climate change midterm2

Dry air (Week 6)

Air with its water vapor left out, because water vapor varies so much from place to place. It is 78.1% nitrogen, 20.9% oxygen, 0.93% argon and about 0.04% everything else.

Well mixed (Week 6)

Having the same composition throughout. Dry air has the same shares from the ground to about 80 km up because winds keep it mixed.

Trace gas (Week 6)

A gas present in tiny amounts, such as CO₂, methane and nitrous oxide. All the greenhouse gases are trace gases, while nitrogen, oxygen and argon let infrared pass straight through.

ppm (parts per million) (Week 6)

The number of molecules of a gas in every million molecules of air. For example, 426 ppm of CO₂ means 426 CO₂ molecules per million, about 0.04%.

Greenhouse gas levels, 2025 vs 1750 (Week 6)

CO₂ was 426 ppm vs 278 ppm (up 53%), methane 1.94 ppm vs 0.73 ppm (2.7 times more) and nitrous oxide 0.34 ppm vs 0.27 ppm (up 25%). Most of the added CO₂ comes from burning coal, oil and gas.

Why a trace gas can matter (Week 6)

What matters is whether a gas absorbs infrared, not how much of it there is. The 99% of air that is nitrogen and oxygen does not absorb infrared but CO₂ does, so a small amount changes Earth's energy budget.

Water vapor (Week 6)

Water as an invisible gas and the most abundant greenhouse gas. Its amount depends on temperature and ranges from almost 0% to about 4% of the air, for example about 0.02% in Antarctica in winter, about 1.4% on a mild LA day and about 3.6% in the Amazon.

Partial pressure (Week 6)

One gas's share of the total air pressure. Of 1,013 mb at sea level, nitrogen supplies about 791 mb, oxygen 212, argon 9 and CO₂ 0.4.

Vapor pressure (Week 6)

Water vapor's share of the air pressure, which is how scientists measure humidity. It is under 1 mb in polar winter, about 14 mb on a mild LA day and up to about 40 mb in the tropics.

Humid air is lighter than dry air (Week 6)

At the same temperature and pressure, water molecules (mass 18) replace heavier nitrogen (28) and oxygen (32). Dry air averages a molecular mass of 29 but with 4% water vapor it drops to about 28.5, which is why moist tropical air rises easily and fuels storms.

Phase change (Week 6)

Water changing between ice, liquid and vapor. Water is the only common substance that changes phase at everyday temperatures, so it moves heat around the planet all the time.

Latent heat (Week 6)

Heat taken in or given off when water changes phase. Melting, evaporation and sublimation take heat in while freezing, condensation and deposition give heat off.

Evaporation (Week 6)

Liquid water turning into vapor, which takes in latent heat. Sweating cools you because evaporating sweat takes heat from your skin, and evaporating 1 g of water takes 2,450 J, as much energy as warming it by almost 600 °C.

Condensation (Week 6)

Water vapor turning into liquid, which gives off latent heat. The heat released as vapor condenses into rain keeps storm air rising and powers thunderstorms.

Sublimation and deposition (Week 6)

Ice turning straight to vapor (takes in heat) and vapor turning straight to ice (gives off heat). Frost is an example of deposition.

Specific heat (Week 6)

The energy needed to warm 1 gram of a substance by 1 °C. Liquid water is 4.18 J per g per °C, ice 2.09, air 1.00, sand and rock about 0.8 and iron 0.45, so water's is about five times that of rock.

Thermal inertia (Week 6)

Resistance to temperature change, which is high for the ocean and the air above it. The reasons are water's high specific heat, sunlight reaching meters deep into water but only centimeters into land, and waves and currents mixing heat downward.

Maritime climate (Week 6)

A climate downwind of the ocean with mild seasons. San Francisco's monthly average temperature ranges only 11 °F (52 °F to 63 °F), and Los Angeles is another example.

Continental climate (Week 6)

An inland climate with large differences between summer and winter. St. Louis ranges 48 °F (32 °F to 80 °F) at about the same latitude and with the same sunshine as San Francisco.

Saturation (Week 6)

Air holding as much water vapor as it can at its temperature. The limit is 6 mb at 0 °C, 12 mb at 10 °C, 23 mb at 20 °C and 42 mb at 30 °C, so every 10 °C of warming roughly doubles it, about 7% more per 1 °C.

