Maritime vs Continental Climates and Microclimates

A maritime climate is one influenced by the sea, with mild winters, cool summers and a small annual temperature range; a continental climate is found far from the sea, with hot summers, cold winters, a large temperature range and less rain; and a microclimate is the local climate of a small area that differs from its surroundings. In brief: the difference arises because water heats and cools much more slowly than land. Continentality is the degree to which a place’s climate is affected by the land mass. Microclimates occur on the scale of a field, a valley or a city, caused by slope, vegetation, water bodies and buildings; the urban heat island is a well-known example.

Places at the same latitude can have very different climates depending on their position relative to the sea. This guide explains continentality, maritime and continental climates and microclimates, compares them in a table, works through numerical comparisons, outlines the history of the concepts and corrects common mistakes.

What is continentality?

Continentality describes how much the climate of a place is controlled by the land rather than the sea. Two properties of water cause the contrast. Water has a very high specific heat capacity (4,186 J per kg per °C, about five times that of dry soil), so it needs a lot of energy to warm up and loses a lot to cool down. The sun’s heat also penetrates and is mixed through a depth of water, and not just the surface layer, and some energy goes into evaporation. The land surface heats rapidly in summer and cools rapidly in winter. A continental climate, found in the interior of large land masses, such as the Great Plains of North America, central Asia and Siberia, therefore has: hot summers, very cold winters, a large annual temperature range (often over 25–30 °C or much more in Siberia), and low rainfall, mostly in summer, because moist air from the sea is far away. Moving inland from a west coast in the mid-latitudes, the temperature range generally increases and the rainfall falls.

What are maritime climates?

A maritime (oceanic) climate is found near the sea, especially on the western sides of continents in the mid-latitudes, where the prevailing westerly winds blow from the ocean (for example Ireland, the United Kingdom, the Pacific Northwest of North America and southern Chile). The sea acts as a heat store: it keeps the winters milder and the summers cooler, so the annual range is small, often only 10–15 °C. The rainfall is generally higher, well distributed across the year, with frequent cloud, and the weather is changeable. The sea’s influence decreases inland, with the effect extending farther where the wind blows from the sea and where there is no mountain barrier. Warm ocean currents add to the effect: the North Atlantic Drift keeps the coast of Norway ice-free at latitudes where eastern Canada is frozen in winter.

What are microclimates?

A microclimate is the climate of a small area, from a few meters to a few kilometers, that differs from the general climate of the region. Causes: slope and aspect (in the Northern Hemisphere south-facing slopes receive more sunlight and are warmer, and vineyards are placed on them); valleys where cold dense air drains downhill at night and forms “frost hollows” with fog; vegetation (a forest is shaded, cooler and more humid than open ground); water bodies (lakes moderate the temperature); and buildings and surfaces. The urban heat island is the warming of cities by several degrees compared with the surrounding countryside, because dark materials such as asphalt and concrete store heat, buildings trap heat and block the wind, there is less vegetation to cool by evaporation, and there is waste heat from vehicles and air conditioning. The effect is strongest on calm, clear nights. Gardeners, farmers and city planners use microclimates, for example by adding trees and green roofs to cool cities.

Climates compared

Worked example: coast and interior

1. Specific heat: water: 4,186 J/(kg·°C); dry soil: about 800 J/(kg·°C). The ratio is 4,186 ÷ 800 = 5.2, so the same energy heats a mass of dry soil by about five times as much as the same mass of water.
2. Coastal town (hypothetical): January mean 6 °C, July mean 18 °C. The annual range is 18 − 6 = 12 °C.
3. Interior town at the same latitude (hypothetical): January mean −12 °C, July mean 20 °C. The annual range is 20 − (−12) = 32 °C.
4. Comparison: the interior’s range is 32 ÷ 12 = 2.7 times larger. Its summer is 2 °C warmer (20 − 18), but its winter is 18 °C colder (6 − (−12)), which shows that the main difference is in the winter.
5. Urban heat island: a city center has a night-time temperature of 14.5 °C while the nearby countryside is at 12.0 °C. The heat island intensity is 14.5 − 12.0 = 2.5 °C.
6. Frost hollow: if cold air flows down and a valley floor is 4 °C colder than a hillside 60 m higher on a still, clear night, a gardener should plant frost-tender plants on the slope, not the valley bottom (the opposite of the usual temperature fall with height, since this is a temperature inversion).

How did the concepts develop?

The influence of the sea on temperature was recognized in the 19th century; Alexander von Humboldt drew the first maps of isotherms (lines of equal temperature) in 1817, showing that the temperature depends on more than the latitude. Indices of continentality were developed in the 20th century, based on the annual temperature range and latitude. The term “microclimate” was popularized by the work of Rudolf Geiger in his book The Climate near the Ground (1927). The urban heat island was first described by Luke Howard in London in the early 19th century, and it is now studied with satellite thermal images.

Which mistakes are common?

Mistake 1: Thinking that the sea makes places warmer

The sea makes the climate milder: warmer in winter, but cooler in summer.

Mistake 2: Assuming that all places at the same latitude have the same climate

Distance from the sea, currents, altitude and relief change it a great deal.

Mistake 3: Believing that continental climates are always dry

They usually receive less rain than coastal areas, but there can be a summer maximum and heavy snow in winter.

Mistake 4: Treating a microclimate as a different climate zone

A microclimate is a local variation within a climate zone.