Climate and weather

Summary: Climate is the term for the average weather of a region, observed over a period of at least 30 years. Climatology is the scientific study of climate conditions, including temperature and precipitation of a defined climate zone, global atmospheric and oceanic currents.

Climatology comprises the recording of climate data (actual and historical), climate diagnostics regarding physical and chemical atmospheric processes, development of climate models, and forecasting consequences of anthropogenic impacts. Characteristic courses of one year describing temperature and precipitation are displayed in climate diagrams. Weather is based on similar data measurements but describes short-term events.

Weather

Meteorology is the scientific study of the atmosphere, investigating mainly the dynamics of weather events. Etymologically, the root of the term can be traced back to the Greek meteoron, referring to “above Earth floating air and phenomena.” As a branch of geosciences, meteorology studies physical and chemical atmospheric events and phenomena, including interactions with the Earth’s surface and solar radiation. Spatial-temporal studies range from microturbulences to general weather to climatology; the latter comprises more comprehensive interdisciplinary research.

The atmosphere is the most unstable and fastest-changing subsystem of the climate system, although it is relatively thin: The stratosphere reaches about 31 miles high, which is less than 1 percent of the Earth’s radius of 3,963 miles. The atmospheric layers, from the Earth’s surface upward, are the troposphere, the stratosphere, the mesosphere, the ionosphere, and the exosphere. These layers can be classified with regard to their chemical, dynamic, thermal, and optical properties. Their vertical formation influences global weather and climate processes. With regard to meteorological research—which concerns mainly tropospheric and stratospheric events, because 99 percent of the entire atmospheric air mass is concentrated in these layers—temperature is the most important factor. Air density decreases with altitude. While at the surface a density of 1.225 kilograms per cubic meter can be measured, at the tropopause (the border between the troposphere and the stratosphere) air density is only 0.36 kilogram per cubic meter. Simultaneously, atmospheric pressure decreases from 1.013 hectopascals at the ground to about 200 hectopascals at the tropopause and only 1 hectopascal at the stratopause (the border with the mesosphere). Regardless of their other properties, the atmosphere’s layers are defined by their temperature profiles, and both density and pressure change depending on weather conditions. In extreme cases, the tropopause at Earth’s mid-latitudes is about 500 hectopascals.

The chemical composition of the atmosphere impacts its characteristic vertical temperature profile. Within the troposphere, from the surface up to the tropopause, temperature sinks from on average 15 degrees Celsius to minus 50 degrees Celsius, because the troposphere is heated primarily by absorbed solar radiation from the surface. Furthermore, reemitted thermal radiation is absorbed by greenhouse gases and thus is kept in the lower troposphere. In the stratosphere, temperature increases again, because solar radiation is partially absorbed by the ozone layer. Ozone results from photolysis of oxygen molecules because of very energetic ultraviolet radiation in the stratosphere. The higher temperatures in an upper atmospheric layer block cooler air from the lower troposphere from rising further.

Essential weather processes include evaporation, condensation, and atmospheric dynamics based on differences in temperature. Radiation energy not only heats the land surface but also causes evaporation: 90 percent of water vapor in the atmosphere is evaporated from oceans, rivers, and lakes. To evaporate one gram of water, 2.45 kilojoules of energy are required. Upon warming, humid surfaces form water vapor, rise up with warm air, condense into water upon cooling, and form clouds and precipitation. Converging air masses can result into heavy weather events, such as thunderstorms. Evaporation consumes energy, stored in water vapor as latent heat and released during condensation and rainfall.

Solar Radiation

Solar radiation and the resulting atmospheric interactions determine the climate of Earth. Interactions include atmospheric heat storage, heat convection, global wind systems, and ocean currents, distributing heat and precipitation over the planet’s surface. Altogether, the global climate system can be understood as a giant thermal power plant driven by solar energy. The radiation budget is the most important part of the Earth’s energy budget. About 30 percent of the incoming solar radiation (that is, an Earth’s albedo of 0.30) is reflected by clouds, air, and land surfaces, especially snow, to outer space. The remaining 70 percent is absorbed: about 20 percent by the atmosphere and 50 percent by land. The latter is reflected as thermal radiation and convection, heating the lower atmosphere.