Relative humidity (Week 6)

The water vapor in the air as a percentage of the most it could hold (vapor divided by maximum times 100%). Air with 12 mb of vapor is about 100% at 10 °C at dawn but only about 38% at 25 °C in the afternoon, and cold winter air at 90% can hold less water than hot summer air at 40%.

Dew point (Week 6)

The temperature at which air becomes saturated. Cooling saturated air gives dew, fog or cloud.

Expansion cooling of rising air (Week 6)

Air pressure drops with height, so rising air expands and cools about 10 °C per km. A spray can gets cold for the same reason.

Cloud (Week 6)

Tiny liquid droplets or ice crystals that form when rising air cools to its dew point and vapor condenses on particles of dust, sea salt or smoke. Water vapor itself is invisible, so a cloud is not water vapor.

Rain shadow (Week 6)

The dry side of a mountain range, where air sinks, warms and dries. Moist Pacific air over the Sierra Nevada drops 62 inches a year at Blue Canyon but only 5 inches at Bishop in the Owens Valley, about 13 times less.

Water vapor feedback (Week 6)

Warming adds water vapor, which traps more heat and adds more warming. Warmer air holds about 7% more vapor per °C, and the feedback roughly doubles the warming from CO₂ alone but does not start it, since vapor stays in the air only about 10 days.

Water cycle (Week 6)

The flow of water between ocean, air and land. Ocean evaporation is 413 thousand km³ per year while 373 falls back on the ocean, and the extra 40 is blown over land and returned by rivers.

Air pressure (Week 7)

The push of the air on each unit of area, caused by the weight of the air above. About 10 tonnes of air rests on every square meter at sea level, and we are not crushed because air pushes in every direction and the fluids in our bodies push back.

Millibar and hectopascal (Week 7)

Units of pressure where 1 mb equals 1 hPa. Average sea-level pressure is 1,013 mb, 29.92 inches of mercury, 760 mm of mercury, 14.7 psi, 101.3 kPa or 1 atm.

Barometer (Week 7)

An instrument that measures air pressure. A glass tube of mercury turned upside down in a dish of mercury is held up by the air to about 76 cm (29.92 in) at sea level, and since mercury is 13.6 times as dense as water a water barometer would need a tube about 10.3 m tall.

Typical pressures (Week 7)

A strong high is about 1,030 mb, the average is 1,013 mb and a deep storm is 1,000 mb or lower.

Pressure falls with height (Week 7)

The lowest layers are the densest, so pressure falls fastest near the ground. It is 1,013 mb at Santa Monica beach, 789 mb at Big Bear Lake (22% less), 583 mb on Mt. Whitney, 314 mb on Mt. Everest and 226 mb at jet cruising height, and half of all air is below about 5.5 km.

Dalton's law (Week 7)

Each gas in a mixture pushes in proportion to how many of its molecules are present, and the pushes add up to the total pressure. CO₂ supplies only 0.4 of the 1,013 mb, so it matters for climate because it absorbs infrared, not because of its weight.

Sea-level pressure (Week 7)

Air pressure adjusted to sea level so places at different heights can be compared on a map.

Density (Week 7)

Mass per volume, such as kilograms per cubic meter. 1 m³ of air weighs 1.225 kg at 15 °C but 1.146 kg at 35 °C, about 6.5% lighter.

Ideal gas law (Week 7)

Pressure goes up and down with density times temperature (PV = nRT, or p = ρRT for air) with temperature in kelvin. Heating a sealed can raises its pressure because the density cannot change, which is why spray cans say do not heat.

Convection (Week 7)

Warm, less dense air rising and cooler air sinking. At the same pressure warm air is less dense and floats up like a hot-air balloon, and convection powers thunderstorms and the rising air over the tropics.

Compression warming (Week 7)

Air warming as it is squeezed, for example when it sinks. A bike pump gets hot, and sinking air warms about 10 °C per km.

Expansion cooling (Week 7)

Air cooling as it expands, for example when it rises, which makes clouds.

500 mb level (Week 7)

The height where the pressure is 500 mb with half the air below, which is higher over warm places because a warm, less dense column must be taller to weigh the same. It is about 5.9 km over the equator, 5.8 km over LA and 5.2 km over the North Pole, so air aloft is pushed poleward.

Column water vapor (Week 7)

All the water vapor above a spot, measured as the millimeters of water it would make if it all fell as rain, also called precipitable water. It is about 42 mm near the equator versus 11 mm at 60°N, about four times as much.

Low pressure (Week 7)

An area where air rises and surface pressure is lower. Air flows in at the ground, expands and cools, and the weather is often clouds and rain.

