of distance above MSL. One mnemonic that can be used is: if it's an altitude you can fly there, if it's an elevation you can walk there, and if it's height that's how far a rock will fall before it hits the ground. Atmospheric sciences In weather and climate studies, measurements or simulations often need to refer to a specific height or altitude, which is naturally AGL. However, the values of geophysical variables measured in various places on the natural (ground) surface may not be easily compared in hilly or mountainous terrain, because part of the observed variability is due to changes in the altitude of the surface. For this reason, variables such as pressure or temperature are sometimes 'reduced' to mean sea level. In general circulation models and global climate models, the state and properties of the atmosphere are specified or computed at a number of discrete locations and heights. When the topography of the continents is explicitly represented, the altitudes of these locations are set above the simulated ground level. This is often implemented using the so-called sigma coordinate system, which is the ratio of the pressure at a location (latitude, longitude, altitude) divided by the pressure at the nadir of that location on ground surface (same latitude, same longitude, altitude AGL = 0). Broadcasting In broadcasting, altitude AGL has relatively little direct bearing on the broadcast range of a station. Rather, it is HAAT (the height above the average terrain (in the surrounding area)) which is used to determine how far a broadcast station (or any other sort of VHF or higher radio-frequency) transmission will travel. From aviation safety perspective though, the more important aspect is the height of the radio tower used to support the radio antenna. In this case, height AGL is the only important measurement for aviation authorities, which require that some tall towers have proper Aircraft warning paint and lights to avoid collisions. See also Minimum safe altitude References External links AGL in aviation Altitudes in aviation Aviation meteorology Clouds Broadcast engineering