Why does it seem that my air conditioning does not work as well on hot days when I really need it?

As the temperature and humidity increase the air conditioner actually has to work a lot harder to pull the moisture and the heat out of the air to make it feel cool in the cabin.  Typically and efficient and effective vehicle AC would see a 30 degree drop from the outside temperature.  So when the temperature reaches upper 90 the 30 degree difference doesn't feel as cool as it did when it was only 80.  The higher the humidity the less of a temperature difference you will see.