High pressure (Week 7)

An area where air sinks and surface pressure is higher. Air flows out at the ground, is squeezed, warms and dries, and skies are usually clear.

Wind (Week 7)

Air flowing at the ground from high pressure toward low pressure. Winds are named for where they come from, so a west wind blows from the west.

Thermal circulation (Week 7)

A loop of air set up by heating one place more than another. The warm column expands upward, pressure surfaces tilt, air aloft is pushed toward the cold side and surface air flows from high to low.

Sea breeze (Week 7)

An afternoon wind from the cooler sea onto the warmer land. Land heats faster than the ocean so air rises over the land and cool ocean air flows in, strongest in mid-afternoon at Santa Monica.

Land breeze (Week 7)

A night-time wind from the cooler land out to the warmer sea. It is weaker than the sea breeze because the temperature difference is smaller.

Sea surface temperature (Week 7)

The temperature of the top layer of the ocean. Air rises most readily in towering storm clouds over water warmer than about 28 °C, which gives the most rising air and heaviest rain.

Pressure gradient force (Week 7)

The push from high toward low pressure, stronger where pressure changes quickly over a short distance. The same 10 mb difference over a third of the distance gives a push three times stronger.

Isobar (Week 7)

A line on a map joining places with equal pressure. Packed-close isobars mean a strong pressure gradient and strong winds, while far-apart isobars mean gentle winds.

Santa Ana wind (Week 7)

A hot, dry wind blowing down to coastal Southern California from high pressure over the Great Basin. A sinking parcel goes from 1.5 km (10 °C, 30% relative humidity) to 0.1 km (24 °C, 12%) with the same vapor, so it arrives hot because of compression warming and not because desert air is hot.

Pacific High (Week 7)

A large area of high pressure over the northeast Pacific, strongest and farthest north in summer at about 1,026 mb in July. Its sinking air keeps skies clear and steers storms north, so LA gets almost no rain from May to October.

Aleutian Low (Week 7)

A large area of low pressure over the North Pacific, deepest in winter. It is about 997 mb in January and almost gone (about 1,008 mb) in July.

Heat low (Week 7)

Low pressure over hot land in summer where heated air rises, as over the desert Southwest in July.

October 2026 heat wave (Week 7)

Downtown LA hit 104 °F on Oct 3 against a normal October high near 79 °F. A dome of high pressure made the 500 mb level about 100 m higher, and air sinking from 850 mb to the ground warmed from 76 °F to about 103 °F while surface pressure stayed near normal.

Newton's first law (Week 8)

A moving object keeps going in a straight line at the same speed unless a force acts on it. If the wind curves, either a force is turning it or we are watching from something that turns, and we are, the spinning Earth.

Coriolis effect (Week 8)

The apparent turning of anything moving freely over the spinning Earth, to the right in the Northern Hemisphere and to the left in the Southern, with no turning of east-west motion at the equator. It changes a wind's direction but never its speed, is stronger for faster winds and nearer the poles, and matters for big long-lasting flows like storms but not for sink drains (about 0.3° in a minute). The turn is 2 × Ω × v × sin(latitude), which is 0 at the equator, 0.56 at LA and 1 at the poles, and it swings a 10 m/s wind at LA by about 50° in 3 hours.

Speed of the ground (Week 8)

Earth's surface moves eastward about 1,670 km/h at the equator, about 1,380 km/h at LA (34°N), about 835 km/h at 60°N and 0 at the North Pole. This is why the Coriolis effect varies with latitude.

Geostrophic wind (Week 8)

Wind high above the ground that blows along the isobars, with the push toward low pressure balanced by the Coriolis turn. The January wind at about 5.5 km follows the 500 mb contours to within about 2°, and in the Northern Hemisphere if you stand with the wind at your back low pressure is on your left.

Friction and boundary layer (Week 8)

Friction is the drag of the ground on the air, and the boundary layer is the lowest kilometer or so where it slows the wind. Slower wind has a weaker Coriolis turn so the wind crosses isobars toward low pressure, at about 2° aloft, about 11° near the surface over the North Pacific in January and typically 30–45° over land.

Cyclone (Week 8)

An area of low pressure where air near the ground spirals in, piles up and rises, bringing clouds and rain. It spins counterclockwise in the Northern Hemisphere and clockwise in the Southern, as hurricanes do.

Anticyclone (Week 8)

An area of high pressure where air near the ground spirals out, sinks and spreads, giving clear skies. It spins clockwise in the Northern Hemisphere and counterclockwise in the Southern.

