The k-factor represents the earth bulge atmospheric effect. It refers to the circular segment of earth profile that blocks off long distance communications. Since the geometric line of sight passes at varying heights over the Earth, the propagating radio wave encounters slightly different propagation conditions over the path. The usual effect of the declining pressure of the atmosphere with height is to bend radio waves down towards the surface of the Earth, effectively increasing the Earth's radius, and the distance to the radio horizon. This k-factor can change from its average value depending on weather.
k-factor < 1: increased earth bulge and shorter service range
k-factor > 1: less earth bulge.
The lower the k-factor is, the higher the towers need to be to successfully create a link that by-passes this earth bulge. The most common k-factor used for link profiles is 1.33, but weather can contribute to changing k-factor values. For example, storms create conditions for a lower k-factor and increased earth bulge.