So my question which part of the Em spectrum(sun) is responsible for heating of the atmosphere ? Is it visible or infrared or both ?
The solar spectrum is such that most of the heating by direct absorption of solar photons by atmospheric gases occurs in the UV (oxygen) and in the visible (ozone).
But, why does infrared contribute more compared to visible ? Is there a difference?. I was thinking whether a particular part of em spectrum caused majority of heating.
Light hits ground... ground gets warm and emits infrared... sky absorbs re-emitted infrared. Greenhouse Effect.
Chemisttree is close. It is important to remember that "Heat" and "Infrared" are totally different things. "Heat" is the vibration of molecules. Since there are (almost) no molecules in space, heat cannot travel from the Sun to the Earth directly. Hot molecules sometimes cool off by emitting a photon. Photons are the particle that visible light is made of, but other frequencies, such as radio, infrared, and ultraviolet are also photons. Infrared and visible light behave mostly the same way. They can travel easily from the Sun to the Earth, and pass right through transparent things like air and glass. But when they hit something opaque, such as the ground, the photon can re-enter those molecules and heat them up. Vibrating molecules in the ground then heat the air that touches the ground. That is why air at high altitudes is so cold - the photons pass through without heating the air, and the ground is too far away! BTW, this also tells you how a greenhouse works! Photons pass through the air and glass, and heat up the ground and plants inside the greenhouse. The vibrating molecules in the ground and plants then warm the air, but only the small amount of air inside the greenhouse - not the whole atmosphere. So the photons can easily enter, but vibrating molecules have a hard time getting out, so the inside stays warm!
You are confusing heat with thermodynamic energy. Heat is energy other than work that is transferred between a system and the environment that surrounds it. Radiative heat transfer is one of the mechanisms by which a system exchanges energy other than work (i.e., heat) with its surrounding environment. There's a lot more to thermodynamic energy than "vibrations of molecules." That concept works somewhat for a solid, but not for a monatomic gas. Kinetic energy is a much better term, but even that's not quite right. Consider an ice/water mix at 0 °C. Transfer heat to that mix and some of the ice will melt, but the temperature will remain at 0 °C until all of the ice has melted. Because the atmosphere is more or less transparent to visible, less so to near infrared. The image below depicts solar radiation at the top of the atmosphere versus that at sea level. The difference between the two is incoming solar radiation that is either absorbed by the atmosphere or reflected back into space by the atmosphere or clouds. Only 19% of the incoming solar radiation is absorbed by the atmosphere; 30% is reflected back into space. Most of the incoming solar radiation is absorbed by the ground, not the atmosphere. This energy is transferred to the atmosphere in the form of rising air, latent heat in water vapor, and thermal radiation. Only a tiny bit of the thermal radiation emitted by the ground makes it all the way through the atmosphere into space.
The atmosphere of the earth extends much farther out than the troposphere.The troposphere is heated mainly by the surface of the earth and the temperature decrease with height. Above the tropophere, the ozone in the stratosphere absorbs UV radiation, and temperature increases with height. The mesophere, where temperature decrease with height, has the coldest temperature on the planet where at the top it averages -85C. As Chestermiller stated, oxygen and ozone molecules absorb radiation. At the surface, most of the UV and above frequencies such as x-ray have already been absorbed in the atmosphere.
So the short answer to thorium1010's question is that the ground heats the lower atmosphere, (troposphere) and UV and X-rays heat the parts above the ozone layer?
Sorry algr. I was only trying to comment on that the variation of temperature with elevation is not a direct relation. The upper atmosphere will absorb most of the shorter wavelengths direct from the sun, of which there is not all that much if you look at the graph of the Solar Radiation Spectrum provifded by DH, and the lower atmosphere by ground heating as per the explanation by DH directly below the graph. cheers
I think you have that around the wrong way Its the Ozone layer that protects us from UV radiation hence why us poor people in NZ and Australia get hit with more UV because of the Ozone hole look at the chart DH posted O3, Ozone is up in the UV section cheers Dave
Look at that chart again, Dave. What you're talking about is but a part of the ozone-oxygen cycle. Ozone molecules absorb incoming UV-B (mid-energy UV) and split into ordinary oxygen molecules and atomic oxygen. That atomic oxygen usually reacts with other ordinary oxygen molecules to re-form ozone. This makes for a nice catalytic cycle that removes UV-B (and some UV-A) from the incoming solar radiation. Sometimes two oxygen atoms will collide and form oxygen, sometimes with something else. If the cycle above were all there was to the ozone-oxygen cycle these depletion events would quickly drain the stratosphere of ozone. Ozone can also be depleted. Ozone can combine with a monatomic halogen to form a temporary species, which in turn combines with another ozone molecule to form ordinary oxygen and monatomic halogen. Yet another catalytic reaction, only this time the catalyst removes ozone. This is the primary manmade cause of ozone depletion. Fortunately, there are also ozone creation events. Look at the chart. High energy UV photons dissociate ordinary oxygen molecules into monatomic oxygen. That monatomic oxygen combines with ordinary oxygen to produce ozone. Unfortunately, ozone is only created when the Sun shines. The Antarctic has a six month long winter when the Sun doesn't shine and the monatomic halogens have free reign to deplete the ozone from the far southern stratosphere.
Heat budget studies by Kiehl and Trenberth (1997) and others put the atmosphere's total heat budget at 390 Watts per square meter. Some 102 Watts comes from direct absorption of solar radiation, 268 Watts from terrestrial longwave radiation, 17 Watts from conductive heat transfer from the surface to the atmosphere, and the remaining 3 Watts from hydrologic cycling. Some 270 Watts is absorbed by water in its three phases, 53 Watts by carbon dioxide, 36 Watts by ozone and oxygen, and 11 Watts by methane and nitrous oxide. Conductive heat transfer and hydrologic cycling are shared by all of the atmosphere's gases in proportion to their abundance.
Of the 31 Watts per square meter of ultraviolet radiation that reaches the outside of the Earth's atmosphere from the Sun, only 5 Watts reaches the surface of the Earth. Most of the remaining 26 Watts is scattered by the atmosphere, but a small amount is absorbed--primarily by ozone and oxygen. Of the 154 Watts of visible sunlight at the outside of the atmosphere, 88 Watts reaches the surface. Most of the remaining 66 Watts is absorbed by clouds and particulates and the rest is scattered. Some of this scattered visible light reaches the surface as "skylight". Of the 157 Watts of infrared solar radiation, some 70 Watts reaches the surface. Most of the remaining 87 Watts is absorbed by water vapor and other "greenhouse" gases. All of this is put in the shade compared to the 268 Watts of terrestrial radiation that heats the atmosphere. Hard numbers are difficult to come by, but it is obvious that the bulk of atmospheric heating comes from the infrared portion of the spectrum, the next share from the visible portion, and the least from the ultraviolet portion.
Solar Constant and Heat Budget Indeed they do. For historical reasons, heat budgets are usually give in units of W/m^2, averaged over the Earth's entire surface. Since the Earth's surface area is exactly four times its disc area, we divide the Solar Constant of 1,366 joules per square meter per second [Scaffeta & West, 2005] by four. This gives us 342 W/m^2--more or less. That's the number I used for the total incoming solar radiation at the top of the atmosphere. 31 + 154 + 157 = 342