# Solar flux on (not at) Earth

I'm trying to find out if there are reliable values for the solar flux received on Earth. I realise this will change with location, time of day, season, weather etc. but I'm surprised that I can find anything at all. For space calculations of solar flux on solar panels I typically use a value of 1372 W/m2 at 1 AU from the Sun. I understand that this value will be significantly reduced once the light has travelled through the atmosphere but I can't find a lookup table or anything.

You can use an online calculator like the one at Meteo Exploration. Input parameters are classic lat / lon / alt / time and:

Visibility (km): it is the maximum distance in km where large objects can be distinguished in the horizon. The value given (50 km) is for a clean atmosphere. This is a proxy for atmospheric turbidity, it is choosen as many airports report visibility, while other parameters, such as Linke Turbidity, are more difficult to obtain. Airport reports (METAR) are given here (choose 'Decoded').

Temperature °C: temperature in degrees centigrade, use CT to convert from Farenheit.

RH (0-100): Relative Humidity in percentage, values from 0 to 100. Airport reports (METAR) normally include RH.

Ozone thickness: ozone layer thickness in cm. Divide Dobson Units by 1000 to get the equivalent in cm. Check the TOMS (Total Ozone Mapper Spectrometer) pages for current values.

Albedo ground (0-1): albedo of the surrounding terrain, e.g. 0.8 to 0.95 for fresh snow, 0.17 for deciduous forest, 0.35 for sand, etc.

Timezone (timezone map): the timezone, a good approximation is (-1)*longitude/15, longitude in degrees and west is negative.

Slope Orientation (0-360): orientation of the surface, solar panel, roof, etc. If looking north it will be 0°, if looking south will be 180°. Range 0 to 360 degrees

Slope Tilt (0-90): inclination of the surface or panel with respect to the horizontal, 0° is flat 90° is completely vertical. Range is from 0 to 90 degrees.

References:

Note that the Earth receives energy from the Sun, but also radiates energy (infrared). Some of this radiated energy is reflected by the atmosphere (greenhouse effect) and is reabsorbed by the Earth. The infrared energy radiated is indeed usable. See Climate and Earth’s Energy Budget at Nasa:

To convert between the energy radiated by the Sun at 1AU and the insolation, see this article on Wikipedia. Extract:

The Earth receives a total amount of radiation determined by its cross section (π·RE²), but as it rotates this energy is distributed across the entire surface area (4·π·RE²). Hence the average incoming solar radiation, taking into account the angle at which the rays strike and that at any one moment half the planet does not receive any solar radiation, is one-fourth the solar constant (approximately 340 W/m²). The amount reaching the Earth's surface (as insolation) is further reduced by atmospheric attenutation, which varies. At any given moment, the amount of solar radiation received at a location on the Earth's surface depends on the state of the atmosphere, the location's latitude, and the time of day.

• -1. Seriously? After I submitted my answer, you edited your initial answer (which was very different and rather non-responsive) to incorporate exactly what I wrote. – David Hammen Feb 19 '15 at 13:37
• @DavidHammen. Request: "I'm trying to find out if there are reliable values for the solar flux received on Earth. I realise this will change with location, time of day, season, weather etc. but I'm surprised that I can find anything at all." I don't think your answer actually covers this, mine yes, from the initial post. Conversion irradiance --> insolation, Wikipedia is clearer and mentions Earth rotation and night. For the Earth budget (which is secondary to the question), I added it after you, you are right. Is this a bad practice? – mins Feb 19 '15 at 18:45

You are asking about the Earth's radiation budget. (Google that phrase. There's a lot of information out there on this topic.) NASA and NOAA have been using remote sensing to study the Earth's radiation budget for at least forty years.

Note that the above has the incoming solar radiation at 340.4 watts/meter2. Compare that with your value of 1372 (which is a bit high). Your value and the NASA value differ by a factor of four. The reason is simple: The flux intercepted by the Earth is proportionate to $\pi {R_e}^2$, but the surface area of the Earth is $4 \pi {R_e}^2$. Dividing the solar constant by four yields the incoming solar flux to the top of the atmosphere, averaged over ten years and over the surface of the Earth. About 29.3% of the incoming solar radiation is reflected back into space. Most of the reflection is by clouds. The atmosphere isn't perfectly clear. It absorbs another 22.6% of the incoming solar radiation. A bit less than half (48.0%) makes it's way all the way to the surface.