The Aeon Index is the UV index for aging: a measure of the ultraviolet A (UVA) light reaching the skin, the light that ages it, on a scale from 0 to 10, shown to one decimal place.
The UV index, the international standard, weights ultraviolet light by how strongly it reddens skin, so it measures the risk of sunburn and is driven mostly by UVB. Most of the ultraviolet light that ages skin is UVA, which the UV index largely leaves out. The Aeon Index measures UVA alone.
Its quantity is the UVA, from 315 to 400 nanometres, that arrives from the sun and the sky on open, level ground, in watts per square metre. One point on the index is 6.5 watts per square metre. The scale is fixed: a value means the same in every place and at every time of year.
What it is calculated from
The index is calculated for a location and a moment from three inputs.
Input
Where it comes from
What it gives
The sun’s height
Calculated from the location, the date and the time
How much UVA a clear sky lets through
The cloud cover
Apple Weather’s forecast for the location
How much of that UVA gets through the cloud
The UV index
Apple Weather’s forecast for the location
A check on the cloud: a high UV index shows that the sky is brighter than its cover suggests
The clear sky. The sun’s height gives the UVA a clear sky lets through, from the TUV model of the atmosphere: about 62.7 watts per square metre with the sun overhead, falling as the sun lowers.
The cloud. The cloud cover gives the share of that UVA that gets through, by the cloud factors the US Environmental Protection Agency uses for the UV index: all of it under a clear sky, about 31% under full overcast. Where the UV index shows that the sky is brighter than that, the brighter share is used.
The scale. The UVA that gets through is divided by 6.5 watts per square metre and shown to one decimal place.
The UV index can only raise the share that gets through, never lower it, because a low UV index can come from thick ozone, which barely affects UVA. It counts only when the clear sky’s UV index would be 3 or more, where its whole numbers are precise enough to read.
The Aeon Index under a clear sky, by the sun’s height.
The sun’s height
Aeon Index, clear sky
90°, overhead
9.6
75°
9.2
60°
7.9
45°
6.0
30°
3.6
15°
1.4
The scale and its words
Three words divide the scale, at 3 and 6.
Range
Advice
0
Low. The sun is down.
Below 3
Low. Sunscreen still recommended.
3 to 6
Moderate. SPF 30 or higher, and a hat or an umbrella.
6 and above
High. Stay in the shade if you can. If you go out, wear SPF 50 and a hat or an umbrella.
The UV index rule. The word is never Low while the UV index is 3 or more, whatever the Aeon Index reads.
The shadow rule. On a clear day the index reaches 6 when the sun is about 45° above the horizon, when a person’s shadow is as long as they are tall, and 3 when the sun is about 26° up, when the shadow is about twice their height.
The model can show how common forms of protection change the index. The table gives the index at a clear summer noon, with the sun 60° above the horizon.
The Aeon Index at a clear summer noon, by protection.
Protection
Aeon Index, 0 to 10
Bare skin
7.9
SPF 30, as people typically apply it
2.4
A sun umbrella (provisional)
1.9
SPF 30 and an umbrella
0.6
The sunscreen figure reflects two effects: people typically apply far less sunscreen than the amount used in laboratory testing, and in the European Union the UVA protection of a sunscreen is required to be at least one third of its SPF.
Limits
The Aeon Index is calculated, not measured: it comes from forecasts for a location, not from a sensor near the person. It is for general information, not medical advice.
Some constants, including the value for umbrellas, are marked as provisional and are stated as such wherever they appear. The model assumes the same skin for everyone. It allows for the height of the place above sea level, where UVA rises by about 3% for each kilometre. It does not account for light reflected from snow, water or sand, for shade, or for haze.
When the sun is below about 40° and thin cloud covers the whole sky, the UV index cannot tell thin cloud from thick, and the index can read lower than the light that arrives.
Sources
1International Commission on Illumination (CIE) and ISO 17166, the erythema reference action spectrum.
2National Center for Atmospheric Research (NCAR), the Tropospheric Ultraviolet and Visible (TUV) radiation model, version 5.3.
3US Environmental Protection Agency, the cloud factors of its UV index forecast.
4European Commission Recommendation 2006/647/EC, on sunscreen products and the UVA protection they should give.
5Alfio Parisi and colleagues, measurements of the UV that reaches people in shade and under umbrellas.