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.