Assuming that this energy would be completely reradiated to the outer space, the average temperature on Earth would be minus 18 degrees Celsius; the average is actually 15 degrees Celsius. At minus 18 degrees Celsius, water would be frozen and life as we know it would not exist. Also, without the thermal regulation by the atmosphere and the oceans, temperature would fluctuate and would be much more dependent on solar radiation (on the moon, with virtually no atmosphere, for example, the surface temperature fluctuates about 300 degrees).

Three factors influence the radiation balance. First, the amounts of incoming radiation can fluctuate because of changes in the sun’s activity or alterations of Earth’s orbit. Second, reflection can vary—for instance, because of aerosol particles released from volcanic eruptions. Third, concentrations of greenhouse gases change; again, volcanic eruptions can release carbon dioxide and sulfate compounds, for example. Earth’s actual average temperature of about 15 degrees Celsius is possible thanks to the natural greenhouse effect of the atmosphere. At this point it is worth noting that the main components of the atmosphere—nitrogen (78.1 percent), oxygen (20.9 percent), and argon (0.93 percent)—do not contribute to the greenhouse effect; instead, trace gases, such as water vapor, carbon dioxide, methane, and nitrous oxide, have a big impact on climate. These greenhouse gases reemit infrared radiation into the atmosphere as well as back toward Earth’s surface. Furthermore, the trace gas ozone plays an important role for life on Earth. In the lower atmosphere, it acts as a greenhouse gas; in the stratosphere, it absorbs the hazardous ultraviolet radiation of the sun.

Local and regional climate conditions depend on many factors. Earth’s surface may reflect an average 30 percent of solar radiation back into space, but regional conditions significantly affect local areas. When there is snow, reflected radiation may be as high as 40 to 90 percent; in deserts, 20 to 45 percent; in forests, 5 to 20 percent. Other factors that may affect reflected radiation include the wave angle of solar radiation reaching the surface as well as its intensity and duration, clouds and humidity, heat transport by wind, atmospheric layers, and so forth.

The particular wave angle of solar radiation at different latitudes causes warmer or colder temperatures and variable precipitation, resulting in the classification of distinct climate zones: the tropical rain climate zone, dry climate zone, temperate humid climate zone, cold-dry climate zone, boreal climate zone, polar climate zone, and mountain climate zone. Solar radiation and availability of water determine the plant cover essentially, and thus diverse habitats. Differences in temperature between cold and warm climate zones relate to congruent differences in air pressure, influenced by the Earth’s rotation, leading to the atmospheric circulation system, which transports energy from tropical latitudes to higher latitudes. Differences in radiation between night and day and summer and winter are taken into account for descriptions of atmospheric circulation. The term solar constant, given as 1,368 watts per square meter (W/m²) and describing the amount of energy reaching the outer atmosphere from the sun, suggests a constant solar radiation. In fact, solar radiation has increased during the past 4 billion years by about 25 to 30 percent, and it also undergoes periodic fluctuations.

Clouds influence climate a great deal. Lower clouds have a cooling impact because their reflection of solar radiation is higher than their absorption, whereas higher ice clouds (cirrus clouds) have a warming impact due to their absorbing properties. Aerosol particles often function as condensation cores during cloud formation. They also have a cooling effect because they reemit solar radiation. Their climatological effects are most significant during and shortly after volcanic eruptions, when sulfate aerosols are blown up into the lower stratosphere and there absorb solar radiation, causing temperature increases at that altitude while temperature at the surface decreases.

