previously inhabited place on Earth. Between April and September, the UV index typically ranges from 1 to 2, with May, June, and July having the highest UV index of 2. All other months average at 0, giving Longyearbyen a mean UV index of 0.75 over 12 months. Demographics , Longyearbyen had a population of 1,753 people. The largest regional group of Norwegians are from Northern Norway, particularly Nordland and Troms, who make up more than 40 percent of the population. Roughly 300 people (16 percent) are non-Norwegian citizens, with the largest nationalities being from Thailand, Sweden, Russia and Ukraine. Because of the dominance of the mining industry, the gender distribution is skewed, with 60 percent of adults being males. Longyearbyen has an over-average share of its population between 25 and 44 years old, but nearly no residents over 66. The number of children in relation to the population is at the national average, but Longyearbyen has significantly fewer teenagers than the national average. As of 2014, Thai people, numbering 120, were the second largest group of residents after Norwegians; there were 60 in 2006. Thais first came to Svalbard when men brought their wives from Thailand during the 1970s. In 2006, most of the Thai residents worked as cleaners, and as of 2011 most stay for terms of two to three years to save funds. In 2007, 10 students at the Longyearbyen School were Thai. The Thai community is active in numerous cultural events annually and motivated the establishment of a Thai supermarket. Longyearbyen experiences a very high turnover; in 2008, 427 people (23 percent) moved away from the town. The average person lived in Longyearbyen for 6.3 years, although it is 6.6 years for Norwegians and 4.3 years for foreigners. In 2009, about a quarter of the population had lived in the town since before 2000, and can thus be regarded as its permanent population. The longest-residing people tend to work in the mining industry, followed by local government employees. The shortest tenures are held by students and employees in higher education, tourism and the state. The turnover has created what the Norwegian government calls a "revolving door society". Most young newcomers leave within seven years, a turnover rate far greater than in any municipality on the mainland. Seventy percent of households consist of a single person, compared to forty-one percent on the mainland, giving an average 1.6 people