To better their standard of living, climate or environmentally induced migration, exile, escapes from prejudice, conflict or natural disaster, or simply the wish to change one's quality of life. Commuters, tourists and other short-term stays in a destination country do not fall under the definition of immigration or migration, seasonal labor immigration is sometimes included. As of two thousand fifteen, the number of international migrants has reached 244 million worldwide, which reflects a 41% increase since the two thousand. One third of the world's international migrants are living in just twenty countries. Push factors refer primarily to the motive for immigration from the country of origin. In the case of economic migration, differentials in wage rates are common. If the value of wages in the new country surpasses the value of wages in one's native country, he or she may choose to migrate, as long as the costs are not too high. As transportation technology improved, travel time and costs decreased dramatically. When the opportunity cost is lower, the immigration rates tend to be higher. Escape from poverty (personal or for relatives staying behind) is a traditional push factor, and the availability of jobs is the related pull factor. Natural disasters can amplify poverty-driven migration flows. Research shows that for middle-income countries, higher temperatures increase emigration rates to urban areas and to other countries. For low-income countries, higher temperatures reduce emigration. Emigration and immigration are sometimes mandatory in a contract of employments: religious missionaries and employees of transnational corporation, international non-governmental organizations, and the diplomatic service. This is expecting, by definition, to work overseas. They are often referred to as expatriates, and their conditions of employment are typically equal to or better than those applying in the host country. For some migrants, education is the primary pull factor