Exploring Our Fluid Earth

Weather and Climate

Large-scale atmospheric and water circulation patterns affect weather events, such as hurricanes, tornadoes, and monsoon rains. Weather is the condition of the atmosphere over a short period of time—typically day-to-day activity in terms of precipitation and air temperature. A weather forecaster or meteorologist might report the expected high and low temperature, cloud cover, or expected precipitation for a specific region for the next few days (Fig. 3.29 A). Weather reports describe the behavior of the earth’s atmosphere on the time scale of minutes to possibly months (e.g., comparing air temperatures at noon and midnight; comparing rainfall measurements in April and August).

In contrast, climate refers to general, average trends or patterns in weather over long periods of time. Climate scientists, or climatologists, study average temperatures, precipitation, wind, humidity, and other weather properties over large timescales in a given area. These long-term time scales are usually a minimum of decades, and can examine a particular area, or even the entire global climate. In contrast to a weather report, a climate scientist might compare air temperature in July over the past 100 years or even the past 1000 years (Fig. 3.29 B).

Fig. 3.29. (A) A weather forecaster describes local air temperature and precipitation conditions in the near future.

Image courtesy of National Oceanic and Atmospheric Administration (NOAA)

Fig. 3.29. (B) NASA climate data reporting average global temperature from the years 1880 to 2012

Image courtesy of Goddard Institute for Space Studies, National Aeronautics and Space Administration (NASA)

Surface Currents Affect Climate

Fig. 3.30. Water temperature affects the distribution of coral reefs worldwide. The dots on the map indicate areas where coral reefs are found.

Image courtesy of National Oceanic and Atmospheric Administration (NOAA)

Even in the summertime, swimming in the ocean off the coast of California is chilly. This is because the North Pacific Current sweeps downward along the west coast and brings cold water with it. Upwelling along the California coast also brings water up from the deep ocean and keeps water near the shore cool. On the Asia side of the Pacific ocean basin, coral reefs are found farther north than on the American side. This is in part because the warm Kuroshio Current sweeps north from the equator, bringing warm water with it. In contrast, on the American West Coast coral reefs do not occur until Baja California, Mexico where water temperatures are warm enough to sustain coral animals (Fig. 3.30).

In Great Britain, winter temperatures rarely drop below than –12°C (1°F). These winters are especially mild when you consider that the British Isles are found between 50° and 56° N latitude. This is about the same latitude as Moscow, Russia, which is known for an average winter temperature in Moscow as low as –10˚C (14˚F) and record cold temperatures of –42˚C (–44˚F). The relative mildness of British winters is because, even though Britain is situated on the cool side of the Gulf Stream, the current brings water that is much warmer than the cold air carried by the winter westerly wind. The warm side of the Gulf Stream also affects the Atlantic Coast of North America. Tropical fish can often be found off Long Island in New York because they are carried there by the warm water originating in the Caribbean.

Water Resists Temperature Change

Fig. 3.31. Tidepools at Barber’s Point, O‘ahu, Hawai‘i

Image by Joanna Philippoff

The oceans play a large role in climate at both a regional and global scale because water has an ability to resist sudden changes in temperature. Because of this, water is said to have a high specific heat. Specific heat is the amount of heat it takes to raise or lower the temperature of one gram of a substance by 1°C.

Because of the high specific heat of water, the ocean and other bodies of water warm much slower when the sun shines on them than nearby rocks, soil, and buildings For example, the rocks around a tide pool may be painfully hot to bare feet while the shallow water in the pool feels cool (Fig. 3.31). During the night, rocks, soil, and buildings lose their heat faster than bodies of water do. During the night, the rocks around a tide pool may feel chilly, whereas the water remains relatively warm.

Because water absorbs and releases heat at a rate much slower than land, air temperatures in areas near large bodies of water tend to have smaller fluctuations. The high specific heat of water, and the mixing that occurs as a result of wind interaction with water, help to moderate the temperature of land located near an ocean. Water temperatures near the ocean surface range from a little below 0°C in Arctic and Antarctic regions to about 30°C in the Red Sea. Most tropical waters stay at a fairly constant 28°C. By contrast, temperatures over the land range from –70°C to 60°C. In the desert, daily temperature variations are extreme. It can be very hot during the day and very cold at night. This is because deserts, by definition, lack large amounts of liquid water. Non-desert areas not located near bodies of water, such as Wyoming in the United States, can have wide ranges of temperatures throughout the course of the year compared to coastal cities.

Though at one location the water receives the same amount of sunlight as the land, water’s high specific heat keeps its temperature within a relatively narrow range. The daily temperature fluctuations of our entire planet are more moderate than they would be if we were devoid of water. Even individual organisms benefit from the high specific heat of water. Most living things are comprised of a high proportion of water and this helps them resist changes in their body temperatures.

Coastal Breezes

Water has an overall moderating effect on climate in terms of annual temperature ranges. Winds that come off the ocean tend to keep nearby coastal areas warm in winter and cool in summer.

In coastal regions along the ocean or a large lake, the same phenomenon operates on a shorter time scale. During the day, cooler, denser air above the ocean or lake sweeps in over the shore as air warmed by the land rises. Because of these breezes, on a hot summer day the beach area is much cooler than inland areas. During the night, breezes blow off the land as the warmer air over the ocean rises (Fig. 3.32).

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Image caption

Fig. 3.32. (A) Sea breeze, normally seen during the day and (B) land breeze, normally seen at night

Image copyright and source

Image by Byron Inouye

Winds Assist Heat Transfer on Earth

The surface of the planet is not evenly warmed by sunlight. Sunlight is concentrated at the equator and diffuse at the poles. However, the equator does not keep getting warmer nor are the poles getting colder. Wind mixing and the circulation of the world ocean regulate the inequality of heat reaching earth.

The surface of the ocean is kept relatively warm because sunlight is converted to heat. Sunlight is absorbed rapidly as depth increases. However, although the amount of available light decreases with depth, the temperature does not decline as rapidly. This is because extensive mixing occurs at the surface of the ocean. The ocean is stirred by friction created by wind at the surface, which helps to mix warmer surface water with cool deeper water. Langmuir currents are an example of wind-driven mixing or water circulation near the ocean surface. Mixing not only helps to regulate water temperatures, but also redistributes dissolved nutrients, oxygen, and living microorganisms within the water column. Mixing in the ocean typically occurs between 25 and 200 m in depth but can extend even deeper. The amount of wind-driven mixing is affected by both water depth and wind speed. The stronger the wind at the surface of the ocean, the more intense and deeper the oceanic mixing. In shallower areas, such as continental shelves, the effects of mixing may reach all the way to the ocean floor

Ocean Circulation Moderates Climate

The world ocean and atmosphere transport are responsible for regulating heat across the planet through large-scale ocean and atmospheric circulation. Ocean circulation regulates heat so that the poles and equator remain at relatively constant temperatures, with the equator being generally warmer and the poles being generally cooler. In general, warm water is transferred from the equator to the poles and cooler water is transferred back towards the equator through ocean currents, wind, and thermohaline circulation (for a review of thermohaline circulation refer to the topic on Density Driven Currents). Wind currents can absorb or diffuse heat when they blow over the ocean, bringing relatively hot or cool air when they travel across continents. For example, Europe’s mild climate is due in part to westerly winds that blow across the continent after absorbing heat from warm ocean surface currents traveling towards the poles. This important, yet invisible, ocean current system is critical in making many places in the world habitable and keeping our climate comfortable.