What might be responsible for the observed difference in temperature between the coastal cities and the valley cities?

Altitude is responsible for the difference in temperature

Explanation:

Height from the sea level is responsible for the difference in temperature between the coastal cities and the valley cities. The temperature of the cities which are located on the coast having moderate temperature due to low altitude while the temperature of valley cites are low due to height from the sea level. So we can say that altitude is the main factor of changing temperature between the coastal cities and the valley cities.