Meteorology – Clouds | Astrolabe Sailing

Clouds can give big clues about the weather, so it makes sense for sailors to be able to identify the different types of clouds, and what they might mean for the people sailing below them.

While it is easy to focus on what the weather is happening at sea level, clouds give an indication of what is happening above us and provide a 3D picture of the movement of the atmosphere.

Lets look at some clouds!

A cloud is defined as suspended water in liquid droplet or ice crystal form. Clouds are condensed water vapour – i.e. it has changed from a gas to a liquid state. For this condensation to occur there must be some kind of condensation nuclei – these are small particles of dust, salt or smoke which have a suitable surface for water to stick to.

You can read more about how clouds are formed here. Basically they are formed by air rising and cooling until the dew point is reached – this is when the water condenses and becomes visible – as a cloud.

Clouds show us the stability of the atmosphere, and indicate where we are in a system. They can give the first indications of an approaching depression, and indicate future changes in the wind and reduced visibility in rain.

Clouds are classified by:

Shape
Vertical extent
Altitude

Terminology:

Cirro = High Level
Alto = Mid Level
Nimbo/Nimbus – High moisture content
Cumulus – heaped or lumpy cloud
Stratus – layers of flat cloud
Cirrus – high wispy

High Cloud

High Cloud is defined as clouds over 17,000ft and above

Cirrus (Ci)
Cirrostratus (Cs)
Cirrocumulus (Cc)

Cirrus (Ci)

Cirrus clouds are the highest and are made up of ice crystals. They indicate the presence of a jet stream and the possible approach of a warm front. No rain.

Cirrus above and thick stratus below – which was very challenging to drive through on the way back down the mountain.

Cirrostratus (Cs)

These clouds are also made up of ice crystals. As the cloud is very high, it is also very cold, and so the moisture content is also low. It is thin and the sun can still be seen – sometimes it is refracted creating a halo around the sun. No rain

Cirrocumulus (Cc)

This cloud resembles sheep’s wool. This cloud is also made up of ice crystals and the low temperature means low moisture. No rain.

Middle Cloud

Middle cloud is defined as clouds around 7,000 – 17,000ft and is called Alto

Altostratus (As)
Altocumulus (Ac)

Altostratus (As)

Mid level, featureless grey and overcast layer which extends for many miles. It produces lingering drizzle or light snow.

Altocumulus (Ac)

Mid level cloud made up of joined or separated cloudlets. As it is lower it is warmer and will have a higher moisture content. Can be ice crystals and water droplets. This cloud comes in many different formations. Light rain possible.

Low Cloud

Low cloud is considered to be from the ground to 7,000ft

Stratus (St)
Cumulus (Cu)
Stratocumulus (Sc)
Towering Cumulus (Tcu)
Cumulonimbus (Cb)
Nimbostratus (Ns)

Stratus (St)

Featureless grey overcast layer cloud – this forms very low and when it is at ground or sea level it is called fog or mist.

Cumulus (Cu)

Fair weather clouds that form on sunny days. They generally have flat bases and cauliflower shaped tops. Very light or no rain.

Stratocumulus (Sc)

Very common cloud – low layer, individual or joined up clumps or ‘cloudlets’. Stratocumulus and Altocumulus look very similar – the only difference is the base of the cloud height. Can have light showers.

Towering Cumulus (Tcu)

When a normal fair weather cumulus grows taller than they are wide, then they become Towering Cumulus. They can then form in to Cumulonimbus clouds. They can produce heavy showers and have strong up draughts or wind beneath them.

A beautiful Towering Cumulus cloud photo from the Bahamas courtesy of Byn Always from Oh Sail Yes blog.

A beautiful towering cumulus in the Florida Keys by Jennifer MacLean from Sunshine Coast Adventures.

Cumulonimbus (Cb)

This is an enormous storm cloud which has a top shaped like an anvil. This indicates the direction in which the cloud is travelling. These clouds can form in to thunderstorms and have heavy showers and strong winds. The picture above probably isn’t a great example, but it shows a Southerly front approaching over Lyttelton Harbour.

Thunder Clouds over Lake Macquarie photo by Penny Anne Hodgson

Nimbostratus (Ns)

A depressing lingering cloud. It is thick, grey and featureless rain cloud. It is usually formed from thickening & lowering Altostratus clouds. Comes with moderate to heavy widespread rain.