Why is the air pressure higher in the morning than at noon?

The explanation should be: in the morning, because the temperature is low, the air belongs to gas, and it contracts when it is cold, so the density is high; At noon, the temperature is the highest during the day, and the density is low due to thermal expansion, so the air pressure is relatively low; There is an unchangeable theorem in geography: density is positively related to air pressure. So the air pressure is high in the morning and low at night.