The atmosphere
Weather happens in the troposphere, the lowest layer of the atmosphere, about 8–16 km deep. It contains most of the air’s mass and almost all its water vapour. On average the temperature falls by about 6.5 °C for every kilometre you climb, which is why mountain tops are cold. At the top of the troposphere, thetropopause, the temperature stops falling, acting as a lid on most clouds.
The ingredients of weather
- Temperature: how warm the air is, set mainly by sunshine and where the air came from.
- Humidity: how much water vapour the air holds. Relative humidity compares this with the maximum the air could hold at that temperature.
- Air pressure: the weight of the air above, about 1,013 hPa at sea level on average.
- Wind: air moving from high pressure towards low pressure.
How clouds form
When the Sun warms the ground, the air above it warms, becomes less dense and rises. As it rises the pressure around it falls, so it expands and cools: about 9.8 °C per kilometre while it is unsaturated. Eventually it cools to its dew point, the temperature at which its water vapour condenses onto tiny particles of dust and salt, forming billions of cloud droplets. That height is the flat cloud base you often see under cumulus clouds. Air can also be forced up over mountains or along weather fronts.
Stable and unstable air
Whether a cloud stays small or grows into a giant depends on the surrounding air. If the rising air stayswarmer than its surroundings, it keeps rising, like a hot-air balloon: the atmosphere isunstable. Inside a cloud, condensation releases latent heat, so cloudy air cools only about 4–7 °C per kilometre and can keep rising for many kilometres. If the surroundings are warm aloft, the air isstable: rising air soon becomes cooler than its surroundings and sinks back, so clouds stay flat or do not form.
High and low pressure
In a high-pressure system (anticyclone), air slowly sinks and warms. This creates a warm layer, aninversion, that caps rising air, so highs bring settled, often sunny weather, and calm nights with fog or frost. In a low-pressure system (depression), air converges and rises, so clouds build easily and the weather is unsettled, wet and windy.
Rain, snow and storms
Cloud droplets are far too small to fall. In deep clouds, droplets collide and merge, and ice crystals grow at the expense of droplets, until they are heavy enough to fall as precipitation. If the air below the cloud is below about 2 °C, it reaches the ground as snow. In the tallest clouds, cumulonimbus, strong updrafts separate electric charges, producing lightning and thunder, and can carry ice up and down until it grows into hail.
The numbers behind the model
Td = c·γ / (b − γ), γ = ln(RH/100) + b·T/(c + T) · cloud base ≈ 125 m × (T − Td)
The simulator uses the Magnus formula for the dew point (b = 17.62, c = 243.12 °C), the dry rate of 9.8 °C per km below the cloud, and the moist adiabatic rate inside it, which depends on temperature and pressure.
Key terms
- Relative humidity
- Water vapour in the air as a percentage of the maximum it could hold at that temperature.
- Condensation
- Water vapour turning into liquid water droplets.
- Lapse rate
- How fast temperature falls with height.
Common misconceptions
- “Clouds are made of water vapour.” Vapour is invisible; clouds are tiny liquid droplets or ice crystals.
- “Higher up is warmer because it’s closer to the Sun.” The air is heated mainly by the ground, so it gets colder with height.
- “100 % humidity means it is raining.” It means the air is saturated; you might have fog or cloud without rain.
Key takeaways
- Rising air expands and cools; clouds form when it reaches its dew point.
- More humid air means a lower cloud base.
- Unstable air (often low pressure) builds tall clouds and storms; stable air (high pressure) keeps skies clear.
- Latent heat released by condensation powers thunderstorms.
Model note: a single bubble of air rising through a simplified atmosphere. Real forecasts use measured temperature and humidity at every height, wind, fronts and much more.