What Are Clouds?
Clouds are visible masses of tiny water droplets or ice crystals suspended in the atmosphere. They form when water vapour in the air rises, cools, and condenses around tiny particles of dust, salt, or smoke. The temperature at which this happens is called the dew point.
Clouds are essential to the water cycle — they are the stage between water vapour in the air and precipitation (rain, snow) falling to the ground.
How Are Clouds Classified?
Clouds are classified by height and shape:
Height:
- High clouds: Above about 6000 m (contain mainly ice crystals)
- Middle clouds: About 2000–6000 m
- Low clouds: Below about 2000 m
- Vertical clouds: Extend through multiple levels
Shape:
- Cumulus (Latin: heap) — puffy, heaped clouds
- Stratus (Latin: layer) — flat, layered clouds
- Cirrus (Latin: curl) — thin, wispy, high-level clouds
- Nimbus (Latin: rain cloud) — rain-producing clouds
Luke Howard, a British pharmacist and amateur meteorologist, devised the modern cloud classification system in 1803 — using Latin names that are still in use today. He identified three main types (cumulus, stratus, cirrus) and showed that clouds follow recognisable patterns that can be used to predict weather.
Common Cloud Types
| Cloud | Height | Appearance | Weather clue |
|---|---|---|---|
| Cirrus | High | Thin, wispy, hair-like streaks | Unsettled weather may follow in 24–48 hours |
| Cirrostratus | High | Thin veil covering the sky; halos around Sun/Moon | Rain likely within 12–24 hours |
| Altocumulus | Middle | Grey/white patchy rows | Often appears in settled spells; some rain risk |
| Altostratus | Middle | Grey uniform sheet; Sun dimly visible | Rain or snow likely soon |
| Stratus | Low | Grey, flat, foggy layer | Drizzle or overcast conditions |
| Stratocumulus | Low | Lumpy, rounded rows | Common in winter; light rain possible |
| Cumulus | Vertical base | White, puffy "cauliflower" | Fine weather (small); heavy showers (large) |
| Cumulonimbus | Vertical, very tall | Massive, dark anvil shape | Thunderstorms, heavy rain, hail, lightning |
Cumulonimbus — Storm Clouds
Cumulonimbus (Cb) clouds are the most dramatic clouds. They extend from low altitudes to over 12,000 m and contain powerful updrafts and downdrafts. They bring:
- Heavy rainfall or hail
- Lightning and thunder
- Strong gusty winds
You can predict a storm is coming by watching cumulus clouds. If small white cumulus clouds are building upward rapidly — growing taller rather than spreading out — they may develop into cumulonimbus storm clouds within 20–30 minutes. A darkening base is a warning sign.
Real world: Reading the sky before a storm
Experienced farmers, sailors, and hikers have long used clouds to predict weather. When you see high cirrus clouds (feathery streaks) appearing, then thickening into cirrostratus (a milky veil), then altostratus (grey sheet), and finally dark nimbostratus or cumulonimbus — you are watching a weather front approach. Each cloud type tells you where you are in that sequence. This is why old sailors said "Red sky at night, sailor's delight; red sky in the morning, sailor's warning."
Real world: Cumulonimbus and aviation safety
Cumulonimbus clouds are some of the most dangerous hazards for aircraft. They contain powerful updrafts, severe turbulence, icing, lightning, and hail — all of which can damage planes. Pilots actively avoid flying through them and must navigate around them. Weather radar on planes detects these clouds by their moisture content, shown in red and orange on cockpit displays. Airlines' flight planning departments specifically route flights to avoid regions of high cumulonimbus activity.
Real world: Cloud types and weather forecasting apps
When you use a weather app on your phone, it is using satellite and radar images to detect cloud types over large areas. Meteorologists at weather centres watch cloud patterns forming and moving across the country to predict rain days in advance. Satellites distinguish cirrus (ice clouds, high altitude) from cumulonimbus (large, tall, bringing storms) and feed this into numerical weather prediction models used by the Met Office, NOAA, and similar services worldwide.
Where do you see this in real life? Look up next time you are outside — identify the cloud types overhead. Wispy cirrus high up? No immediate rain. Dark, towering cumulonimbus on the horizon? Find shelter soon.
Practice Problems
Why do cirrus clouds appear white and wispy while cumulonimbus clouds appear dark and grey?
Show answer
Name two types of cloud that bring rain, and describe them.