When does sky become space?

The Kármán line (Karman line) is an official boundary between the Earth’s atmosphere and space, lying 100km (approximately 62 miles) above sea level. The governing body for air sports and aeronautical world records, Federation Aeronautique Internationale (FAI), recognizes it as the Line where aeronautics ends and astronautics begins.   

The line was named after Theodore von Kármán, (1881–1963) a Hungarian-American engineer and physicist who was active primarily in the fields of aeronautics and astronautics. He first calculated that around this altitude the Earth's atmosphere becomes too thin for aeronautical purposes (because any vehicle at this altitude would have to travel faster than orbital velocity in order to derive sufficient aerodynamic lift from the atmosphere to support itself, neglecting centrifugal force). There is an abrupt increase in atmospheric temperature and interaction with solar radiation just below the line, which places the line within the greater thermosphere.

Thin air also explains why the Earth‘s sky looks blue and space is black. Atmospheric gases scatter blue light more than other colours, turning the sky blue. At higher altitudes, less air exists to scatter light.

Some people (including the FAI in some of their publications) also use the expression "edge of space" to refer to a region below the conventional 100 km boundary to space, which is often meant to include substantially lower regions as well. Thus, certain balloon or airplane flights might be described as "reaching the edge of space". In such statements, "reaching the edge of space" merely refers to going higher than average aeronautical vehicles commonly would.