Module 25: Weather

This is dew — water condensed directly from water vapour in the air onto the surface. The mechanism: during a clear night, surfaces (car bonnet, grass blades) radiate their heat to the sky without a cloud "blanket" to reflect it back (this is radiative cooling). The surface temperature drops below the dew point — the temperature at which the surrounding air is 100% saturated and can no longer hold its water vapour. Water vapour then condenses onto the cooler surface as liquid water. Clear nights produce the most dew because cloud cover prevents radiative cooling. On a cloudy night, surfaces stay warmer because clouds absorb and re-emit longwave radiation back to the surface — the same mechanism that makes cloudy nights warmer than clear nights. If the surface temperature drops below 0°C before moisture condenses, you get frost (ice crystals) rather than dew. Dew is not "falling" from the sky — it forms directly on the surface from local air moisture.

Lightning and thunder occur simultaneously — but light travels at ~300,000 km/second while sound travels at ~340 m/second. Light arrives essentially instantaneously regardless of distance. Sound takes about 3 seconds per kilometre. The rule: count the seconds between the flash and the thunder, then divide by 3 for kilometres (or divide by 5 for miles). 6 seconds = 2 km. 15 seconds = 5 km. When lightning and thunder are simultaneous, the strike is directly overhead. Thunder is the acoustic shockwave from the rapid superheating of air by the lightning channel — the air expands explosively at ~30,000 K (five times hotter than the Sun's surface), creating a pressure wave. The rumbling that follows a single flash occurs because the lightning channel is kilometres long — you're hearing thunder from different parts of the channel arriving at different times, and reflections off clouds and terrain.

A rainbow appears at exactly 42° from the antisolar point (the point directly opposite the Sun from your perspective). When sunlight enters a water droplet, it refracts (bends), reflects off the back of the droplet, and refracts again as it exits. Different wavelengths (colours) refract at slightly different angles — red at 42°, violet at 40°. You see red on the outside and violet on the inside of the primary bow. This 42° geometry is fixed by physics — only droplets that happen to be at that exact angle from the antisolar point send their light to your eye. The "bow" shape is because the antisolar point is directly behind you and you're tracing a 42° cone in all directions — which is a circle (or semicircle above the horizon). No two people see the same rainbow because each person's antisolar point is different — you see light from different sets of droplets than the person standing next to you. You can never reach a rainbow's base; as you move toward it, the antisolar point shifts with you and the rainbow remains at the same angle ahead.

Water has a very high specific heat capacity (~4× that of land) — it takes much more energy to change its temperature. This creates the maritime moderating effect. The sea heats up slowly in summer and cools slowly in winter. Coastal areas therefore have smaller temperature ranges: cooler summers and milder winters compared to inland areas at the same latitude. Sea breezes add to this: during the day, land heats faster than sea → air rises over land → cooler sea air flows in (sea breeze) — creating a refreshing coastal breeze even when inland is still. At night, the reverse happens (land breeze). High coastal humidity also makes temperatures feel different from what a thermometer shows: 25°C at 80% humidity feels much hotter than 25°C at 40% humidity, because sweat evaporates slowly when the air is already nearly saturated. Inland continental areas experience larger diurnal (day-night) and seasonal temperature swings — 40°C summers and −20°C winters in places like central Russia or the American Midwest are normal, while oceanic islands rarely see either extreme.

A single record-hot summer alone doesn't "prove" global warming in the sense of being conclusive proof of anthropogenic climate change — but it is consistent with it, and the pattern of record-hot summers occurring with increasing frequency is highly significant. Individual events are weather (variable). The statistical pattern over decades is climate. What does provide compelling evidence: the global average temperature anomaly trend over 150 years; the fact that the 10 warmest years globally have all occurred since 2005; the shrinking Arctic sea ice measured over decades; the poleward shift of climate zones; attribution studies that calculate how much more likely specific events are due to climate change. As for a cold winter: this is exactly the loaded dice problem. Climate change shifts the probability distribution toward hotter outcomes, but the distribution doesn't disappear. You still get cold winters — they just become less frequent and less extreme. Moreover, some patterns of warming (weakening of the Arctic polar vortex) can paradoxically push cold Arctic air southward in winter, causing cold snaps in mid-latitudes. Cold winters and record-hot summers can co-exist in a warming world.

Take shelter when you can hear thunder — if you can hear it, you're within 10 miles (16 km) of lightning and at risk. The rule of thumb used by safety organisations is "when thunder roars, go indoors" — don't wait for rain to start. A growing cumulonimbus signals increasing danger: flat base (condensation level), vigorous cauliflower-shaped top expanding rapidly upward, and darkening base indicating heavy precipitation. The anvil shape (glaciated top spreading horizontally) forms at the tropopause — the boundary between the troposphere and the stratosphere. When the rising updraft reaches the tropopause, it hits a stable layer (temperature stops falling or begins rising) that acts like a lid. The air can no longer rise, so it spreads outward horizontally, carried by upper-level winds (typically westerlies), creating the characteristic anvil shape pointing downwind. The anvil tells you the storm is mature. Ice crystals at −50°C or colder give it the fibrous, wispy appearance distinct from the sharp cauliflower edges of lower levels. A storm with an "overshooting top" — a dome of cloud punching above the anvil — indicates an exceptionally powerful updraft and is associated with the most severe storms.

Two separate limitations are at work. The first is observational: we never perfectly know the current state of the atmosphere. Temperature, pressure, wind, and humidity observations have measurement error, and the global observing network (weather stations, radiosondes, aircraft, satellites, buoys) has gaps — especially over oceans and polar regions. Better computing allows us to initialise models more accurately from these imperfect observations. The second limitation is fundamental chaos: the atmosphere is a nonlinear dynamical system. Small differences in initial conditions grow exponentially over time — the famous "butterfly effect" described by Edward Lorenz in 1963. Two model runs started from slightly different initial conditions will produce similar forecasts for 3–5 days, begin to diverge around days 5–7, and be effectively unrelated by days 10–14. No amount of computing power can overcome this because it's an intrinsic property of the physics, not a technical limitation. Modern ensemble forecasting (running 50+ models simultaneously with perturbed initial conditions) quantifies this uncertainty: when all ensemble members agree, you have high confidence; when they diverge, uncertainty is high. This is why you can have high confidence in a 3-day forecast and essentially no confidence in day 12 — and why a forecast that says "probably warmer than normal next month" is the honest limit of what physics allows.