In January, the majority of the land in the northern hemisphere is very cold, while the ocean is relatively warm. In fact, the warm ocean currents reach all the way up past the arctic circle and keep the United Kingdom and Norway relatively warm, while inland areas at the same latitude (Central Russia, Canada) are very cold. Why is that? Note also the map of ‘Normal annual range in temperature’. The greatest temperature ranges can be found on land, specifically in the center of continents. Why is this?
One part of the answer can be found in the differing abilities of land and water to absorb isolation from the sun. One property of every substance (such as rock or water) is its ability to absorb energy without heating up. Objects that can absorb a lot of energy without heating up very much are said to have a 'high specific heat'’ or 'high heat capacity'. Substances that can only absorb a little energy before they heat up are said to have a 'low heat capacity' or 'low specific heat' (you may recall the concept of specific heat from chemistry class). Heat capacity is the amount of energy needed to raise 1 gram of substance by 1º C. Water has a high heat capacity, while ‘land’ (rocks, vegetation, soil etc.) has a low heat capacity. In fact, the heat capacity of water is nearly four times greater than that of land. Recall our analogy from last week about walking barefoot on a concrete pool deck on a sunny day, then feeling the water, you will have an appreciation for this difference. The pool deck may be very hot to the touch, yet the water is chilly, though both have received the same amount of isolation!
Another reason that the temperature variations on land are much greater than those on water is that the water in the ocean circulates vertically. Fresh water added to the surface of the ocean from rivers, melting glaciers, and precipitation is less dense than the salty ocean water. This drives powerful vertical circulation currents that mix the surface water with cooler water from deep in the ocean.
The ocean and continent effects (called ‘oceanality’ and ‘continentality’) are the reason that coastal areas usually have a very mild climate (it does not change significantly year round) while inland areas have drastic swings in their annual temperatures. The ocean acts to regulate the temperature of seaside cities because the wind which blows over the ocean and onto the city is a uniform temperature year-round (because the ocean that it blows over is a uniform temperature). This is why cities that are on the ocean, such as San Francisco are cold and foggy year-round. Here in Los Altos Hills, we are in a valley, protected from the brunt of the ocean breezes by the coast mountains. Therefore, the land around us heats up and cools down more than in San Francisco or Half Moon Bay. San Francisco is at about the same latitude as Salt Lake City, Utah, an inland city. What are the differences in annual temperature between these two cities?