Describe the atmosphere: its layers and composition, why it circulates, and how El Niño and La Niña reach the other spheres

Learning goal: Describe the layers of the atmosphere with their temperature trends and chemical composition; explain how uneven solar heating, convection and Earth's rotation produce global atmospheric currents; explain the mechanism of the greenhouse effect; and describe the causes of El Niño and La Niña and compare their regional effects on the hydrosphere, biosphere and biogeochemical cycles.

Composition. Dry air near the surface is roughly 78% nitrogen, 21% oxygen and a little under 1% argon by volume, with carbon dioxide at a few hundredths of one percent and traces of neon, helium, methane, ozone and others. Water vapor is left out of that list because it varies so much: from almost nothing over a cold desert to about 4% in warm, humid air. The trace gases matter far more than their percentages suggest, because nitrogen, oxygen and argon are nearly transparent to the infrared radiation Earth emits, while water vapor, carbon dioxide, methane, nitrous oxide and ozone absorb it. Those absorbers are the greenhouse gases.

Layers, defined by temperature. The atmosphere is divided by how temperature changes with height, not by composition, which is nearly uniform up to about 80 km because winds mix it. The troposphere runs from the surface to about 8 km at the poles and 16 to 18 km at the equator (a working average is about 12 km). Temperature falls with height through it, on average by about 6.5 °C per kilometer, because the air is heated mainly from below by the sun-warmed ground. Nearly all water vapor and weather are in the troposphere. At the tropopause the fall stops. In the stratosphere (to about 50 km) temperature rises with height, because the ozone layer, concentrated roughly 15 to 35 km up, absorbs ultraviolet radiation and warms the air around it. Warm air over cold makes the stratosphere stable and stratified, hence the name; long-distance jets cruise near the tropopause, sometimes just inside the lowest stratosphere, above most of the weather. In the mesosphere (to about 85 km) temperature falls again, to the coldest values in the atmosphere; most meteors burn up here. In the thermosphere, above about 85 km, the thin gas absorbs very energetic solar radiation and its temperature climbs to hundreds or thousands of degrees, though the gas is so sparse that it would not feel hot. The aurora forms here, and the outermost fringe, the exosphere, fades into space. Pressure falls with height through every layer because pressure at any level is the weight of the air above it: roughly half of the atmosphere's mass lies below about 5.5 km.

A worked model in the prose: lapse rate. Suppose a hypothetical weather balloon leaves the ground at 20 °C and the air cools at the average 6.5 °C per kilometer. At 2 km the temperature is 20 − 13 = 7 °C; at 5 km it is 20 − 32.5 = −12.5 °C; at 10 km it is 20 − 65 = −45 °C. Real profiles differ from the average, and an inversion, where temperature rises with height near the ground, traps pollutants beneath it; a lapse-rate calculation is a first estimate, not a measurement.

The greenhouse effect, mechanism first. Sunlight arrives mostly as visible and near-infrared radiation, which the clear atmosphere largely lets through. The ground absorbs it, warms, and radiates energy back, but because the ground is far cooler than the sun it radiates at longer, infrared wavelengths. Greenhouse gases absorb much of that outgoing infrared and re-emit it in all directions, including downward, so the surface receives energy from the sky as well as from the sun and settles at a higher temperature than it would with a transparent atmosphere. This natural effect is what keeps Earth's average surface temperature above freezing. Adding more absorbing gas thickens the blanket. Two clarifications: the name is imperfect, because a glass greenhouse warms mainly by stopping convection, not by absorbing infrared; and the greenhouse effect and the stratospheric ozone hole are different problems with different gases and different remedies.

Atmospheric properties and currents. The equator receives more solar energy per square meter than the poles because sunlight strikes it more directly. Warm equatorial air rises, spreads poleward aloft, cools, sinks near 30° latitude, and returns toward the equator along the surface as the trade winds. That loop is the Hadley cell. Sinking dry air at 30° produces the belt of subtropical deserts; rising air at the equator produces a belt of heavy rain. The Ferrel cell (between about 30° and 60°) and the polar cell (from about 60° to the pole) complete the pattern. Earth's rotation deflects moving air: to the right in the Northern Hemisphere and to the left in the Southern (the Coriolis effect), which is why the trade winds blow from the northeast and southeast rather than straight toward the equator, why mid-latitude winds are westerlies, and why storms spin. High in the troposphere, fast jet streams ride the boundaries between cells and steer weather systems across Ohio from west to east. Local currents follow the same physics at small scale: a sea breeze forms because land warms faster than water by day.

