“The atmosphere is a continuous mass resting on the earth and the sea, and they react upon each other.” ~ Swedish meteorologist Hugo Hildebrandsson
All the planets in the solar system have atmospheres. Venus is covered by a dense, stifling blanket of carbon dioxide, with an atmospheric pressure 90 times that of Earth. Mars too has an atmosphere of CO2, but it is a thin coat: only 1% the pressure on this planet.
The gaseous onion of Earth’s biosphere is the least weighty: one billion megatons. The ocean weighs 1,000 times as much; the physical earth a million times more.
Like all other aspects of the planet over its 4.55 billion-year existence, Earth’s atmosphere has evolved. Its present composition is 78% nitrogen, 21% oxygen, 0.93% argon, and trace gases, most notably carbon dioxide (0.04%).
Earth’s atmosphere is especial in the solar system, for the relative abundance of oxygen and scarcity of carbon dioxide. This owes to photosynthetic life, particularly plants.
The atmosphere acts as an exchange medium for Earth’s elemental cycles, such as carbon and oxygen. A viable atmosphere, as much as clean water, is a requisite for life.
The atmosphere protects life by absorbing ultraviolet radiation. The atmosphere’s greenhouse effect retains heat, warming Earth’s surface, as well as reducing diurnal temperature variation: the temperature extremes between night and day.
Given its orbital station about the Sun, Earth is naturally given to frigidity. Its atmosphere breathes the possibility of life in temperate climates.
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The atmosphere has 5 layers: the exosphere, thermosphere, mesosphere, stratosphere, and troposphere. While the exosphere is sparse, the other layers have varying layers of density which turbulently fluctuate via internal waves that propagate between high- and low-density layers. Internal waves are a natural fluid coherency that also appears in the ocean and other waters.
In the atmosphere, internal waves are the reason why weather forecasts go awry. These waves deposit and move energy. Through the butterfly effect, small internal waves can create significant, weather-changing dynamics.
“When forecasters don’t account for them on a small scale, then the large-scale picture becomes a little bit off, and sometimes being just a bit off is enough to be completely wrong about the weather.” ~ American mechanical engineer Julie Crockett
The outermost exosphere interfaces outer space, theoretically halfway to the Moon (190,000 km). The exosphere is mostly dispersed hydrogen and helium, along with stray molecules of carbon dioxide and atomic oxygen toward the lower boundary, which ranges from 200–500 km, depending upon the solar wind.
The thermosphere begins 80 km above the Earth’s surface. At such high altitude, residual gases sort into strata according to molecular mass.
Solar radiation excites the residual oxygen ever more as altitude climbs, heating the sparse gas in the upper thermosphere to 2,800 K, depending upon the flux of solar activity. Yet a thermometer would measure the thermosphere below freezing, because it is so near vacuum.
Above 160 km, gas density is so low that molecular interactions are too infrequent to carry sound (the anacoustic zone).
The thermosphere is vital to life on Earth. Conditions there determine the net loss of hydrogen: the hotter, the greater the loss. Since water is the atmospheric source of hydrogen to the thermosphere, losing hydrogen means planetary water loss.
The dynamics of the lower thermosphere are driven by the atmospheric tide, analogous to the ocean tides. Variations in diurnal heating swagger the atmospheric tide. The tide rides below 120 km; above that, gas concentration is too thin to support coherent fluid flow.
At 53–85 km, the mesosphere is atmospheric drama incarnate. Most meteors burn up there. Fierce sunlight cracks air molecules like a squirrel cracks nuts. Water molecules violently split into hydroxyl radicals (HO) and hydrogen atoms (H). CO2 cleaves into CO and a lonely O.
The stratosphere is 10–50 km above Earth’s surface. Living up to its name, the stratosphere is stratified. Higher up, the stratosphere hots up.
As to the heat: it’s the ozone (O3); or not. In the lower strata none forms. The energy of sunlight is not strong enough to form ozone there.
