An overcast sky, falling pressure, a rising wind: the words of weather are familiar, yet we often use them without knowing what they measure or why they change. This Atlas gathers 14 short, precise pages to help you understand what you see overhead and what the forecasts are saying.
Each cloud page describes one genus of the International Cloud Atlas: how it forms, how to recognise it, what weather it brings and what risks it carries, with a drawing specific to that genus. Each page on a measurement explains the quantity, its unit and typical values, then applies it to today's forecast for the country's largest city.
Clouds
- Cloud types: ten genera, three levelsA table of the ten genera: level, base height, appearance, associated weather.
- Cirrus: the wispy cloud of iceWhite wisps of ice, very high: fair weather, unless they thicken.
- Cirrocumulus: the finest dappled skyTiny white grains with no shading, very high: a dappled sky that does not last.
- Cirrostratus: the veil of ice that makes the haloA milky veil very high up: the halo often signals rain.
- Altocumulus: the woolly sky of the middle levelWhite and grey patches at mid-height: sometimes a sign of summer storms.
- Altostratus: the grey sheet that dims the sunA grey sheet where the sun looks like it is behind frosted glass: rain is near.
- Nimbostratus: the cloud of steady rainA very thick grey layer: rain or snow falling steadily for hours.
- Stratocumulus: the grey sky in rollsLarge grey rolls, low: the gloom of a winter anticyclone.
- Stratus: the low, uniform cloud that hugs the groundA very low, uniform grey layer: drizzle possible, a close relative of fog.
- Cumulus: the cauliflower cloud of fair weatherA flat base, a domed top: cumulus is born of rising warm air.
- Cumulonimbus: the thunderstorm cloudThe storm tower: violent showers, hail, lightning, gusts, from a low base to the anvil.
The measurements of weather
- Atmospheric pressure: what a barometer measuresThe hectopascal, the standard value, the fall with height and reading the trend.
- Air humidity: relative humidity and dew pointRelative humidity, dew point and what temperature changes about saturation.
- Precipitation: rain, snow, hail and intensitiesDrizzle, rain, snow, hail: formation, units and intensity classes.
- Wind: speed, gusts and the Beaufort scaleThe Beaufort scale, mean wind and gusts, direction and units.
Where the text and figures come from
Cloud genera, levels and heights follow the International Cloud Atlas of the World Meteorological Organization (WMO). The heights given are those for temperate regions. Scales (Beaufort, rainfall intensity) and formulas (pressure with height, humidity) come from the references named at the foot of each page.
The worked examples are recalculated at each update from the Open-Meteo forecast for the country's largest city: they are model values on a grid, not measurements. The drawings are original and schematic; they show the typical shape of a genus, not a photograph.