El Niño and La Niña. In an ordinary year the trade winds push warm surface water across the tropical Pacific toward Indonesia, and cold, nutrient-rich deep water rises along the coast of Peru to replace it (upwelling). That upwelling feeds plankton and one of the richest fisheries on Earth, and the warm pool in the west feeds heavy rain over Indonesia and northern Australia. Every few years the trade winds weaken. Warm water sloshes back east, upwelling off South America is suppressed, and the rain follows the warm water into the central and eastern Pacific. That is El Niño. Its opposite, La Niña, is an intensification of the normal pattern: stronger trades, colder eastern Pacific, stronger upwelling. Because the atmosphere and ocean are coupled, the change spreads through the spheres. Hydrosphere: in strong events, often floods on the normally dry Peruvian coast during El Niño and drought in Indonesia and eastern Australia. Biosphere: when upwelling brings up only warm, nutrient-poor water the Peruvian anchovy catch can collapse, sea birds that depend on it can starve, coral reefs bleach in unusually warm water. Biogeochemical cycles: with less upwelling, fewer nutrients reach the surface, so less carbon is fixed by plankton, and drought-driven fires release stored carbon. La Niña tends to bring roughly the reverse at the same places: stronger upwelling and cooler water off Peru, which usually favors plankton, anchovy and the sea birds that eat them; drier conditions on the Peruvian coast; heavier rain and higher flood risk in Indonesia and eastern Australia; and, with more cold, nutrient-rich water at the surface, more carbon fixed by plankton in the eastern Pacific. Like El Niño, its effects vary from event to event. The regional comparison is the point: the same event brings too much water to one coast and too little to another, so a statement about "the effect of El Niño" is meaningless without a location. In the United States the influences are statistical tendencies rather than certainties, and this lesson does not assign specific outcomes to Ohio.

What the layers model cannot show. A stacked-layer diagram gives the impression of sharp floors and ceilings; in reality the boundaries move with season and latitude, and the layers exchange air, especially at the tropopause during large storms. It also cannot show that nearly all the mass is in the bottom layer.

Worked composition reasoning: A student argues that carbon dioxide cannot matter to Earth's temperature because it is only a few hundredths of a percent of the air. Reasoning: nitrogen, oxygen and argon, which make up over 99% of dry air, are almost transparent to the infrared radiation the surface emits, so they play no part in trapping it. The temperature effect depends on which molecules absorb infrared, not on which are most abundant. A small quantity of an absorber in a large quantity of a transparent gas still absorbs; a few drops of ink color a glass of water.

Worked pressure and altitude: A hypothetical mountain station at 5.5 km reports about half the surface pressure. Reasoning: pressure at a level is the weight of the air column above it. Half the atmosphere's mass lying below 5.5 km means half the weight is below and half above, so pressure there is about half of surface pressure. Pressure keeps falling above that, and each further halving takes roughly the same additional thickness (about another 5.5 km), because pressure falls by a roughly constant fraction for each kilometer climbed.

Worked El Niño regional comparison: Compare two hypothetical coastal towns during a strong El Niño: one on the coast of northern Peru, one on the north coast of Java. Peru: eastern Pacific surface water is unusually warm, upwelling is suppressed, plankton and anchovy catches fall, rain and flooding increase on a normally arid coast. Java: the warm pool has shifted east away from it, so rainfall is below normal, drought stresses crops and raises fire risk. Same event, opposite water outcomes. A complete answer names the mechanism (weakened trade winds) and states that the sizes of the effects vary from one event to the next.

Worked lapse-rate reading: A hypothetical sounding gives 15 °C at the surface, 9 °C at 1 km, 12 °C at 2 km and 5 °C at 3 km. Between 1 and 2 km temperature rises with height: that is an inversion. Air below it is cooler and denser than the air above, so it cannot rise through it; smoke and exhaust released below 1 km accumulate. Above 2 km the ordinary lapse resumes. The overall change from 0 to 3 km (15 to 5 °C) averages 3.3 °C per km, but the average hides the inversion, which is why the layer-by-layer reading matters.

Representation task: Make a foldable or labeled column of the atmosphere with the four main layers, their approximate altitude ranges, the direction of temperature change in each, the main chemical composition (78% nitrogen, 21% oxygen, about 1% argon, trace carbon dioxide, variable water vapor), and one feature per layer (weather, ozone layer, meteors, aurora). Mark the height below which about half of the atmosphere's mass lies. Then add arrows for the Hadley cell between the equator and 30° and label the trade winds with their Northern Hemisphere direction.

Safe investigation, a model of trapped energy: Place two identical clear jars containing equal amounts of dry soil, each with a thermometer, in the same sunlight. Cover one with clear plastic wrap and leave the other open. Record both temperatures every two minutes for 20 minutes, then move both into shade and keep recording. Repeat on another day. Result that supports the claim: the covered jar warms faster and cools more slowly. What the model cannot show: the plastic wrap works mainly by stopping warm air from escaping by convection, not by absorbing infrared radiation, so this demonstrates that a barrier can retain energy but does not demonstrate the radiative mechanism of greenhouse gases. Say so in your write-up.

Independent performance: Write an explanation of at least 250 words for a reader who knows no science, titled "Why the same El Niño floods one coast and dries another." Success criteria: it describes the normal trade-wind pattern and upwelling; it states what changes during El Niño and what La Niña is; it compares effects in two named regions on the hydrosphere and the biosphere; it links suppressed upwelling to the carbon cycle; it uses the words troposphere and Coriolis correctly at least once each; and it states one limit on predicting the effects of a particular event.