San Francisco sits beside the ocean. Sacramento is 90 miles inland.

The beach in San Francisco in July: cool, foggy air from over the ocean.

Ocean water warms up slowly in summer and cools down slowly in winter.

So in July the ocean beside San Francisco is still cool, and in January it is still a little warm.

The air over the ocean is never very hot or very cold.

That air moves onto the land nearby, and it keeps San Francisco mild.

Air from over the ocean moves onto the land at San Francisco. Sacramento, 90 miles inland, does not get that air.

Sacramento is far from the ocean. Its land heats up fast in summer and cools down fast in winter.

A great lake does the same for the towns on its shore, a little.

Chicago sits on Lake Michigan, and the lake keeps Chicago's summers a little cooler than towns farther inland.