2.3 — Water in the Air: Clouds and Rain

A typical cumulus cloud — one of the fluffy white ones on a pleasant afternoon — is about a kilometre across and contains on the order of half a million tonnes of water. It stays up.

It stays up because that water is divided into droplets a hundredth of a millimetre across, and a droplet that small falls through air at about a centimetre per second, which the gentlest updraft in the cloud beats easily. The cloud is not floating. It is falling, extremely slowly, into air that is rising slightly faster.

Understanding how that half-million tonnes got up there, and what has to change before any of it reaches the ground, is this page.

What humidity actually means

Air can hold water vapour, and there is a limit to how much. The limit rises steeply with temperature — roughly doubling for every 10 °C.

Temperature	Maximum water vapour (g per kg of air)
0 °C	about 3.8
10 °C	about 7.7
20 °C	about 14.8
30 °C	about 27.7
40 °C	about 49.8

Why the limit exists. Water molecules constantly escape from a liquid surface into the air, and constantly return. At a given temperature there is a balance point where escapes equal returns, and the amount of vapour present at that balance is saturation. Heat the water and more molecules have enough energy to escape, so the balance shifts to a higher vapour content.

Relative humidity is the ratio of the vapour actually present to the maximum at that temperature, expressed as a percentage. This is why the number can be misleading on its own. Air at 30 °C and 50 percent humidity holds nearly twice as much water as air at 20 °C and 100 percent humidity. Relative humidity tells you how close the air is to saturation, not how much water is in it.

Dew point is the more useful number, and it is the one meteorologists actually use: the temperature to which the air must be cooled for it to reach saturation. It is an absolute measure of how much moisture is present. A dew point above about 24 °C is oppressive to almost anyone, which is exactly the Kolkata or Chennai pre-monsoon experience, and it is why a dry 40 °C in Jaisalmer is more bearable than a humid 34 °C on the coast.

And it explains a thing everyone has seen. A cold bottle taken out of a fridge gets wet on the outside. The water is not leaking through the glass. Air touching the cold surface is chilled below its dew point, and its vapour condenses. Dew on grass at dawn and mist on a bathroom mirror are the same event.

It also explains why humid heat is genuinely dangerous. The body sheds heat mainly by evaporating sweat, and evaporation slows as the air approaches saturation. At high humidity, sweat runs off instead of evaporating and does nothing. The combined measure is the wet-bulb temperature — what a thermometer reads with a wet cloth around the bulb — and a sustained wet-bulb temperature above roughly 35 °C is unsurvivable for a healthy person even at rest in shade, because there is no longer any route for the body to dump heat. Parts of the Gangetic plain and the Persian Gulf have now recorded brief excursions close to that. Part 14 returns to it.

Why rising air cools without losing any heat

This is the single most important mechanism in weather, and it is not obvious.

Air pressure falls with height (Chapter 2.1). So a parcel of air that rises expands, because there is less pressure squeezing it. Expanding means pushing outward against the surrounding air, which is work, and doing work costs energy. The parcel has no other source of energy, so it takes it from its own internal store — and internal energy in a gas is molecular motion, which is temperature.

So the parcel cools purely by expanding. No heat is removed. This is called adiabatic cooling, from the Greek for "not passing through", meaning no heat crosses the parcel's boundary.

You can feel it work in reverse. A bicycle pump gets hot at the barrel when you compress air into a tyre. A deodorant can gets cold as the propellant expands out of it. Same physics, both directions.

The rate for dry air is 9.8 °C per kilometre of ascent. That is fixed by the physics of air and gravity, and it is called the dry adiabatic lapse rate.

But once the parcel reaches saturation and starts condensing, the rate drops — to somewhere between 4 and 7 °C per kilometre depending on temperature. The reason is latent heat: turning vapour back into liquid releases the energy that was absorbed when it evaporated, and that release partly offsets the cooling. This is the moist adiabatic lapse rate, and the difference between the two rates is the engine of every storm on this planet. Chapter 2.4 uses it directly.

Stability: whether the air will rise on its own

Compare two numbers. The environmental lapse rate is how fast temperature actually falls with height in the air around you on a given day, measured by a weather balloon. The adiabatic lapse rate is how fast a rising parcel cools itself.

