# I Estimating UVA & UVB from UVI index

1. Mar 2, 2017

### simonbour

Hi all,

I am interested in estimating the amount of UVA and UVB from the UVI index for parctical purposes.
I have found 2 research papers that shed some light on the matter (see [1][2]).
Let us define first UVB as the solar irradiance integral in the range 280-315mm
and UVA as the solar irradiance integral in the range 315-400mm

From [1] it is established that for zenith angles < 70deg the following relationship hold with an accuracy > 90%:
UVB[W/m2] = 18.9 UVI

From [2] it is established, for a specific region (Kuwait), that the relationship between UVA and UVB for zenith angles < 50deg is:
UVA = 41 UVB

Here comes the part where I hit the wall. Doing a simple google search on weather condition today in Kuwait, I get and UVI = 6, this would imply (for zenith angle < 50deg):

UVB = 113 W/m2
UVA = 4649 W/m2

This is way to much radiation in the UV band. Even at the Atacama Desert, full-spectrum irradiance values only go as high as ~1300W/m2

So I am left with 3 options (not mutually exclusive):
1. Paper [1] is wrong
2. Paper [2] is wrong
3. I am wrong

Any thoughs?

Simon.

References:
[1] McKenzie et al. Relationship between UVB and erythemally weighted radiation
[2] Kollias et al. The value of the ratio of UVA to UVB in sunlight

2. Mar 4, 2017

### John Park

From Wikipedia "Ultraviolet":

"Sunlight in space at the top of Earth's atmosphere (see solar constant) is composed of about 50% infrared light, 40% visible light, and 10% ultraviolet light, for a total intensity of about 1400 W/m2 in vacuum.[20]

"However, at ground level sunlight is 44% visible light, 3% ultraviolet (with the Sun at its zenith), and the remainder infrared."

This suggests no more than about 40 W/m2 total uv at the surface.

Are you sure those units were W/m2?

Since everything hinges on the calibration of UVI, I'd look there first. A factor of nearly 20 doesn't leave much room for UVI to grow. Could the relations be for example latitude dependent?

3. Mar 5, 2017

### simonbour

Thank you for the answer John. I actually got in touch with the author paper [1] and there was a numeric error that propagated throughout the publication. The correct
relationship is:
UV-B [W/m2] = 0.189 UVI
So for the previous example (UVI = 6) , the correct result would be:
UV-B [W/m2] = 1.3
UV-A [W/m2] = 46.5

I would expect it to be dependant upon several factors but this are rough approximations. In the first case it has an error of 10% (assuming sza<70deg and 250<DU<400), I don't remember the details about the second.

All the best,

Simon.

4. Mar 6, 2017

### John Park

I assume that second UV-B value should be 1.13 not 1.3?

Glad that things seem to be resolved. And good luck.