Free-space optical (FSO) communications links are envisioned as a viable option for the provision of temporary high-bandwidth communications links between mobile platforms. The introduction of mobility to a technology originally intended for fixed communications links introduces several interesting challenges. Two of those challenges are the variation in receiver beam profile of the FSO link and variation in received optical power due to the constantly changing transmitter-receiver separation. This paper investigates how the provision of mobile communications using FSO technology is challenged by beam divergence and optical power levels. An analytical model of the receiver beam profile of a mobile FSO link is presented, along with simulation results of both beam profile and optical power levels for a temporary mobile FSO communications link.