# What are the units which I should use for these Rayleigh scattering equations?

1. Mar 1, 2012

### Jonsson

Hello there,

After chapter 4.2, it mentions some equations for approximating Rayleigh scattering. That is all well, however, it doesn't mention which units of measurement I should use, nor the value of the constants. This causes problems because I find it hard for me, because I have never worked in the field before.

So,

- What is the average value and unit of measurement for I at the top of the atmosphere for visible electromagnetic radiation?
- K, constant for the standard atmosphere. I could not find this anywhere.
- Molecular density at sea level.
- Wavelength.

Kind regards,
Marius

2. Mar 1, 2012

### Born2bwire

The units are whatever you wish to use. Looking at say, Equation (1) we notice that the right-hand side is a product of intensity and other variables and functions. Since the left-hand side is intensity too, you can easily see that your choice of units cancel out as long as you remain consistent.

As for the molecular number density, you could probably get it from the table on this page:

http://en.wikipedia.org/wiki/Number_density

As for the rest, they give you the equations to calculate K.

3. Mar 2, 2012

### Jonsson

Thanks for that.

I am not sure I understand the implications regarding 'remaining consistent', in this case.

Can I for example use an RGB colour model where wavelength 450 is blue, 550 is green and 650 is red, and calculate using intensity at the top of the atmosphere of 255?

Or it is necessary to use a metric intensity unit, and scale the value up to my 8bit 0-255 value?

Which unit do you recommend for intensity?

Kind regards,
Marius

Last edited: Mar 2, 2012
4. Mar 2, 2012

### Jonsson

Upps, double post. My mistake. I am sorry.M

5. Mar 2, 2012

### Born2bwire

You can use any unit for intensity since the right hand side is intensity multiplied by a dimensionless factor. You input an intensity and get out an intensity. You just need to be consistent in the units you use. If you decide to use meters as you unit of length, then all variables that use length (wavelength, distance) must be in meters so that the units cancel out accordingly.

6. Mar 2, 2012

### Jonsson

Thank you.

Can you think of a seach phraze which I can type into google where I may find an article explaining the relevance of dimensionless factor?M