Question 1

Why do clouds form, and why do they stay up?

Accepted Answer

Clouds form when **air cools to the point where it can no longer hold its water vapour**, and they stay up because the droplets are so small that air resistance almost entirely offsets their weight.

**Why cooling causes condensation.** Warm air holds more water vapour than cold air. When air cools to its **dew point**, the vapour condenses onto tiny particles — dust, salt, pollen, pollution — called **condensation nuclei**. Without those particles condensation is remarkably difficult, and clean air can become substantially supersaturated without forming anything.

**What makes air cool, which is the same in every case: it rises and expands.** Rising air moves into lower pressure, expands, and **cools without losing heat to anything** — adiabatic cooling. That single mechanism explains almost all cloud:

**Convection**, where the sun heats the ground and warm air rises — producing the lumpy cumulus of a summer afternoon.

**Orographic lift**, where air is forced up over hills and mountains, which is why cloud sits on high ground and why one side of a range is wet and the other dry.

**Frontal lift**, where a warm air mass rides over a cold one, producing the layered sheets of an approaching front.

**Convergence**, where air flows together and has nowhere to go but up.

**Why they do not simply fall.** Cloud droplets are around a hundredth of a millimetre across. At that size the **ratio of surface area to volume is enormous**, so air resistance dominates and terminal velocity is a few centimetres per second — slower than the gentle updraughts holding them. A cloud is not floating; its droplets are falling extremely slowly through rising air.

**Why they are white, and grey.** Droplets are large compared with the wavelength of light, so they scatter all colours roughly equally, which appears white. A thick cloud **absorbs and scatters so much light** that little reaches the bottom, which we see as grey.

**Rain requires droplets to grow** roughly a million times in volume, by collision or via ice crystals — which is why not every cloud rains.

Question 2

What causes lightning?

Accepted Answer

A vast electrical discharge between regions of separated charge — and the honest position is that how the charge separates in the first place is still not fully settled, which surprises people about

Question 3

How does weather forecasting actually work?

Accepted Answer

By solving physics equations on a grid, starting from the best available picture of the atmosphere right now — and the limits of forecasting come from the fact that small errors in that starting

Question 4

How do stars form, and what happens when they die?

Accepted Answer

Stars form when gravity overcomes pressure in a cloud of gas and dust, and they die when they run out of fuel to resist gravity with. The entire life of a star is a standoff between those two forces,

Question 5

Why does ice float on water?

Accepted Answer

Almost every substance is denser as a solid than as a liquid, because cooling slows molecules and lets them pack more tightly. Water is a notable exception, and the reason is the geometry of hydrogen

Question 6

How do mountains form, and why do they stop growing?

Accepted Answer

Mostly through tectonic collision thickening the crust, and they stop growing because erosion and gravity eventually balance uplift — so a mountain range's height reflects an equilibrium rather than

Question 7

What causes rainbows, and why are they always curved?

Accepted Answer

Refraction, reflection and dispersion inside raindrops — and the curve is a consequence of geometry rather than of anything about the sky. What happens in a single drop: Light enters the drop and

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