It's the warmest in the middle of the day for the reason that the sun typically tends to be brighter at that time. While in the evening, the sun is getting ready to go down and in the mornings the sun just came up, so it is not at its highest point so the rays are not as strong. So since the sun's rays are striking part of the planet straight during noontime, therefore giving us more heat, but is striking us at a viewpoint at night or in the morning, therefore giving us less heat.