  Unmanned aerial vehicles~(UAVs) are envisioned to be an integral part of
future 5G communication systems. The agile nature of UAVs for serving users at
different locations can help to dynamically optimize coverage and
quality-of-service (QoS) in future networks. %However, there is very limited
literature available for mmWave communications using UAVs. In this work, we
explore the small scale temporal and spatial characteristics of mmWave
air-to-ground~(AG) line-of-sight~(LOS) propagation channels at $28$~GHz in
different environmental scenarios: dense-urban, suburban, rural, and over sea
using omni-directional antennas employing Wireless InSite ray tracing software.
We classify the received multipath components~(MPCs) into persistent and
non-persistent components. The small scale temporal and spatial characteristics
of the AG propagation channel are found to be dependent on the scatterer
properties: number, distribution, and geometry. Additionally, clustering of
MPCs in the time and spatial domain for different environments is found to be
dependent on the scatterer properties and receiver sensitivity. When the height
of the UAV is comparable to the height of the scatterers, we observe large
temporal and angular spreads.
