Question

re word on a hot day, the air is much warmer than the air above it. Warm air is less dense than cool air, so
light waves travel faster through

Answer 1

Sure — here are several reworded versions you can use (short, formal, and explanatory options):

1. On a hot day, the air near the ground is warmer and therefore less dense than the air above it, so light travels faster through that lower layer.
2. When the surface heats the air, the warmer, less-dense air close to the ground lets light propagate faster than the cooler air higher up.
3. Warm air is less dense than cool air, so light waves move more quickly through the warmer layer of air near the ground than through the cooler air above.
4. Because heated air is less dense, light travels faster through the warm air at the surface than through the cooler air overhead, which can bend the light rays.
5. The air just above a hot surface is warmer and less dense than the air above it, causing light to travel faster through that lower layer.

Want one phrased for a specific audience (kids, scientific report, casual blog)?