In the mid-strata, the ozone layer forms as O and O2 combine when they meet. Higher up, O3 is whacked into diatomic (O2) and atomic (O) oxygen by sunlight. At different altitudes, depending upon its intensity, sunlight creates and destroys ozone.
Stratospheric ozone provides a protective layer to life below. O3 absorbs nearly all shorter wavelength ultraviolet sunlight (< 32 nm). The ozone layer is at a higher altitude in the tropics, and at its lowest in the polar regions.
The strata of the stratosphere are generally stable, with little convective turbulence. Variations in the jet stream, along with other wind shear, stir the lower stratosphere.
That stir can be quite significant at times. Gyral variations in stratospheric winds directly perturb the layer below: the troposphere. They also affect deep-ocean currents.
The stratosphere is somewhat different at the north pole. The polar vortex comprises the stratospheric winds that encircle the Arctic. These winds can extend from the lowest stratospheric layer to beyond the top of the stratosphere.
Polar vortex patterns typically last 2 years. Sudden warming events can occur, with these winds weakening or even changing direction. Sporadic breakdown can last for up to 2 months, affecting air circulation all the way to the surface, and the ocean below.
The North Atlantic Ocean circulation pattern influences Earth’s oceans by moving water around the planet like a conveyor belt. Hence, changes in circulation pattern speed have a cascading effect on the rest of the world.
South of Greenland lies Earth’s most important seawater downwelling region: the sinking of cold, salty water that drives the oceanic conveyor belt. This downwelling area is quite susceptible to temperature changes in the troposphere there. Even modest warming or cooling in the local troposphere can trigger or delay downwelling. Hence the subtle gyral relationship between events in the polar stratosphere and ocean currents worldwide.
Bacteria live in the lower stratosphere. Some birds reach the lower stratosphere, notably the Bar-headed goose, which routinely flies over Mount Everest’s summit.
Aerosols layer in the stratosphere, especially sulfuric and nitric acids, which are naturally deposited from volcanic exhaust. Other aerosols and gases make their way into the stratosphere. Some pose a threat to the ozone layer upon which life depends.
The troposphere is the atmospheric layer of life, with 80% of the atmosphere’s mass, and 99% of its water vapor and aerosols. The troposphere averages 17 km, though its height variation is considerable. The tropic troposphere reaches 20 km. Near the poles in summer, one can go troppo for only 7 km up. In winter, the polar troposphere becomes indistinct.
The composition of the lower atmosphere is dynamically defined by the lithosphere – most dramatically by volcanoes, but most continuously by dust; most subtlety and constantly by evaporation from the hydrosphere; and constitutionally, by the most powerful of biota: plants, as well as by recently-descended hominids.
Plants have long poured their exhausts into the atmosphere, to the ultimate beneficence of all other life. Much more recently, humans have done the same, to quite the opposite effect.
The temperature of the troposphere drops with altitude, as does water vapor saturation, because saturation vapor pressure weakens with chill. Atmospheric pressure also decreases with altitude. The turbulent and thick portion of the troposphere is near the surface.
Warmth is spread through the troposphere by atmospheric circulation. While chaotic turbulence plays a large role locally, wind belts girdle the Earth and create global patterns.
Solar heating primarily drives atmospheric circulation. The hot equator and cold poles move thermal energy poleward.
The Coriolis effect caused by planetary rotation tends to move air mass flow latitudinally, in contrast to the thermal longitudinal inclination. Thus, 2 countervailing dynamics affect the atmospheric circulation gyre.
The uneven heat distribution of solar radiation and Earth’s rotation cause differences in air pressure. Aiming at equalization, wind flows from high- to low-pressure areas.
The Earth’s wind belts are organized into 3 different cell types, based upon latitude. These cells are characterized by the bands of pressure which drive them.
The equator is a low-pressure area. Prevailing winds are calm: the doldrums.
The tropical Hadley cell carries the vast bulk of vertical circulation in Earth’s atmosphere. The Hadley cell mechanism characterizes the trade winds.
