station. The transition altitude (TA) is the altitude above sea level at which aircraft change from the use of local barometer derived altitudes to the use of flight levels. When operating at or below the TA, aircraft altimeters are usually set to show the altitude above sea level. Above the TA, the aircraft altimeter pressure setting is normally adjusted to the standard pressure setting of 1013.25 hectopascals (which is equivalent to millibars) or 29.92 inches of mercury and aircraft altitude will be expressed as a flight level. In the United States and Canada, the transition altitude is . In Europe, the transition altitude varies and can be as low as . There are discussions to standardize the transition altitude within the Eurocontrol area. In the United Kingdom, different airports have different transition altitudes, between 3000 and 6000 feet. On 25 November 2004 the Civil Aviation Authority of New Zealand raised New Zealand's transition altitude from and changed the transition level from FL130 to FL150. The transition level (TL) is the lowest flight level above the transition altitude. The table below shows the transition level according to transition altitude and QNH. When descending below the transition level, the pilot starts to refer to altitude of the aircraft by setting the altimeter to the QNH for the region or airfield. The transition layer is the airspace between the transition altitude and the transition level. According to these definitions the transition layer is thick. Aircraft are not normally assigned to fly at the "'transition level'" as this would provide inadequate separation from traffic flying on QNH at the transition altitude. Instead, the lowest usable "'flight level'" is the transition level plus 500 ft. However, in some countries, such as Norway for example, the transition level is determined by adding a buffer of minimum (depending on QNH) to the transition altitude. Therefore, aircraft may be flying at both transition level and transition altitude, and still be vertically separated by at least . In those areas the transition layer will be thick, depending on QNH. In summary, the connection between "transition altitude" (TA), "transition layer" (TLYR), and "transition level" (TL) is TL = TA + TLYR Semicircular/hemispheric rule The semicircular rule (also known as the hemispheric rule) applies, in slightly different version, to IFR flights in the UK inside controlled airspace and generally in the rest of the world. The standard rule defines an East/West