Convergence (Week 8)

Air flowing together, which forces it to rise as in a low.

Divergence (Week 8)

Air spreading apart, which draws air down from above as in a high.

Heat transport (Week 8)

The carrying of heat from the tropics toward the poles by the air and the oceans. The tropics absorb more sunlight than they give off and the high latitudes give off more than they absorb, so circulation balances this.

Hadley cell (Week 8)

The tropical loop from the equator to about 30° where air rises near the equator (rainy) and sinks near 30° (dry). George Hadley proposed one loop per hemisphere in 1735 for a non-spinning Earth, and Earth's spin breaks it into three loops.

Ferrel cell (Week 8)

The middle loop of the circulation between about 30° and 60°, where air rises near 60° (stormy) and sinks near 30°.

Polar cell (Week 8)

The loop between about 60° and the pole, where air rises near 60° and sinks at the pole (cold and dry).

ITCZ (Intertropical Convergence Zone) (Week 8)

The belt of rising air and heavy rain near the equator where the trade winds meet. Rain peaks near 6°N at about 6 mm a day, and the belt follows the overhead Sun north in July and south in January, swinging farthest over land (monsoon rains in India, West Africa and Mexico).

Subtropical high (Week 8)

The belt of high pressure near 30° under sinking air, about 1,022 mb, where rain dips to about 2 mm a day. The Pacific High is one, and great deserts such as the Sahara, Arabia, Australia, the Atacama and the Sonoran sit under these highs.

Subpolar low (Week 8)

The belt of low pressure near 60° with storms, such as the Aleutian and Icelandic lows and the ring of low pressure around Antarctica.

Trade winds (Week 8)

Steady surface winds blowing toward the equator from the northeast in the north or the southeast in the south, between the equator and 30°.

Westerlies (Week 8)

Surface winds in middle latitudes (30° to 60°) blowing from the west and carrying storms eastward, including LA's winter storms. Winds are named for where they come from, so westerlies do not blow toward the west.

Polar easterlies (Week 8)

Surface winds blowing from the east near the poles, from 60° to the poles.

Jet stream (Week 8)

A narrow band of very fast westerly wind near the top of the troposphere, where cold polar air meets warm tropical air. The January average peaks at about 74 m/s (165 mph) over Japan, and jets are strongest in winter and steer storms.

Mediterranean climate (Week 8)

Dry summers and wet winters on west coasts near 30–40°, as in Los Angeles, Chile, South Africa, Perth and the Mediterranean itself. USC averages 14.25 inches of rain a year, 82% from December to March and 0.00 inches in August, because the Pacific High blocks storms in summer and weakens and moves south in winter.

Atmospheric river (Week 8)

A long, narrow plume of moist air carrying water vapor from the warm subtropical Pacific, thousands of km long and a few hundred wide, carrying more water than two Amazon Rivers. It is invisible vapor, not liquid, and rain falls only when the air is lifted and cooled over mountains. The February 4–6, 2024 storm gave USC 8.51 inches (about 60% of a normal year), and atmospheric rivers bring 20–50% of California's precipitation and caused 84% of western US flood damage in 1978–2017.

Climates follow the circulation (Week 8)

Rising air near the equator gives rainforest (Amazon, Congo, Indonesia), sinking air near 20–30° gives desert (Sahara, Arabia, Australia), west coasts at 30–40° are Mediterranean (LA, Chile, Perth), 40–60° is wet all year (Seattle, London) and the poles are cold deserts (Antarctica).

Front (Week 8)

The boundary between two air masses of different temperature, listed as for the curious. At a cold front cold air plows under warm air giving tall clouds and short heavy showers, while at a warm front warm air glides over cold air giving wide cloud layers and steady light rain.

Heat capacity (Week 9)

How much heat it takes to warm one gram of a material by 1 °C, where heat equals mass times heat capacity times temperature change. 1,000 J into 100 g of water gives +2.4 °C but into 100 g of sand gives +12.5 °C, and warming the top 3.5 m of ocean by 1 °C takes as much heat as warming the whole atmosphere by 1 °C.

Ocean heat content (Week 9)

The total heat stored in the ocean, measured by thermometers on ships and Argo floats. The ocean took about 89% of the extra heat in 1971–2020 (land 6%, melting ice 4%, air 1%) and has gained about 11 ZJ a year since 2005, roughly 18 times all human energy use.

