The mean is often known as the average, but it is only one of the measures of central tendency. To find it, add up all the values, and then divide by the number of readings you have made. For example, if you have forecasts for 6, 10, 7, and 2 for a four-day period, you would add those values together (25) and then divide by the number of readings (4), to get a mean UV Index forecast of 6.25.