Low- and high-pressure areas on the surface are balanced by their opposite relative pressures in the upper troposphere; hence a circulating cell is defined.
Elevated temperatures at the equator cause surface air to expand and become lighter. This warm, moist air at the equator is lifted aloft, and carried poleward 10–15 km above the surface. The warm air is replaced by cooler, heavier air, flowing in from the north and south.
At the high-pressure Horse Latitudes, between 30°–35° north and south, the air descends. Some of the descending air travels along the surface, closing the Hadley cell loop and creating the trade winds.
The trade winds generally flow in the same direction due to the Coriolis effect: from the northeast in the northern hemisphere, and from the southeast in the southern hemisphere.
Owing to chronic high pressure, land in the Horse Latitudes receives little precipitation. Winds there are variable, mixed with calm.
Over the oceans, the Westerlies have the strongest winds, especially between 40°–50° latitude, and particularly in the southern hemisphere, where there is less land in the middle latitudes. The Westerlies blow from west to east.
The Polar cell is the belt between latitude 60° and the pole. Like the Hadley cell, air masses at the 60th parallel are still warm and moist enough to undergo convection, and thereby drive a thermal loop. By the time relatively warm air masses reach the polar region they have cooled considerably, and so descend as a dry high-pressure area. This wind moves away from the pole near the surface, but twists westward because of the Coriolis effect, producing the polar easterlies.
The polar front is the boundary between the polar cell and the Ferrel cell in each hemisphere. A secondary mechanism, the Ferrel cell depends upon the Hadley cell and Polar cell for its existence, acting like a ball bearing between the 2.
While the Hadley and Polar cells are closed-loop systems, the Ferrel cell is not. The Westerlies are created from eddy circulations between high- and low-pressure regions in the middle latitudes.
Just as the trade winds blow beneath the Hadley cell, the Westerlies are found under the Ferrel cell. The base of the Ferrel cell has moving air masses which are influenced by the jet stream. Thus, winds created by the Ferrel cell are highly variable.
Jet streams are fast-flowing, narrow air currents near the tropopause, caused by solar radiation and Earth’s rotation. The tropopause is the transition layer between the troposphere, where the temperature drops with altitude, and the stratosphere, where temperature rises with altitude.
Earth’s major jet streams are westerly winds: flowing west to east. Jet stream paths may meander. Their shapes are highly variable.
The strongest jet streams are the polar jets, 7–12 km above sea level. At 10–16 km, subtropical jets are higher and somewhat weaker. Both the northern and southern hemispheres each have a polar jet and a subtropical jet.
While the polar jet in the southern hemisphere mostly circles Antarctica year-round, the northern hemisphere polar jet flows over middle to northern latitudes in North America, Europe, and Asia, and their intervening oceans. A weakened polar jet from climate change has delivered bursts of frigid winter weather to northern continents in the 2010s.
Climate & Weather
“Earth’s climate system has sensitive triggers that can cause abrupt and dramatic shifts in global climate.” ~ American geological oceanographer Matthew Schmidt
All the talk of the troposphere brings us to climate: a characterization of a region over a long (though not geological) time frame, focused on atmospheric measurement. The standard averaging of a climate is 30 years but is often adjusted to suit reportage.
Climate is a sketch of the effect that biospheric elements have on atmosphere, with weather as a snapshot. Climate is to weather what a movie is to stills (individual film frames).
The term climate comes from klima, the ancient Greek word for “inclination.” The term weather has more recent roots: sometime shortly after the Black Death in Europe, when one’s sense of time ran short.
Climate is what you expect. Weather is what you get.
“We’re just starting to understand the effects of dust.” ~ American climatologist Wallace Broecker
The wind carries sediment – fine particles of rock – much the way that water does. Both water and wind flow in currents, carrying ever larger particles along as current flow increases. In that air is much less dense than water, wind must move much faster to porter particles of equivalent size.
Water’s power is immense in crafting geological features, but wind is by far the major mover of sediment on a global scale.