Carbon uptake (Week 9)

Of every 10 tonnes of carbon we emit (2015–2024), about 5 stay in the air, about 3 go into the ocean and about 2 go into plants and soils.

Henry's law (Week 9)

The amount of a gas dissolved in water is proportional to the amount of that gas in the air above it, with a solubility constant K that is bigger in cold water. 0 °C seawater dissolves about 2.4 times as much CO₂ as 28 °C water, so cold polar oceans take up the most, as a warm soda goes flat faster than a cold one.

Biological pump (Week 9)

Plankton in the sunlit top 100 m turn CO₂ into food, and some sinks and decays at depth back to CO₂ and nutrients, storing carbon for centuries until upwelling returns it.

Southern Ocean flip (Week 9)

The Southern Ocean changed from a CO₂ source to a CO₂ sink. In 1850 upwelled water around Antarctica had slightly more CO₂ than the air (280 ppm) and gave it off, but air CO₂ is now about 429 ppm so the water takes it in, and south of 30°S the ocean takes up about 40% of the human CO₂ the whole ocean absorbs.

Ocean acidification (Week 9)

The fall in seawater pH as the ocean takes up CO₂, which makes carbonic acid. Surface pH has dropped about 0.1 since the 1800s, about 30% more acidic on a log scale, and it uses up carbonate ions that shells and corals need, though seawater is still alkaline (pH above 7).

Gyre (Week 9)

A large loop of ocean currents filling an ocean basin, driven by the wind belts. Trade winds push water west near the equator and westerlies push it east near 40–50°, piling a hill of water about 1 m high in the middle of a subtropical gyre that flows clockwise in the north.

Western boundary current (Week 9)

A narrow, fast, warm current on the west side of an ocean basin, such as the Gulf Stream and the Kuroshio. East-side currents like the California Current are broad, slow and cool.

Ekman transport (Week 9)

The net movement of the wind-driven layer of water, 90° to the right of the wind in the Northern Hemisphere and to the left in the Southern. In the Ekman spiral the surface water turns 45° right of the wind and each deeper layer is slower and turns further right, adding up to 90° over the top 100 m.

Upwelling (Week 9)

Cold, deep water rising to replace surface water pushed away by the wind. North winds along California drive Ekman transport offshore and cold, nutrient-rich water from 50–200 m rises, so plankton bloom and upwelling coasts give about a fifth of the world's fish from under 1% of the ocean, while the cold water also makes the marine layer fog of May gray and June gloom.

Cold tongue (Week 9)

The band of cool water along the equator in the eastern Pacific, kept cold by upwelling. Trades blow west and Ekman transport turns water away from the equator on both sides.

Warm pool (Week 9)

The very warm water above 28 °C piled up in the western tropical Pacific by the trades, with rising air and heavy rain over Indonesia.

Marine heat wave (Week 9)

A patch of unusually warm ocean. In September 2026 the sea surface between 60°S and 60°N averaged a record 21.05 °C for September, with warm water along the eastern equatorial Pacific and across the North Pacific.

El Niño (Week 9)

A warming of the central and eastern tropical Pacific lasting about a year when the trade winds weaken. It is not a storm. By mid-October 2026 the Niño-3.4 box was about 3 °C above normal, NOAA issued an El Niño Advisory with an 83% chance of the strongest since 1950, and effects include floods in Peru and Ecuador, drought in Indonesia and Australia, coral bleaching and about 0.1–0.2 °C extra global warmth the next year.

La Niña (Week 9)

A cooling of the central and eastern tropical Pacific when the trade winds are stronger than normal and warm water is pushed far west.

Niño-3.4 box (Week 9)

The standard place in the tropical Pacific to measure El Niño, tracked with the RONI index, the official index since February 2026.

ENSO (Week 9)

El Niño and La Niña in the ocean together with the seesaw of air pressure across the Pacific, called the Southern Oscillation. It recurs every 2–7 years, usually peaks in Northern winter and is forecast 6–9 months ahead using about 70 buoys, about 4,000 Argo floats and satellites.

Thermocline (Week 9)

The boundary between warm surface water and cold deep water. It is normally deep in the west and shallow in the east and flattens during El Niño.

Walker circulation (Week 9)

The loop of air along the equator that rises and rains over the warm western Pacific, flows east aloft and sinks over the cool east.

Bjerknes feedback (Week 9)

Why El Niño grows. Weaker trades mean less upwelling so warm water slides east, the east warms and rain moves east, the pressure difference across the Pacific shrinks and the trades weaken more.