The Meath Weather Cam

An Insight to Irish Weather, 24/7

Cloud Index

Saw a cloud on the Meath Weather Cam, in your garden, or on holiday? Do you want to be able to identify it, and know what it's called? Then you've come to the right place!

Click one of the categories below to see a description of different clouds types!

Low Level Clouds            Mid Level Clouds           High Level Clouds

                      Vertical Clouds                Other Clouds

 

A cloud is a visible mass of droplets or frozen crystals floating in the atmosphere above the surface of the Earth or another planetary body. A cloud is also a visible mass attracted by gravity (clouds can also occur as masses of material in interstellar space, where they are called interstellar clouds and nebulae.) The branch of meteorology in which clouds are studied is nephology.

                                                 Dramatic clouds over Mauna Loa

On Earth the condensing substance is typically water vapor, which forms small droplets or ice crystals, typically 0.01 mm in diameter. When surrounded by billions of other droplets or crystals they become visible as clouds. Dense deep clouds exhibit a high reflectance (70% to 95%) throughout the visible range of wavelengths: they thus appear white, at least from the top. Cloud droplets tend to scatter light efficiently, so that the intensity of the solar radiation decreases with depth into the gases, hence the gray or even sometimes dark appearance of the clouds at their base. Thin clouds may appear to have acquired the color of their environment or background, and clouds illuminated by non-white light, such as during sunrise or sunset, may be colored accordingly. In the near-infrared range, clouds would appear darker because the water that constitutes the cloud droplets strongly absorbs solar radiation at those wavelengths.

                                                       Rain clouds over the North Sea,                                                                                       taken from the coast of Herne Bay, Kent

Clouds are divided into two general categories: layered and convective. These are named stratus clouds (or stratiform, the Latin stratus means "layer") and cumulus clouds (or cumuliform; cumulus means "piled up"). These two cloud types are divided into four more groups that distinguish the cloud's altitude. Clouds are classified by the cloud base height, not the cloud top. This system was proposed by Luke Howard in 1802 in a presentation to the Askesian Society.
 
 
Colours
 
                                            An example of various cloud colors
 
Cloud iridescence occurring in clouds
 
The color of a cloud tells much about what is going on inside the cloud. Clouds form when relatively warm air containing water vapor is lighter than its surrounding air and this causes it to rise. As it rises it cools and the vapor condenses out of the air as micro-droplets. These tiny particles of water are relatively densely packed and sunlight cannot penetrate far into the cloud before it is reflected out, giving a cloud its characteristic white color. As a cloud matures, the droplets may combine to produce larger droplets, which may combine to form droplets large enough to fall as rain. In this process of accumulation, the space between droplets becomes larger and larger, permitting light to penetrate much farther into the cloud. If the cloud is sufficiently large and the droplets within are spaced far enough apart, it may be that a percentage of the light which enters the cloud is not reflected back out before it is absorbed (Think of how much farther one can see in a heavy rain as opposed to how far one can see in a heavy fog). This process of reflection/absorption is what leads to the range of cloud color from white through grey through black. For the same reason, the undersides of large clouds and heavy overcasts appear various degrees of grey; little light is being reflected or transmitted back to the observer.

Other colours occur naturally in clouds. Bluish-grey is the result of light scattering within the cloud. In the visible spectrum, blue and green are at the short end of light's visible wavelengths, while red and yellow are at the long end. The short rays are more easily scattered by water droplets, and the long rays are more likely to be absorbed. The bluish color is evidence that such scattering is being produced by rain-sized droplets in the cloud.

A greenish tinge to a cloud is produced when sunlight is scattered by ice. A cumulonimbus cloud which shows green is an imminent sign of heavy rain, hail, strong winds and possible tornadoes.

Yellowish clouds are rare but may occur in the late spring through early fall months during forest fire season. The yellow color is due to the presence of smoke.

Red, orange and pink clouds occur almost entirely at sunrise/sunset and are the result of the scattering of sunlight by the atmosphere. The clouds are not that color; they are reflecting the long (and unscattered) rays of sunlight which are predominant at those hours. The effect is much the same as if one were to shine a red spotlight on a white sheet. In combination with large, mature thunderheads this can produce blood-red clouds. The evening before the Edmonton, Alberta tornado in 1987, Edmontonians observed such clouds — deep black on their dark side and intense red on their sunward side. In this case the adage "red sky at night, sailor's delight" was wrong.The cloud was white because the reflection of the sun.