Rise in Temperature

The most remarkable climate feature during the last 1,000 years is the net increase in temperature by the end of the twentieth century caused by anthropogenic emissions of greenhouse gases. Apart from this peculiarity, global average temperatures fluctuated only about 0.5 degree Celsius. Prior to the Industrial Revolution, climatic changes were caused only by volcanic activities or changes in solar radiation, and the latter had an impact of about 0.2–0.4 degree Celsius. Volcanism may well have caused the Medieval Warm Period (between 950 and 1250), as well as the Little Ice Age (between 1550 and 1850). Studies of ice cores from Greenland and Antarctica demonstrated atmospheric impacts of volcanic eruptions.

Sulfate concentrations in certain ice layers show evidence of the eruptions of Mount Tambora in 1815 and Mount Krakatoa in 1883, as well as another extremely strong eruption, in 1259. There were two significant maxima of solar radiation during the past 1,000 years, one during the early Middle Ages and the other during the 20th century, while radiation was relatively low during the Little Ice Age. Its minimum occurred in the 15th century.

Atmospheric Oscillation and Wind Systems

With respect to atmospheric dynamics of the last decades, consequences of global warming include a fortification of the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) and a reduction of planetary waves. This causes less warm and ozone-rich air to reach the polar stratosphere during winter months, resulting in what is called climatic isolation of the stratosphere. Similar trends can be described for the Antarctic Oscillation. Altogether, global warming amplifies ozone depletion and retards the regeneration of the ozone layer, even after the banning of chlorofluorocarbons in the late 1980s.

Any change in ozone concentrations has a direct impact on temperature in the stratosphere. Reduced ozone causes a cooling in the stratosphere, significantly above the poles. During 1969 and 1998, the temperature at the higher latitudes of the Southern Hemisphere decreased about 6 degrees Celsius in 11 miles (18 kilometers) altitude. The lower polar stratosphere even cooled between October and November 1985 by about 10 degrees Celsius. Such changes in temperature have also had an influence on atmospheric dynamics. For instance, a significant amplification of the polar vortex was observed to be combined with a retarded timing of its collapse about one month later. Because of this feedback effect, given reduced ozone concentrations additionally amplify the polar conditions, leading to further ozone depletion.

The amplification of the polar vortex in the spring of the Southern Hemisphere one to two months later results in a stronger tropospheric Antarctic Oscillation, again causing a cooling above Antarctica, with the exception of the west Antarctic Peninsula where strong west winds reduce polar cold air. This cooling is also a result of lower thermal radiation resulting from stratospheric ozone depletion. Similar trends have been described for polar latitudes of the Northern Hemisphere. Furthermore, the fortification of circulation around Antarctica causes less precipitation between latitudes 35 and 50 degrees south. Most alarming is the reduction of rainfall by 15 to 20 percent in southwest Australia in the latter twentieth and early twenty-first centuries as a result of replacements of southward rain-laden cyclones. These examples demonstrate the influence of the stratosphere on climate.

At the equator, radiation is most intense; therefore, heating and evaporation at the equator are most intense too. Warm air expands, is less dense, and rises, thus transporting heat away from the surface, a process that is called convection. As it rises, this air cools down, water vapor condenses, and heavy tropical rainfalls occur. This area of daily, heavy rainfall is called the Intertropical Convergence Zone. The rising warm air leads to low pressure, causing suction of air from regions with higher air pressure. Equatorial air flows in this direction until it is chilled and sinks, inducing one of the global circulation systems, the Hadley cell, more populary known as the trade winds (based on its surface characteristics). The result for climate is that the circulating air transports heat from the equator toward the poles.

Because the rotation of the Earth redirects the circulation to the right (eastward), there are two Hadley cells, causing the northeast trade winds in the Northern Hemisphere and the southeast trade winds in the Southern Hemisphere. At the poles, similar circulations occur but are caused by reverse conditions: Cold air sinks above the poles and moves toward warmer regions, until it is warm enough to rise. The resulting polar cells are the reason for polar east winds. Between both systems lies a third, the Ferrel cells, where west winds are characteristic.

