41 N America: Physical Geography II – Maritime and Continental Climates

Two of the most important influences on the temperature of a location are discussed elsewhere in this book. In Chapter 6, we examine the relationship between latitude and temperature. Generally, places at lower latitudes (closer to the equator) tend to be warmer than places at higher latitudes (closer to the poles). In Chapter 68, we analyze the relationship between elevation and temperature. Places at higher elevations are usually cooler than places at lower elevations. There is a third important influence on temperature – whether or not a location has a continental climate or a maritime climate.

Continental climates are found in locations that take on the temperature characteristics of a large land mass. They are characterized by extreme variations in temperatures throughout the year, featuring hot summers and cold winters. Maritime climates are found in locations that take on the temperature characteristics of a large body of water. They are characterized by much less extreme annual temperature variations, featuring relatively mild summers and winters. Maritime climates are sometimes called “marine” climates. Both terms are derived from the Latin word mare, which means “sea.”

To get a sense of the difference between continental and maritime climates, we’ll examine the temperature patterns of three American cities: Seattle, Fargo, and Boston. Seattle, Washington, is located in the northwestern U.S., situated on Puget Sound, just inland from the Pacific Ocean. Fargo, North Dakota, is in the north-central U.S., close to the dead center of the North American continent. Boston, Massachusetts, is located in the northeastern U.S., on the Atlantic Ocean. Seattle is the northernmost of the three cities, and Boston the southernmost, but they’re all located within a few degrees of latitude of one another. None of them are at significantly high elevations. So, based on latitude and elevation, they should have relatively similar climates. But one of them is quite different from the other two – Seattle has a maritime climate, while Fargo and Boston have continental climates.

City Temperatures

In the table above, you can get a sense of Seattle’s relatively mild maritime climate. Its average January low is 36˚F, which is not terribly cold, particularly given the city’s northerly location. The coldest it has ever gotten in Seattle is 16˚F, a temperature most Midwestern cities plunge below every single winter, often for days at a time. Now, consider Fargo. On a typical January day, Fargo drops to a frigid -2˚F, usually running about 38˚ colder than Seattle. And Fargo can get much colder than that. Its record low is a stunning -48˚F, which is 64˚ colder than Seattle’s record. The pattern is reversed in the summer. In July, Fargo is usually running several degrees warmer than Seattle, and regularly approaches or exceeds 100˚F. Seattle has never broken the 100˚F mark. The difference between a typical January day in Seattle and a typical July day is only 39˚F, while the difference in Fargo is 84˚F. The difference between Seattle’s record high and low is just 60˚F, while the difference in Fargo is a whopping 162˚F.

We’ll consider Boston’s numbers in a moment. But first, to get a sense of why Fargo and Seattle have such different temperature patterns, let’s take trip to the beach. Imagine that you are in Chicago in May, and you head to one of the beaches along Lake Michigan. It is the first truly hot day of the year, with plenty of sunshine. You take off your shoes and start walking across the sand. Even though it’s been cold for months, you find that the sand is blazing hot. You go sprinting across the sand, wade into the water, and get quite a shock. Even though it’s a hot day, the water is freezing cold.

Now, imagine that you return to the beach a few months later. It’s a cloudy, chilly day in late September. You take off your shoes and start walking across the sand. Even though it’s been hot for months, the sand is cool. You stroll across the sand, and wade into the water. Even though it’s a cool day, the water is relatively warm, much warmer than it was back in May.

What your two trips to the beach have taught you is that the sand tends to heat up very quickly in the spring, and cool down very quickly in the fall. The lake tends to heat up very slowly in the spring, and cool down very slowly in the fall. On a global scale, continents behave like the sand on the beach, and oceans behave like the water in the lake. Both Seattle and Fargo are located in the westerlies, so most of the air in these cities arrive from the west. That means that the air in Seattle arrives directly off the Pacific Ocean. The Pacific heats up very slowly in the spring and summer, keeping temperatures in Seattle relatively cool. Then, in the fall and winter, the Pacific loses its heat very slowly, keeping temperatures in Seattle relatively warm.

Fargo is located more than a thousand miles from the Pacific. By the time air masses arrive in North Dakota, they have lost any memory they had of the Pacific, and have taken on the temperature characteristics of the continent they’ve crossed. As soon as the sun starts rising higher in the sky in the spring, the North American landmass begins to heat up. It bakes in the sun all summer long. As a result, the air that arrives in Fargo is quite warm. In the fall, the continent loses heat rapidly, and turns frigid in the winter. Consequently, the air that arrives in Fargo is quite cold.

Much of the west coast of North America has a maritime climate. San Francisco, Portland, and Vancouver have climates that are much like Seattle, with mild winters and mild summers. Heat waves and snowstorms are rare in these cities. The American Midwest has a continental climate, as does much of Canada’s interior. Chicago, Toronto, Detroit, St. Louis, Cincinnati, Kansas City, Minneapolis, and Montreal are all similar to Fargo, well known for hot summers and cold winters.

Which brings us back to Boston. On the table above, you can see that Boston’s temperature patterns, while not quite as extreme as Fargo’s, more closely resemble Fargo than Seattle. Despite being located on the Atlantic Ocean, Boston still has a continental climate, as do other cities along the northeastern seaboard of the United States, such as New York, Washington, Philadelphia, and Baltimore. All of these cities experience hot summers and cold winters. That is because locations with maritime climates aren’t just on the ocean, they receive their weather from the ocean. Because the northeastern U.S. is located in the westerlies, it still receives most of its air masses from the vast continent to its west.

Cited and additional bibliography:

Bahamontez H, Manuel. 2017. Seattle. https://tinyurl.com/seattleharbor. Attribution-NonCommercial-NoDerivs 2.0 Generic (CC BY-NC-ND 2.0).