Part A · what weather actually is — energy redistribution in the troposphere
The atmosphere as an engine
The troposphere
All weather happens here — the bottom 8–15 km of atmosphere. Temperature decreases ~6.5°C per km of altitude (the environmental lapse rate). Above it: stratosphere (ozone layer, no weather, very stable).
The engine
The Sun heats the Earth's surface unevenly (equator more than poles, land more than sea). This creates temperature differences → density differences → pressure differences → wind. Weather is the atmosphere attempting to redistribute this energy imbalance.
The water cycle's role
Water evaporates (absorbs energy) → rises (cools) → condenses into clouds (releases latent heat — this energy powers storms) → falls as precipitation → evaporates again. This cycle moves enormous amounts of energy.
The Coriolis effect: Earth's rotation deflects moving air to the right in the Northern Hemisphere and left in the Southern. This is why low-pressure systems spin anticlockwise in the Northern Hemisphere (cyclones) and clockwise in the Southern. At the equator, the Coriolis effect is zero — which is why powerful rotating storms (hurricanes, typhoons) cannot form within about 5° of the equator.
Part B · pressure systems — high and low, what they mean and why
High pressure (anticyclone)
Air sinks from above → warms as it descends (compression) → reduces relative humidity → clouds evaporate → fair weather. Winds spiral outward (clockwise in N. Hemisphere).
Air rises → cools as it ascends → relative humidity increases → water vapour condenses → clouds form → precipitation. Winds spiral inward (anticlockwise in N. Hemisphere).
Typical pressure: 980–1,010 hPa. Severe storm: below 960 hPa
Part C · fronts — where weather changes happen
Cold front
Cold air undercuts warm air — sharp, fast
A wedge of cold air advances and undercuts warmer air ahead of it. Warm air is forced sharply upward → rapid condensation → tall cumulonimbus clouds → intense but brief rain, thunderstorms, gusty winds. After passage: rapid clearance, temperature drops sharply, pressure rises quickly. On a weather map: blue line with triangular "teeth" pointing in the direction of travel.
Warm front
Warm air overrides cold air — gradual, wide
Warm air slides gently up and over retreating cold air. The lifting is gradual → clouds form over a wide area (500–1,000 km ahead) → prolonged, steady rain. Arrival sequence: cirrus (high wispy clouds, 24h+ before), then altostratus, then nimbostratus (continuous rain). After passage: temperature rises, pressure stabilises. On a map: red line with rounded bumps (semicircles) pointing forward.
Occluded front
Cold overtakes warm — complex, often intense
As a depression matures, the faster-moving cold front catches up with the warm front. The warm air is lifted entirely off the surface → occluded front. Often produces persistent rain and unsettled weather. Marks the stage where a depression is weakening. On a map: purple line with alternating triangles and semicircles.
Stationary front
No movement — prolonged same weather
When neither air mass advances. Can produce days of persistent rain or fog over a region. On a map: alternating cold and warm front symbols facing opposite directions. Often responsible for prolonged flooding events when stationary fronts stall over a region for days.
Part D · atmospheric phenomena — what causes each
Part E · what "60% chance of rain" actually means
Forecast probability of precipitation (PoP) — the most misunderstood number in meteorology
What 60% does NOT mean
It does NOT mean: "it will rain for 60% of the day." It does NOT mean: "60% of the city will get rain." It does NOT mean: "there's a 60% chance it rains somewhere in the region at some point."
What 60% actually means
PoP = C × A, where C = confidence (probability that rain occurs somewhere in the forecast area) and A = the area coverage fraction if rain does occur. A 60% PoP typically means: there is a 60% probability that at least 0.01 inches (0.25mm) of precipitation will fall at any specific point in the forecast area during the forecast period.
Practical interpretation
A 60% PoP means: if you were to experience this forecast 10 times, it would rain on about 6 of those days (at your specific location). It says nothing about duration or intensity. You might get 5 minutes of drizzle or 2 hours of heavy rain — the probability is the same.
The decision threshold
Research shows: people tend to carry an umbrella when PoP ≥ 50–60%. But whether to carry one depends on the consequences of being wrong, not just the probability. For a formal outdoor event: even 30% might warrant precautions. For a 5-min walk: 80% might not.
Additional forecast numbers you should know: "Isolated showers" = less than 25% area coverage. "Scattered showers" = 25–54% coverage. "Numerous showers" or "widespread" = 55%+. These give area coverage context the PoP number alone doesn't. Humidity percentage is not PoP — it's relative humidity (how saturated the air is relative to its capacity at that temperature).
Part F · climate zones — how location shapes weather character
Part G · weather vs climate — and the global warming question
The most important distinction in climate science
Weather
The state of the atmosphere at a specific place and time. Highly variable day to day, week to week, year to year. Subject to enormous natural variability. A single hot summer, cold winter, or unusual storm is weather. Cannot be reliably forecast more than ~10 days ahead.
Climate
The statistical average of weather over long periods — typically 30 years (the WMO standard climate normal period). Climate describes the pattern, not any individual event. "Climate is what you expect; weather is what you get." (commonly attributed to Mark Twain, though disputed).
The signal vs noise problem — what the data actually shows
Global average surface temperature has risen approximately +1.1–1.2°C since the pre-industrial baseline (1850–1900). The warming trend is clearly visible in 30-year averages. Individual years vary: a cold year doesn't contradict the trend, just as one bad day on the stock market doesn't mean the market is declining.
Annual global average temperature anomaly — illustrative of the trend
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Year-to-year variation is high (some years cooler than the previous, some much warmer). But the 30-year trend is unambiguous. The 10 warmest years on record globally have all occurred since 2005. 2023 and 2024 set successive all-time global temperature records.
The loaded dice analogy (NASA scientist James Hansen, 1980s): Before climate change, rolling a die gave roughly equal chances of a hot, average, or cold summer. Climate change loads the die — you still get variety (some cool summers), but hot summers now come up far more often. A single hot summer might have been equally likely before. A series of record-breaking summers is consistent with loaded dice even though any individual outcome is possible. Statistical attribution science can now calculate: "this specific heatwave was X times more likely due to climate change." For recent European heatwaves, the multiplier has ranged from 2× to 5×.
Part H · test yourself
1. It's a clear night in summer and the forecast says no rain, but in the morning there's moisture on your car and grass. Where did it come from, and why does it appear overnight?
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.
2. Why does thunder follow lightning, and why can you estimate the storm's distance?
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.
3. Why does a rainbow always appear as a semicircle at a specific angle, and why can't two people ever see the same rainbow?
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.
4. You live near the coast and your friend lives 200km inland. The forecast shows the same temperature for both locations, but the coast always "feels" different. Why?
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.
5. This summer was the hottest in your city's recorded history. Does that prove global warming? And if temperatures are rising, why did last winter feel colder than usual?
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.