• If the surrounding air cools slowly with height (less than the parcel does), a rising parcel quickly finds itself colder and denser than its surroundings and sinks back. Stable air. Nothing rises, so no clouds form, or only flat layered ones. Pollution accumulates.
• If the surrounding air cools rapidly with height (faster than the parcel does), a rising parcel stays warmer and lighter than its surroundings and keeps going up on its own. Unstable air. Towering clouds, showers, thunderstorms.

This is why afternoon thunderstorms are a thing. Strong surface heating makes the lowest air much warmer than the air above it — a steep environmental lapse rate — so the atmosphere becomes unstable in the afternoon and stabilises overnight.

A temperature inversion is the extreme stable case: temperature actually rising with height. Nothing can rise through it. This traps smoke and fog beneath a lid, which is the Delhi winter problem named in Chapter 2.2, the London killer fog of 1952, and the reason valley towns get bad air.

The four ways air is forced upward

Rising air is what makes clouds, so every cloud on Earth traces to one of four lifting mechanisms.

Convection. The ground heats, bubbles of warm air detach and rise. Gives puffy cumulus clouds with flat bases, in the afternoon, over land.

Orographic lifting. Air is forced up over a mountain range. This is the mechanism that makes Cherrapunji and Mawsynram in Meghalaya among the wettest places on Earth — moist Bay of Bengal air is funnelled into a narrowing valley and shoved up a steep escarpment. It also makes the rain shadow behind every range: the air comes down the other side dry.

Frontal lifting. Two air masses of different temperature meet and the warm, lighter one is forced up over the cold, denser one. A warm front is warm air riding gently up over retreating cold air — a shallow slope, hundreds of kilometres of gradually thickening cloud, steady drizzle for hours. A cold front is cold air bulldozing under warm air — a steep slope, narrow band, violent lifting, heavy showers and thunder, then a sharp clearance and a temperature drop. This is why the two feel completely different when they pass over you.

Convergence. Air flowing in from several directions has nowhere to go but up. This is what happens along the equatorial trough, and it is the mechanism behind the low-pressure systems that carry monsoon rain across India.

How a cloud actually forms

Water vapour does not condense easily on its own. In perfectly clean air you would need several hundred percent relative humidity before droplets formed spontaneously, because a very tiny droplet has such extreme surface curvature that molecules escape from it faster than they arrive.

So condensation needs a surface to start on. Those surfaces are cloud condensation nuclei — microscopic particles of sea salt, dust, soot, pollen, sulphate, volcanic ash. There are typically hundreds to thousands per cubic centimetre of air, so there is never a shortage in practice.

This has a consequence that shows up in the sky. Clean maritime air has few nuclei, so the available water goes into relatively few, relatively large droplets — and clouds over the ocean rain more readily. Polluted air has enormous numbers of nuclei, so the same water is divided into many more, much smaller droplets. Those clouds are brighter, whiter, longer-lived and less inclined to rain. This is a real and measured effect of air pollution on cloud behaviour, and it is one of the largest single uncertainties in climate modelling, because it works to cool the planet while the gases work to warm it.

The height where condensation begins is the lifting condensation level, and it is why cumulus clouds all have flat bottoms at the same altitude on a given day — every parcel rising from the same surface conditions reaches saturation at the same height.

The cloud families

The cloud classification, arranged by height. The naming system was devised by Luke Howard, a London pharmacist, in 1802, and it has survived essentially unchanged for two centuries. Image: Wikimedia Commons.

Howard's system uses four Latin roots that combine, which is why the names are learnable rather than arbitrary.

• Cirrus — a curl of hair. High, wispy, made of ice.
• Cumulus — a heap. Puffy, convective.
• Stratus — a layer. Flat, spread out.
• Nimbus — rain.

Combine them and the name tells you what you are looking at. Cirrostratus is a high thin ice sheet — it causes the halo around the Sun or Moon, and it is often the first sign of an approaching warm front, twelve to twenty-four hours ahead of the rain. Nimbostratus is a thick low rain layer, the grey all-day drizzle. Cumulonimbus is a heaping rain cloud, which is a thunderstorm, and it is the one Chapter 2.4 is about.

