Low level clouds are those with a cloud base between sea level and about 6,500 feet.
Cumulus are individual, detached clouds that have the appearance of floating cotton wool or cauliflower, with flat bases. The tops are typically bright white with a darker base.
Made mostly of water droplets, cumulus are the result of convection (remember the rising air in an unstable atmosphere?) and most commonly appear on fair weather days.
When small cumulus continue to grow, showers of rain or snow will being to form and the cloud can transition into a towering cumulus. Given the right conditions, they will continue to grow and eventually form cumulonimbus clouds.
Commonly known as thunderstorms, cumulonimbus are convective clouds with extensive vertical development into the middle and high levels. The tops often spread out into smooth anvil-shaped cirrus. The cloud base is typically flat and very dark.
Cumulonimbus begin life as cumulus that continue to grow due to an unstable or conditionally stable atmosphere. They produce heavy showers of rain and are the only cloud that creates lightning, thunder and hail.
Cumulonimbus clouds present many hazards to a pilot and we will talk about them more in the section on thunderstorms.
With a fairly uniform grey or white colour, stratus clouds are made from water droplets and usually appear rather dull, often persisting for long periods of time. They are typically formed through frontal lifting or orographic lifting of stable air, or fog layers lifting as the sun heats the ground.
Stratus is usually associated with only very little precipitation, but if it becomes thick enough it can produce light drizzle or snow.
With a grey or whitish colour, Stratocumulus are a layer of patchy clumps of cloud that typically form when a layer of stratus begins to break apart. This often indicates the change in the weather that is associated with an approaching front.
However, this type of cloud can be present in any weather and it is rare for anything other than light drizzle to be produced by stratocumulus.
The term nimbo refers to a cloud that is rain bearing, so nimbostratus is simply stratus cloud that is rain bearing. These common clouds are grey with a base that is hard to distinguish due to the continuous rain or snow.
Nimbostratus is found where widespread ascent is caused by a convergence or in a stable atmosphere with a slow moving front.
Often there are individual lower hanging, ragged clouds beneath the base of nimbostratus.
Middle level clouds are those with a cloud base between about 6,500 feet and 20,000 feet.
Altostratus are middle level clouds that can be composed of both liquid water droplets and ice crystals, though there is typically more water than ice. Altostratus is a large sheet of thin cloud that often covers the entire sky. It can be semi-transparent, allowing the sun or moon to be visible with a somewhat fuzzy appearance.
Altostratus is typically formed by the widespread ascent associated with frontal lifting or convergence. When thick enough, it can create intermittent or continuous rain.
Altocumulus can be either small patches of cloud, often with the appearance of rounded masses, or can spread to form a wide layer.
They are composed of both water and ice, making them appear less fluffy than low level cumulus.
Altocumulus is usually formed by turbulence in the middle level but they can also be the result of convection or orographic lifting when relative humidity is low.
High level clouds are those with a cloud base above roughly 20,000 feet.
Cirrus are thin, wispy clouds that form in narrow bands with a hair-like appearance. They are composed of ice crystals and are typically a result of frontal lifting or a convergence, though they also form at the top of large cumulonimbus clouds.
Cirrus are the most common type of high level cloud.
Also composed of ice crystals, Cirrostratus are sheet-like high level clouds that can cover the entire sky and produce a ‘milky’ appearance. However, they are relatively transparent so the sun or moon can often be seen through Cirrostratus, sometimes surrounded by a halo.
Like Cirrus, Cirrostratus is often the result of wide spread ascent of an air mass from frontal lifting or convergence.
Also made of ice crystals, Cirrocumulus is a somewhat rare type of high level cloud. They are composed of many small white clouds grouped together and often appearing to form ripples in the sky.
They are formed when vertical movement exists at high altitudes, typically as a result of two horizontal airflows meeting. They can also be a precursor to an advancing storm system.