UV Index - What Is It?

The UV Index is a next day forecast of the amount of skin damaging UV radiation expected to reach the earth's surface at the time when the sun is highest in the sky (solar noon). 

The amount of UV radiation reaching the surface is primarily related to the elevation of the sun in the sky, the amount of ozone in the stratosphere, and the amount of clouds present. The UV Index can range from 0 (when it is night time) to 15 or 16 (in the tropics at high elevations under clear skies). UV radiation is greatest when the sun is highest in the sky and rapidly decreases as the sun approaches the horizon. The higher the UV Index, the greater the dose rate of skin and eye damaging UV radiation. Consequently, the higher the UV Index, the smaller the time it takes before skin damage occurs.

Diurnal Variability

The variation throughout the course of the day (diurnal) of UV radiation is much like that of visible light. UV radiation is much more attenuated at high solar zenith angles in the early morning and late afternoon. This is because the two components which make up UV and all other forms of radiation; the direct and the diffuse, are both greatly affected at these low sun angles. Direct UV radiation is greatly reduced by the increased absorption by stratospheric ozone during its increased path length through the atmosphere (about 6 times more than when the sun is directly overhead). Also, radiation at the UV wavelengths is scattered much more than visible light. This further decreases the direct component and increases the diffuse component. As the Sun rises above the horizon, the amount of absorption in the stratosphere and scattering in the troposphere is reduced. The result is a drastic increase in UV radiation reaching the surface.

The variation of surface temperature differs significantly from both UV and global radiation. Whereas the diurnal variation of incoming infrared radiation is similar to that of global radiation, there exists a delay between the time when the earth-atmosphere system is irradiated to when the temperature begins to increase. This is know as the thermal response. Depending upon the time of the year the lag in time between the peak radiation flux reaching the surface (solar noon) to when the surface temperature reaches its maximum can be as great as 3 to 4 hours. For example, have you noticed that it is hottest in the mid afternoon and not at noon? A typical summer surface temperature plot shows that peak temperature is reached later in the afternoon. By the time of the temperature is at maximum, the amount of UV radiation reaching the surface has decreased almost by half of that at solar noon.

UV Index: How is It Computed?

In the United States (unlike some countries), the NOAA/ EPA UV Index is not based upon surface observations. Rather, it is computed using the forecasted ozone data, a radiative transfer model, forecasted cloud amounts and the elevation of the forecast cities.

Total ozone amounts for the entire globe are obtained via the TOVS (TIROS Operational Vertical Sounder) or the SBUV/2 (Solar Backscatter UltraViolet/2) instruments on board NOAA polar orbiting satellites. The observed data is then used to produce a forecast of the ozone data for tomorrow. This is done using the thermal-dynamical relationship between total ozone and heights at 100 and 500 hPa and temperatures at 50 hPa. NCEP provides the necessary analyzes and forecasts to used to determine the forecasted ozone data. This forecasted ozone data has been shown to be much more accurate than just using persistence. A radiative transfer model is used to determine the UV irradiances from 290 to 400 nm, using the time of day (solar noon), day of year, and latitude. The irradiances are weighted by the McKinlay-Diffey Erythema action spectrum (weighting function) so as to reflect the human skins response to each wavelength. These weighted irradiances are integrated over the 290 to 400 nm range resulting in the erythema dose rate. So from the total ozone amounts, erythema dose rates are determined. The erythema dose rates are adjusted for the effects of elevation( an increase of about 6% per kilometer).

The NCEP forecast models also provide forecasts of the cloud conditions at the time the UV Index would verify at. We have determined from earlier data that:

• Clear skies allow 100% transmission of UV radiation to the surface.

• Scattered cloud conditions allow 89% transmission of UV radiation to the surface.

• Broken cloud conditions allow 73% transmission of UV radiation to the surface.

• And overcast cloud conditions allow 32% transmission of UV radiation to the surface.

Given these transmission amounts, the forecast cloud conditions can be converted into a forecast transmission. This percentage is then applied to the elevation adjusted erythema dose rate at each of the forecast cities. This value is then scaled (divided) by the standard of 25 milliWatts per square meter. This results in a number that usually ranges from 0 (where there is no sun light) to the mid 'teens. This value is the UV Index.

Currently, the computation of the UV Index does not include the effects of variable surface reflection, atmospheric pollutants or haze.

UV Index: How to use it!

Because the UV Index is a forecast of the probable intensity of skin damaging ultraviolet radiation reaching the surface during the solar noon hour (the greater the UV Index is the greater the amount of skin damaging UV radiation). 

How much UV radiation is needed to actually damage one's skin is dependant on several factors. But in general the darker one's skin is, (that is the more melanin one has in his/ her skin) the longer (or the more UV radiation) it takes to cause erythema (skin reddening). As a very general guide: for those who always burn and never tan the times to burn are relatively short compared to those who almost always tan.

The (again very general) guidelines as far as what to do to protect oneself from overexposure to UV radiation are shown in the table below.

Exposure Category UV Index Protective Actions Minimal 0, 1, 2 Apply skin protection factor (SPF) 15 sun screen. Low 3, 4 SPF 15 & protective clothing (hat) Moderate 5, 6 SPF 15, protective clothing, and UV-A&B sun glasses. High 7, 8, 9 SPF 15, protective clothing, sun glasses and make attempts to avoid the sun between 10am to 4pm. Very High 10+ SPF 15, protective clothing, sun glasses and avoid being in the sun between 10am to 4pm.

Effects of Clouds, Elevation, and Surface Pollution?

Clouds, air pollution, haze and elevation all have affects on the amount of ultraviolet (UV) radiation reaching the surface. UV radiation reaches the surface as a sum of its direct component (normal to the sun) and its diffuse component (from all directions). UV radiation reaches the top of the troposphere in mostly its direct component. This is because there are few molecules to scatter the radiation. Decreases in UV radiation intensity has resulted due to absorption by ozone. Once the UV radiation reaches the troposphere it encounters much greater numbers of scattering air molecules and dust. Below are how UV radiation is affected by:

Elevation: In the troposphere, air molecules and dust increase as the UV radiation travels from the stratosphere to the troposphere. The further down in to the atmosphere UV radiation travels, the more the direct component is reduced and the more the diffuse component is increased. As more UV radiation is scattered, the smaller the amount that reaches the surface. As a result, there is more UV radiation at higher elevations than at lower elevations.

Clouds: Made up of millions of water droplets, clouds can transmit, reflect and scatter UV radiation. The amount of each is dependant upon the thickness of the cloud and its morphology. Generally, the larger and thicker the cloud is the lesser amount of UV radiation that is transmitted. UV radiation can and does reflect off the sides of towering cumulus clouds. Such conditions result in actual enhancements of surface UV radiation.

Dust/Haze: These two conditions act on UV radiation the same way. They both scatter UV radiation. Enough UV radiation is scattered that on hazy or dusty days there is less UV radiation reaching the surface than would otherwise be there on a clear day.

Air Pollution/Smog: This encompasses many greenhouse gases. Emissions from traffic and manufacturing plants form smog as UV radiation and heat cause the necessary chemical reactions to take place. As a result, the amounts of UV radiation reaching the surface is smaller under these conditions.