Clouds

Introduction

We are all familiar with clouds, but in this section we will look at the different types of cloud, how they are formed and the kind of weather they tend to be associated with. Clouds are composed of either water or ice depending on their temperature and altitude. Clouds can be divided at the most basic level into two types depending on their form:

• Stratiform Cloud – cloud in the form of a layer and usually of limited vertical extent                           
• Cumuliform cloud – cloud in a heaped (cauliflower like) form and often with extensive vertical development  

Clouds are also divided into 3 types depending on the height of their base.

• Low Level Cloud – has a base below 6500’ amsl
• Medium Level Cloud – has a base from 6500’ – 20000’ amsl. Identified by the prefix alto
• High Level Cloud – has a base above 20000’ amsl. Identified by the prefix cirro

Low Level Cloud

Stratus (St)

Low Stratus

Description: This is a stratiform cloud, which means that it is a layer type cloud with extensive horizontal extent, but limited vertical extent. It is usually grey and has little variation in colour. The sun is often visible through the cloud.
Location: It commonly forms as a very low level cloud, often as low as 200’ aal. It may form as a result of the lifting of fog or mist. It also occurs at a warm front. It forms in stable air.
Flying conditions: Usually smooth with the possibility of icing. Poor visibility expected and sometimes light drizzle.

Nimbostratus (Ns)
Description: This is similar to stratus, but has the Latin prefix nimbo meaning rain-bearing. It is darker in colour, often dark grey to almost black as a result of the large rain drops inside. It is usually thicker than Stratus and will usually completely hide the sun. Continuous rain or snow falls from it.
Location: Often at a warm front where it is responsible for the majority of the precipitation.
Flying conditions: Less smooth than in stratus with a possibility of moderate icing.

Cumulus (Cu)

Description: This are individual, detached cumuliform clouds formed as a result of warm air rising in an unstable atmosphere. They usually have a definite base and top.

Cumulus cloud

The tops are very white and look like cauliflowers and the bases may be flattened and darker. It may start to develop vertically, becoming initially a towering cumulus (TCu) and eventually a cumulonimbus (Cb).
Location: Common over the land on warm days, when it may be called fair weather cumulus. Also found after the passage of a cold front and in the warm sector.
Flying conditions: Usually bumpy below, and within, but smooth to the sides and above. In larger cumulus clouds light showers occur. Visibility between clouds is excellent.

   
                     Fair Weather Cumulus                                                 Towering Cumulus (TCu)

Stratocumulus (SC)

Description: This is cumulus cloud joined together to form a layer.
Flying conditions: Bumpy like cumulus, but usually smooth above. Drizzle possible.

Stratocumulus from above

Cumulonimbus (Cb)

Description: These are individual, detached large cumuliform clouds formed as a result of warm air rising in an very unstable moist atmosphere. They can extend to over 40000’ and are the clouds responsible for thunderstorms. They range in colour from black at the bottom through to white at the top. The top may be flattened over into an anvil as a result of strong upper winds. They can form extremely rapidly, and can merge to form a line of Cumulonimbus clouds.

                          large individual CB                                                            CB with anvil

Flying conditions: Severe turbulence to be expected in and around the cloud. Strong up and down draughts with heavy rain, snow and hail are likely. Severe icing will be encountered inside the cloud as will lightning strikes. Visibility between clouds is excellent.

Underneath a large Cb

For further information see the section on Thunderstorms

Medium Level Cloud

Altostratus (As)

Description: This is a stratus (layer) cloud at medium levels often covering the whole sky. It is usually grey in colour and the sun can normally be seen through it. Light rain or snow can fall from Altostratus.
Location: Often as part of a warm front.

Altocumulus (Ac)

Description: This is similar to stratocumulus but at higher levels.
Location: Forms in unstable middle level air.
Flying conditions: Virga (rain which does not reach the surface) may fall from altocumulus clouds in very dry conditions. There are several special forms of Altocumulus:

• Altocumulus Lenticularis (Lenticular Cloud). Sometimes forms over or near mountain ranges and consists of individual lens shaped clouds. Indicates strong up and down draughts.
• Altocumulus Castellanus. Indicative of extensive mid level instability of the atmosphere. Composed of many individual cumulus elements which start upward growth.

Lenticular cloud

High Level Cloud

Cirrus (Ci)

Description: This is a cloud composed of ice crystals in the form of fibrous filaments of very white cloud. It may weave across the whole sky.

Cirrus

Location: Often the first indication of a warm front.

Cirrostratus (Cs)

Description: This also composed of ice crystals in the form of a thin layer often across the whole sky. The sun will be clearly visible through it and may cause a halo to be seen around the sun and especially the moon.
Location: Often seen early in a warm front’s approach.

Cirrocumulus (Cs)

Description: This also composed of ice crystals in the form of small cumuliform elements. It may have a rippled appearance at times.

22 Mar 04 ©T&FA