Ultraviolet Dose Calculation Overview

Hourly Dose Values
The database generated for this project is derived from spectral irradiance calculations carried out for each hour of daylight for all days during the year at the prescribed locations, for the years 1979-1990. The hourly values are instantaneous estimates of spectral irradiance (W cm^-2 nm^-1) at each hour interval during the day, which are a function of the following atmospheric/geophysical data inputs and orbital parameters:

• daily ozone values from the TOMS gridded data set;
• seasonally- and zonally-averaged vertical ozone profiles from SBUV data;
• hourly cloud cover, ceiling height, and visibility conditions for each location from the SAMSON data archive;
• monthly-averaged cloud optical depth from the ISCCP-C2 gridded data set;
• seasonal profiles of temperature, pressure, and aerosol extinction from the LOWTRAN atmospheric model;
• elevation above sea level from the Rand gridded elevation data set;
• solar zenith angle at each hour;
• orbital eccentricity

More details on these model inputs can be found in the model description document (full text), or by selecting the individual links above. Once the spectral irradiance values are found, they are weighted by a biological action spectrum in order to find the biologically effective irradiance at each wavelength, since some wavelengths are more effective at producing a particular biological effect than others. Finally, these weighted irradiance values are integrated over the UV-B + UV-A wavelength range to determine the total biologically effective UV dose (given in W cm^-2) at the top of each hour. These values are presented as the hourly dose variables in the Ultraviolet Interactive Service (UVIS).

Daily Dose Values
Daily UV dose values given in the UVIS are simply the integrated sum of all the hourly UV dose values calculated initially (described above). The crude assumption being made in this process is that the dose values calculated at the top of each hour is invariant over a time interval spanning the previous half-hour and the following half-hour surrounding the top-of-the-hour estimate. This is, of course, not the case since local conditions vary on much shorter time scales (i.e. cloud cover) in addition to the constantly changing solar zenith angle. Calculations are limited to this time scale by the temporal resolution of the SAMSON meteorological data (hourly).

Monthly Dose Values
Monthly-averaged UV dose values are defined as the average of all daily UV dose values over the course of a given month. This figure is found by summing all daily dose values in the month and dividing by the number of days in the month. With these figures it is easier to qualitatively visualize long-term trends in UV exposure than with daily values alone.

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