Africa’s Lake Chad is the modest remain of what was once an inland sea, covering 400,000 km2, larger than the Caspian Sea is now. But Chad was never very deep, and so, unlike the Caspian Sea, the Chad mega-lake was susceptible to the vicissitudes of climatic change. In more recent times, other forces affected it further.
Lake Chad has dried out over the last few thousand years. Since the 1960s, owing mostly to human extractions for drinking and irrigation, the shrinkage of this endorheic lake accelerated, to being less than 1,300 km2 in 2007. (There has been no update on Lake Chad’s desiccation for a decade (between 2007 and 2017).) Endorheic refers to a closed drainage basin: no outflow to another body of water. Increased aridity from global warming in recent decades has also contributed to the evaporation of Lake Chad.
In the process of desiccating Lake Chad, a gigantic dust bowl was created: the Bodélé Depression, a huge hollow at the southern edge of the Sahara Desert in north central Africa. 100 days a year, massive dust storms sweep the bowl, carrying its dust across continents. The mountains to the north channel the winds into a howl across the big dip, scooping up dried diatoms left over from the former freshwater lake.
Many millions of tonnes of soil and dust circle the globe at any given time: affecting the climate, altering rain patterns, fertilizing the oceans, and pouring nutrients into the Amazon rain forest. Each year, the Earth’s surface emits 2 billion tonnes of dust.
More than half of the world’s dust flows from African deserts and dry lands. Much of it is carried on westward trade winds across the Atlantic to the Americas. The Middle East and Europe get a goodly share of African dust as well.
China emits dust that lands in Hawaii and western North America. Greenland is landfall for Asian dust. Patagonia, at the southern end of South America, ships its dust in a relatively short hop to Antarctica.
Each year, several hundred million tonnes of dust, laden with life-sustaining elements from Africa, fertilize the Amazon rain forest; blessings of nitrogen, iron, and carbon. Half of that may originate from the scoop at the Bodélé.
The Sahara Desert
“North Africa is pumping dust everywhere, all year long.” ~ American meteorologist Joseph Prospero
The Sahara Desert covers most of north Africa. It is the largest subtropical desert, and the 3rd-largest desert in the world, after Antarctica and the Arctic.
The desertification of northern Africa began ~7 million years ago, an outcome of tectonic plate movements. The African plate moved north relative to the Eurasian plate, prompting a shift in weather patterns. At the same time, the Arabian Peninsula uplifted, replacing a broad swath of ocean off northeastern Africa, thereby weakening the African summer monsoon. When the westerly winds waned, the flow of moisture from the tropical Atlantic Ocean that had swept north Africa shifted south.
The Tethys Sea, the predecessor of today’s Mediterranean, gradually shrank. North Africa parched. Once semi-arid, north Africa collapsed into desert.
4,600 years ago, dust from the Sahara Desert fertilized the nutrient-poor wetlands of south Florida. Water lilies and other aquatic flowers dotted the grass carpets of the Everglades. An abrupt shift in winds 2,800 years ago downsized the dusting. The drop in nutrient flow ushered in the sawgrass-dominated ecosystem that characterizes the Everglades today.
Dust from north Africa and the Arabian Peninsula is lofted toward India. The dust absorbs sunshine, warming the air, and strengthening the winds that blow from the Indian Ocean, carrying moisture eastward. Thus, dust affects the intensity of summer monsoon rainfall in India.
The tropics and mid-latitude arid regions are not the only dust source, though they are the major ones. Dust generated in the higher latitudes is not confined to arid regions. Because of strong seasonal winds, humid areas, even near glacial regions, generate dust.
Airborne dust fashions fickle effects. Do-nothing dust for millennia may suddenly start to modulate the global climate: absorbing warmth from the Sun and rays reflected off Earth’s surface, and thus warming the atmosphere. Dust with soot absorbs even more heat.
Dust can have the opposite influence: cooling by reflection rather than absorbing heat. The effect depends upon dust’s chemical composition and size, as well as the wavelength of light that hits it. Generally, dust tends to reflect shortwave solar radiation, while absorbing the longer-wave bounce-back from the Earth.