Ferrel cells, also referred to as midlatitude cells, do not comprise a real wind system; the west winds are more unsteady and stormier than the trades and the polar east winds are. They are much more a chaos of storms and weather systems, driven around the globe by jet streams from both sides of the west wind zones.

Wind systems distribute heat and evaporated humidity, including the absorbed latent heat of water vapor. They also determine precipitation and thus the availability of water. Without the wind systems, the tropical zone would be on average 15 degrees warmer and polar regions would be 25 degrees colder than they are now. The formation of rainfall is also influenced by the characteristics of the Earth’s surface. Where mountains block winds, air rises and it rains; conversely, in the rain shadows of mountains, deserts often develop.

Ocean Currents and Global Conveyors

In the upper 10 feet (3 meters) of the oceans, there is as much energy absorbed as in the entire atmosphere. Thanks to the high heat, the storage capacity of water in the oceans has a balancing impact on climate, mitigating differences in temperature between winter and summer. Wind systems account for about 80 percent of the entire heat distribution; the remaining 20 percent is distributed by oceanic streaming systems. Because of the predominant wind directions and the distracting power of the rotation of the Earth, round vortices occur in the big ocean basins, transporting warm water away from the equator and cold water to the equator.

These surface currents are related to bottom currents; besides being driven by winds, they are moved by means of the sinking of dense, cold, very saline water in the polar sea, forming a global conveyor belt. Climate in the Northern Hemisphere is influenced by this global conveyor belt. Without it, climate in Europe would be much colder, comparable to that of Newfoundland. One important current is the Gulf Stream, which begins in the Gulf of Mexico and transports an estimated 1.3 billion megawatts of energy as North Atlantic current from the tropics northbound. Thanks to the Gulf Stream, the palm trees can grow as far north as Scotland.

The ocean conveyor belt is driven by trades that push water from West Africa to America. It leaves the Gulf of Mexico and moves through the Florida Straits, heating as Gulf Stream water along the U.S. east coast. West winds, the Earth’s rotation, and the cold Labrador Stream distract the current and lead it as the North Atlantic current to Europe. Arriving at the European North Sea, the water is cooled; it also at this point has a high salinity due to evaporation. It sinks because of its increased density, which causes a suction so that the water keeps flowing. If winds are considered as a primary driving force, salinity and temperature determine the secondary driving force, called thermohaline circulation. As a deep current, the water flows back into the southern Atlantic, restarting the circulation.

There are several oceanic regions where cold water sinks: in the Labrador Sea, the Weddell Sea, and the Ross Sea. Hence, periodic temperature fluctuations occur both in the Atlantic and in the Pacific, whose relations to the ocean currents and influences on climate are an object of further research.

Researchers believe Atlantic Ocean currents might be weakening because of climate change. One of the most powerful surface-to-deep currents is the Atlantic Meridional Overturning Circulation (AMOC), which moves warm water north and cold water south. It is a vital component in maintaining climate balance. Researchers believe AMOC is slowing and that weakening could bring about devastating climate change. Hot areas could get hotter, while cold areas might get colder, and patterns of precipitation could be greatly altered. Furthermore, sea levels on the East Coast of North America could rise.

Bibliography

Aguado, Edward, and James E. Burt. Understanding Weather and Climate. 5th ed. Upper Saddle River, NJ: Prentice Hall, 2009.

Barry, Roger Graham, and Richard J Chorley. Atmosphere, Weather, and Climate. New York: Routledge, 2010.

"Decades of Data on a Changing Atlantic Circulation." National Centers for Environmental Information, 24 Apr. 2024, www.ncei.noaa.gov/news/decades-data-changing-atlantic-circulation. Accessed 2 Aug. 2024.

McIlveen, J. F. R. Fundamentals of Weather and Climate. New York: Oxford University Press, 2010.

"Slowdown of the Motion of the Ocean." National Aeronautics and Space Administration, 18 Mar. 2024, science.nasa.gov/earth/earth-atmosphere/slowdown-of-the-motion-of-the-ocean/. Accessed 2 Aug. 2024.