VFR flight is not allowed in airspace known as "class A", regardless of the meteorological conditions except after failure of two way radio communications or during declared emergencies such as VFR traffic attempting to avoid severe weather formations. In the United States, class A airspace is measured using flight levels, and begins at FL180 up to FL600, which is about 18,000 to 60,000 feet as measured using an altimeter at standard pressure (29.92 inHg, 1013 mbar). An exception to IFR-only flying in this airspace is the occasional allowance of sailplanes within designated "wave windows" which may be opened by air traffic control when high altitude flights are to be conducted into mountain Lee waves.