Dust over darker areas, notably the oceans, has a cooling effect, by reflecting light that would otherwise be absorbed at the surface. Dust on ice or snow darkens it, willing warming.
Moisture in the air must attach to particulates to form droplets that lead to rain, or, in colder conditions, hail and snow. Aerial bacteria play this particulate part, as does dust. Exactly how these particulates affect precipitation varies depending upon a variety of factors, including the characteristics of the dust.
Airborne dust has doubled in the past century. While it generally has a cooling effect in the atmosphere, there are too many variables to characterize its temperate effects.
Large areas of the ocean are rich in nitrogen and phosphorus. An iron shortage limits what would otherwise engender massive plankton blooms. African dust has a high iron content.
Not only does dust craft the global climate, but the climate claims a toll on dust. Just as dust modulates climate variability, dust is dependent upon climate.
“If climate change affects wind velocity and rainfall, it can have an immense impact. Dust is extremely sensitive to small changes in wind and rain. It’s the ultimate feedback loop.” ~ Joseph Prospero
Water vapor and ice crystals coalesce in the atmosphere, forming clouds as warm, moist air rises. As atmospheric pressure lowers at higher altitude, an air mass expands as it ascends. During expansion, air cools adiabatically until its temperature falls below dew point. As the air becomes supersaturated, water vapor condenses into microscopic droplets, which are the nuclei of cloud condensation. The now cool, wet air mass forms a cloud.
Cloud cover is especially dominant over the oceans, where less than 10% of the sky is without clouds at any one time. 30% of land surface is cloud-free on average. Overall, 2/3rds of Earth’s surface has cloud cover at any time.
Regardless of rain or snow, clouds themselves have an overall cooling effect that greatly affects global climate. Clouds cool by reflecting incoming solar radiation, and warm by trapping outgoing heat.
The band of clouds that cover the equator is caused by the Hadley cell which dominates the tropics. Winds converge near the equator and rise to 10–15 km before moving toward the poles.
As this warm, moist, tropical air rises and cools it loses its capacity to hold water vapor; condensing into clouds which produce regular thunderstorms. The cold, dry air sinks back to the surface at about 30° latitude.
Clouds also commonly form in the middle latitudes – 60° north and south of the equator – where polar and mid-latitude Ferrel circulation cells collide. This pushes air upwards, fueling the formation of large-scale frontal systems that dominate the weather patterns there.
Ferrel cells are an atmospheric eddy, formed by the upward movement of polar air currents at 60° latitude and the downward flow of cool dry air from the Hadley cycle at 30° latitude.
Clouds are categorized by their appearance. The 8 main cloud families are divided into 3 groups, based upon altitude.
A cloud at the surface is called fog. Low clouds (up to 2 km) are stratocumulus, stratus, and nimbostratus. Mid-altitude clouds (2–7 km) are altocumulus and altostratus. High clouds (5–13 km) are cirrus, cirrocumulus, and cirrostratus. A cloud that extends through all 3 heights is a cumulonimbus.
Cumulus clouds are dense, puffy mounds, with flat bases and cauliflower tops.
Stratus clouds are relatively featureless, with a uniform base and gray, horizontal layering. Altostratus clouds are lighter than nimbostratus, and darker than cirrostratus.
Cirrus clouds are the thin wisps of vapor that streak the sky at high altitudes; covering nearly 1/3rd of the globe at any time. Mineral dust forms the seeds of cirrus clouds. This dust naturally originates from dry surface regions, but human activities have increasingly contributed.
High-altitude clouds that curl at the top like breaking waves indicate disturbance in Earth’s magnetic field. The waves are caused by high-speed winds blowing over more stagnant air masses, forming a distinction pattern known as Kelvin-Helmholtz instability waves. These ultra-low frequency waves occur 20% of the time. They change the energy dynamics in the magnetosphere, affecting the blockage of incoming cosmic radiation.