Why is it that the air conditioning works only when car speeding?

The air conditioning in a car often works better at higher speeds because the car's alternator, which charges the battery and powers accessories like the AC, generates more electricity at higher engine speeds. The AC compressor is an electrically powered device, and needs sufficient power to operate effectively. At lower speeds, the alternator may not produce enough power to fully run the AC compressor, resulting in weaker cooling. Additionally, more airflow over the condenser (the part of the AC system that releases heat) at higher speeds improves its efficiency.