I am trying to understand Greenhouse effect. I have heard that during the day the earth absorbs heat from the sun in the form of visible light, and during the night it looses heat in the form of infrared radiation.

Does earth emit infrared light during the day as well? I mean does it loose heat in the form of infrared light while absorbing visible light?

My assumption is that any object with a temperature above absolute zero emits some sort of radiation and loose heat - is this correct?

If this assumption is correct, why some objects like the sun emit heat in the form of visible light while other objects like earth emit heat in the form of infrared radiation?

• infrared light is around the peak of black body/Maxwell-Boltzmann thermal distribution for earth average temperature, even after considering night and day variations – lurscher Sep 15 '19 at 0:21
• also remember to not feed the trolls – lurscher Sep 15 '19 at 0:23
• @lurscher I am not trolling, it's a legitimate point and I really was wondering if the OP was aware of this. – BioPhysicist Sep 15 '19 at 0:24
• @lurscher You mean the Planck distribution, not the Maxwell-Boltzmann distribution. – G. Smith Sep 15 '19 at 1:00
• – anna v Oct 26 '19 at 5:45

My assumption is that any object with a temperature above absolute zero emit some sort of radiation and loose heat - is this a correct?

Yes, this is correct. Every object with a temperature $$T>0$$ emits electromagnetic radiation.

According to Wien's displacement law this radiation has its maximum at a wavelength $$\lambda_\text{max}$$ depending on the temperature $$T$$. $$\lambda_\text{max} = \frac{b}{T}, \quad \text{with} \quad b = 2.9\cdot 10^{-3}\text{m}\cdot\text{K}$$

• The surface of the sun has a temperature of $$5500$$ °C, i.e. $$T = 5800\ \text{K}$$. Therefore its radiation has its maximum at $$\lambda_\text{max}=500\ \text{nm}$$, which is visible light.
• Let's say the surface of the earth has a typical temperature of $$20$$ °C, i.e. $$T=290\ \text{K}$$. Then its radiation has its maximum at $$\lambda_\text{max}=10\ \mu\text{m}$$, which is infra-red light.

Does earth emit infrared light during the day as well? I mean does it loose heat in the form of infrared light while absorbing visible light?

The earth emits infra-red light all the time, day and night. And because the earth is warmer during day time, it emits more infra-red light at day time than at night time.

The reason for that is the Stefan-Boltzmann law which says that the total radiation power $$P$$ per area $$A$$ increases with temperature $$T$$: $$\frac{P}{A} = \sigma T^4, \quad \text{with}\quad \sigma=5.7 \cdot 10^{-8} \frac{\text W}{\text{m}^2\text{K}^4}$$

• Let's say again the surface of the earth has a typical temperature of $$20$$ °C, i.e. $$T=290\ \text{K}$$. Then its radiation power per area is $$400\ \text{W/m}^2$$.
• @HoomanBahreini Yes, see my amendment to the answer. – Thomas Fritsch Sep 15 '19 at 1:11