First of all, sound is not instant. It has a speed, which is called the speed of sound. Sound travels in air at a speed of around 331 meters per second. This speed is called Mach 1. This is the speed of sound at 0 degrees Celsius. But for each degree of Celsius increase in temperature, the speed of sound speeds up by 0.6 meters per second. So the warmer it is, the faster sound travels. And the colder it is, the slower sound goes. This effect is caused by the movement of air. The warmer the air, the livelier it moves. The colder the air, the more sluggish it moves. So on chilly days, it’s harder for sound to travel.