Reading the Sky
Clouds are the atmosphere's visible language — their shape, altitude, and thickness tell a trained observer everything from the current air mass to the weather likely in 12 hours. The World Meteorological Organization classifies clouds into 10 genera, a system first proposed by Luke Howard in 1803 using Latin names that are still used today: cirrus (curl), stratus (layer), cumulus (heap), and nimbus (rain). This simulation lets you explore how temperature, moisture, and instability determine which clouds form.
Altitude and Composition
Clouds are grouped into three altitude bands. High clouds (cirrus, cirrocumulus, cirrostratus) float above 6 km where temperatures are below -30°C — they are composed entirely of ice crystals and appear thin and wispy. Mid-level clouds (altostratus, altocumulus) occupy the 2-6 km range and may contain a mix of water droplets and ice. Low clouds (stratus, stratocumulus, nimbostratus) form below 2 km and are composed primarily of water droplets. Cumulonimbus, the thunderstorm cloud, spans all three levels.
The Lifting Condensation Level
Every cloud begins at the lifting condensation level (LCL) — the altitude where rising air cools to its dew point and water vapor begins to condense. The LCL can be estimated with a beautifully simple formula: multiply the dew-point depression (surface temperature minus dew point) by 125 to get the height in metres. On a humid tropical day with a 4°C dew-point depression, clouds form at just 500 m. On a dry desert day with a 25°C spread, the cloud base may be above 3 km — or clouds may not form at all.
Stability and Cloud Shape
Whether clouds grow vertically into towering cumulus or spread horizontally into flat stratus depends on atmospheric stability. In an unstable atmosphere, rising air continues to accelerate upward because it remains warmer than its surroundings — producing cumulus and cumulonimbus with dramatic vertical development. In a stable atmosphere, rising air quickly becomes cooler than its environment and stops ascending, spreading horizontally into thin stratiform layers. The instability slider in the simulation above directly controls this vertical-versus-horizontal growth.