Why is the valley cooler in the evening

Well cooler air is heavier or denser therefore it will sink to the lower area. And the reason it is cooler at night than during the day is due to the sun's electromagnetic radiation which does not reach the dark side of the earth. So the atmosphere at night  emits more energy into space than it absorbs therefore the tempature increases.

The changes to agriculture in the united states since 1900 is that increased mechanization has increased food production, thereby increasing the need for labor. there has been an increased need for higher speeds of food production due the fact that the population in the united states continues to rise.