How a raindrop is made — two routes

The problem. A cloud droplet is about 0.02 mm across. A raindrop is about 2 mm. That is a hundred times the diameter, which is a million times the volume. Simple condensation is far too slow to bridge that gap — it would take days, and clouds rain within tens of minutes.

Route one: collision and coalescence, in warm clouds. Droplets are not all the same size. Bigger ones fall faster, sweep up smaller ones on the way down, grow, fall faster still. Once a droplet is large enough, this runs away. This dominates in the tropics, where clouds are warm all the way up, and it is why tropical rain arrives as large drops in heavy bursts.

Route two: the ice process, in cold clouds. This is the more common route worldwide and it turns on a subtle fact. In a cloud that is below freezing, water droplets and ice crystals can coexist — many droplets remain liquid well below 0 °C, which is called supercooling, because freezing also needs a nucleus and suitable ones are scarce.

And ice has a lower saturation vapour pressure than liquid water at the same temperature. In plain terms: at a given temperature, molecules escape more readily from a liquid surface than from an ice surface. So in a mixed cloud the air is simultaneously saturated with respect to water and supersaturated with respect to ice. Vapour therefore deposits onto the ice crystals while the droplets evaporate to replace it. The ice grows at the expense of the water, fast.

This is the Bergeron–Findeisen process, and it explains why most rain in the mid-latitudes starts life as snow, melting on the way down. It also explains why cloud seeding is attempted with silver iodide — its crystal structure resembles ice closely enough to act as a freezing nucleus — and why the measured results of cloud seeding remain modest and hard to verify, since you can never know what that particular cloud would have done unseeded.

Hail is the ice route with a violent updraft attached. A pellet is carried up, accumulates a layer of ice, falls, is caught by another updraft, and goes round again. Cut a large hailstone open and the layers are countable, like tree rings, each one a trip.

Fog, dew and frost

Fog is a cloud at ground level, and the classification is by how the air got cooled to its dew point. Radiation fog forms on clear calm nights when the ground radiates heat away and chills the air above it — this is the fog that closes north Indian airports and roads in December and January, made far denser by the pollution particles providing extra condensation nuclei. Advection fog forms when warm moist air moves over a cold surface, which is San Francisco's summer and the fog banks off Newfoundland. Upslope fog is orographic lifting doing it.

Frost is not frozen dew. When the dew point is below freezing, water vapour goes straight from gas to solid without passing through liquid — deposition. That is why frost is feathery and crystalline rather than a sheet of ice.

Where this shows up in your life

Drying clothes. Clothes dry by evaporation, so the rate depends on how far the air is from saturation and how fast it is replaced. This is why clothes dry poorly on a humid overcast day even in warmth, well on a dry windy day even in cold, and why a fan helps more than heat in a humid climate.

Air conditioning is a dehumidifier. An air conditioner cools a coil below the room's dew point, moisture condenses on it and drains away, and that dripping outdoor pipe is where the comfort actually comes from in a humid climate. A large share of the energy an air conditioner uses in Chennai or Mumbai goes into removing water, not into lowering temperature.

Contrails. An aircraft engine emits water vapour and soot particles into extremely cold air. The vapour condenses on the soot and freezes. Whether the trail vanishes in seconds or spreads into a sheet lasting hours tells you the humidity of the upper air — a persistent spreading contrail means the air up there is already near saturation with respect to ice, which often means a front is on the way.

And the reason a hot day "breaks". Afternoon heating destabilises the air, convection builds a cumulonimbus, and the storm that follows both cools the surface and stabilises the column. The relief you feel after a pre-monsoon thunderstorm in Delhi or Kolkata is the atmosphere resetting itself.

What the next page covers

Take an unstable atmosphere, plenty of moisture, and a mechanism to organise the rising air instead of letting it happen in scattered bubbles, and you get storms — the most concentrated release of energy in the natural world short of a volcano. Chapter 2.4 covers thunderstorms, lightning, tornadoes and tropical cyclones: how each one organises itself, where lightning's charge comes from, why the strongest tornadoes happen in one specific part of North America, how a cyclone builds an eye, and why the storm surge kills more people than the wind.