Although there is no well-defined solid surface within Uranus's interior, the outermost part of Uranus's gaseous envelope that is accessible to remote sensing is called its atmosphere. Remote-sensing capability extends down to roughly 300 km below the level, with a corresponding pressure around and temperature of . The tenuous thermosphere extends over two planetary radii from the nominal surface, which is defined to lie at a pressure of 1 bar. The Uranian atmosphere can be divided into three layers: the troposphere, between altitudes of and pressures from 100 to 0.1 bar (10 MPa to 10 kPa); the stratosphere, spanning altitudes between and pressures of between (10 kPa to 10 µPa); and the thermosphere extending from 4,000 km to as high as 50,000 km from the surface. There is no mesosphere.