This is also a typical temperature pattern observed during a balloon’s ascent into near space. The air temperature of the troposphere (the lowest layer of the atmosphere) decreases as the balloon climbs upward, or away from its source of heat. Once the balloon enters the stratosphere (at around 50,000 feet) however, the air temperature increases due to the presence of ozone and solar ultraviolet radiation. The constant temperature observed between 40,000 and 50,000 feet appears in many flights and marks the boundary between the troposphere and stratosphere, which is called the tropopause.