D What are clouds and why does it rain?

How much of the sky is covered by cloud?

Meteorologists [Weathermen] measure the amount of the sky covered in eighths or oktas. 

What type of clouds are covering the sky?

High Clouds are mainly made up of ice crystals and include the following:

• Cirrus are high altitude wispy clouds. They are usually quite thin and don't really block out the sunshine. The curled up ends as depicted in this picture are commonly seen and show high level winds which can cause a change in the weather.
• Cirrocumulus are high clouds that have a distinct patchy or wavelike appearance, such as, in our patchwork cirrocumulus photo, composed of many individual cloud elements, or in our wavy cirrocumulus photo with its banded linear structure.
• Cirrostratus are high clouds that usually blanket the sky in ill-defined sheets. These clouds are usually thin and the sun and moon can usually shine some light through them.The picture shows the sun shining through grey cirrostratus. It is overcast [8 oktas] and steady rainfall can be expected within the next hour.

"Middle height" clouds come next in the classification scheme. They can either come in layers [stratus or stratiform] or can be heaped up to look a bit like "cotton wool" [cumulus or cumuloform]. These "middle height" clouds can contain a mixture of ice crystals and water droplets and may give light precipitation [rainfall].

• Altocumulus have distinct cloud elements and are either in a patchy, scattered distribution or can appear in linear bands. The altocumulus in this photo by Jay Shafer consists of a number of individual cloud elements. Jay also took this beautiful sunset photo of altocumulus clouds.
• Altostratus are like lower and thicker versions of cirrostratus. They form uniform light grey layers. In this picture, a few altocumulus clouds in the foreground precede a more uniform deck (see arrow) of altostratus.

Low clouds are most often just composed of water droplets. Some of these clouds can develop into the multi-level clouds and can go through changes during the day. For example, early morning stratus clouds can turn into late morning stratocumulus. Early afternoon cumulus can develop vertically with low cloud bases and very high cloud tops. These are called cumulonimbus clouds and can produce heavy rain and possible lightning and thunder.

• Cumulus are usually puffy and often have very distinct edges and usually a clear vertical development. The clouds can either be rather isolated or they can be grouped together in clusters as shown in this photo. The main cumulus cloud pictured in this view was nearly overhead, so the vertical extent is hidden from view. 
• Stratocumulus can be widely scattered (as depicted in this photo, but are usually concentrated closer together in layers. They have very little vertical development. A second photo provides a close-up, side view of a stratocumulus cloud in this cloud complex. 
• Stratus are usually the lowest of the low clouds. Stratus often appear as an overcast deck (as shown), but can be scattered. The individual cloud elements have very blurred edges compared to most low cumuloform clouds (e.g. cumulus and stratocumulus).
• Fog can be considered as a low stratus cloud in contact with the ground. When the fog lifts, it usually becomes true stratus. This photo shows fog hanging over a river with clear skies elsewhere.

Multi-layer clouds are the heavy precipitation [rainfall] producers. The latin words "nimbo" and "nimbus" mean rainfall.

• Nimbostratus are often included in many texts as low clouds, but these clouds often have even taller cumulonimbus clouds embedded within them. The clouds are very dark, usually overcast, and are associated with large areas of continuous precipitation.
• Cumulonimbus, as shown in this photo (with cumulus in the foreground), are the clouds that can produce lightning, thunder, heavy rains, hail, strong winds, and tornadoes. They are the tallest of all clouds that can span all cloud layers and extend above 60,000 feet. They usually have large anvil-shaped tops (as shown) which form because of the stronger winds at those higher levels of the atmosphere.  Another picture shows a view from the ground of a cumulonimbus with a base at around 3,000 feet and vertical development upward to around 30,000 feet - small compared to most thunderstorms which are associated with really severe weather. Sometimes strong cumulonimbus clouds can have curved sections of cloud drooping from the base of the cloud, which are called "mammatus" clouds because they resemble the mammary glands of mammals. They indicate that the atmosphere is quite unstable and can also be an indicator of impending severe weather including tornadoes. 

Orographic clouds are produced by the flow of air often over mountainous terrain.

• Cap clouds form when air containing water vapor is uplifted on the windward slide of the slope and reaches saturation producing liquid water cloud droplets and a cloud which can "cap" the summit. The spectacular pictureshows the US peak of Mt. Ranier.
• Lenticular clouds are lens-shaped clouds that can result from strong wind flow over rugged terrain. 

Another type of cloud can be formed from the vapour contained in the exhaust of a jet engine of an airplane when they are flying at high altitudes where cold temperatures cause the vapour to turn into ice crystals like cirrus clouds. These clouds are called "contrails" (short for "condensation trails") and look like lines in the sky.

Your Task is one which will take you a week of observation! Each day,

1. note the time you are doing your field work
2. observe how much of the sky is covered which cloud (use oktas)
3. what type[s] of cloud are in the sky
4. the present weather conditions

Present your data in table form [use a word document for this]

Extension/project ideas:

• take a digital photograph and add that into your table
• take the current satellite image, copy and crop it to the region you are making your observations
• copy or cut out, either from the met. office site or from a newspaper the "weather chart" for each day.
• see if you can add brief comments linking each aspect of your data collection.

Paul Thompson Ounsdale High Schoo,l Wombourne, Staffordshire