• The waves travelling in the atmosphere where gravity acts as the buoyant force.
• In the neutral atmosphere they are known as gravity waves, and in the ionosphere they are referred as TIDs.
  
• These are divided into two groups: 
  
• Large scale     :  velocity = 400-1000 (m/s)  wavelength = 1000 (km)  Time = 30 min-3 hours
• Medium scale :  velocity = 100-250 (m/s)    wavelength = few hundred (km) Time = 15-60 min

             Large scale waves propagate in the thermosphere while medium scale waves propagate in the 
         lower atmosphere.