In this paper, we experimentally investigate the effect of fast moving object on the RSSI in the wireless sensor networks in presence of the ground effect and antenna orientation in elevation direction. In experimental setup, MICAz mote pair was placed on the ground, where one mote acts as a transmitter and the other as a receiver. The transmitter mote's antenna was oriented in elevation direction with respect to the receiver mote's antenna. The fast moving object i.e. car, was passed between the motes and the fluctuations in the RSSI are observed. The experimental results show some sequential pattern in RSSI fluctuations when car moves at some relatively slow speed. However, some irregularities were also observed when antenna was oriented at 45 and 90 in elevation direction.