solar gain = radiated loss. The exact amount of gain/loss depends on the temperature and other factors.

While temperature is falling solar gain < radiated loss.Consider a house in winter. If you set the thermostat to 18C the house will eventually reach 18C. When its stable at 18C the power going into the house will be equal to the power lost through the walls. Let's say they are both 10kW.

If you turn up the thermostat to 20C the heating system will increase its output delivering more heat than the house is loosing and the temperature will rise and the losses increase.

Once the temperature is stable at 20C the power going into the house will again be equal to the power lost. However the loss will be greater because the temperature gradient through the wall will be higher. They might both be 11kW.

Tom.G

## 1. Why is solar radiation equal to long wavelength emission?

Solar radiation is equal to long wavelength emission because the sun emits radiation across a wide range of wavelengths, including both short and long wavelengths. However, as this radiation travels through the Earth's atmosphere, some of the shorter wavelengths are absorbed or scattered, leaving mostly longer wavelengths to reach the Earth's surface. This is why the majority of solar radiation reaching the Earth's surface is in the form of long wavelengths.

## 2. What is the significance of solar radiation being equal to long wavelength emission?

The significance of solar radiation being equal to long wavelength emission is that it plays a crucial role in regulating the Earth's climate and maintaining a habitable environment for living organisms. The Earth's atmosphere absorbs and re-emits long wavelength radiation, creating the greenhouse effect that keeps the Earth warm. Without this balance of solar radiation and long wavelength emission, the Earth's temperature would be too extreme for life to exist.

## 3. How does solar radiation affect the Earth's temperature?

Solar radiation affects the Earth's temperature by providing the energy needed to heat the Earth's surface and atmosphere. As mentioned before, the Earth's atmosphere absorbs and re-emits long wavelength radiation, which helps to regulate the Earth's temperature. Without solar radiation, the Earth would be a frozen, lifeless planet.

## 4. Is solar radiation equal to long wavelength emission the same as infrared radiation?

Yes, solar radiation equal to long wavelength emission is the same as infrared radiation. Infrared radiation is a type of electromagnetic radiation with longer wavelengths than visible light, and it is a major component of solar radiation. In fact, nearly half of the solar radiation reaching the Earth's surface is in the form of infrared radiation.

## 5. How does solar radiation affect solar panels?

Solar radiation is essential for solar panels to generate electricity. Solar panels convert solar radiation into electricity through the photovoltaic effect, where photons from the sun's rays knock electrons loose from atoms in the solar cell, creating an electrical current. Therefore, the amount of solar radiation directly affects the efficiency and output of solar panels.

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