500
High clouds are composed primarily of of ice crystals, whereas low clouds are composed primarily of water droplets. This difference is most likely because the average air temperature at altitudes from this.
(Science pg 45)
What is 0 km to 3.2 km is above 0 degrees Celsius, whereas the average air temperature at altitudes from 6.0 km to 16.0 km is at or below 0 degrees Celsius?