The troposphere is the lowest and densest part of the atmosphere and is characterised by a decrease in temperature with altitude. The temperature falls from about at the base of the nominal troposphere at −300 km to at 50 km. The temperatures in the coldest upper region of the troposphere (the tropopause) actually vary in the range between depending on planetary latitude. The tropopause region is responsible for the vast majority of Uranus's thermal far infrared emissions, thus determining its effective temperature of .