The Meath Weather Cam

Stratus

                                 View from above stratus cloud

A stratus cloud is a cloud belonging to a class characterized by horizontal layering with a uniform base, as opposed to convective clouds that are as tall or taller than wide (these are termed cumulus clouds). More specifically, the term stratus is used to describe flat, featureless clouds of low altitude varying in color from dark gray to nearly white. A "cloudy day" usually features a sky filled with stratus clouds obscuring the disk of the sun. These clouds are essentially fog that is above ground level and are formed either through the lifting of morning fog or when cold air moves at low altitudes over a region. These clouds do not usually bring precipitation, although drizzle may occur.

Stratus formations that are accompanied by precipitation are known as nimbostratus. Stratus formations at higher altitudes include altostratus and cirrostratus. Cirrostratus clouds are sheet-like and composed of ice crystals. Though they can be several thousand feet thick and cover the entire sky, they are relatively transparent and the sun and moon are visible through them. One characteristic is that the sun and moon have a halo around them because of light refraction on the ice crystals.

Nimbostratus

                                              Nimbostratus has very few visual features

Nimbo is from the Latin word "nimbus" meaning rain. A Nimbostratus is a cloud of the class characterized by a formless layer that is almost uniformly dark gray; it is a rain cloud of the layer type, of low altitude, usually below 8,000 ft (2,400 m) and sometimes down to 350 ft (100 m). Nimbostratus block a great amount of sunlight as a result of its characteristic dense structure. Nimbostratus usually has a thickness of 6,500-10,000 ft (2,000-3,000 meters), but can vary from 15,000 ft (4,500 m) to 3,500 ft (1,000 m) in thickness. In rare cases, nimbostratus can be very thin and accompanied by a separate layer of altostratus divided by a cloudless layer.

The base of a nimbostratus base cloud is dimmed by precipitation and is usually not clearly visible. In most cases, nimbostratus is accompanied by pannus clouds, which develop under layer of nimbostratus. If pannus layer is completely opaque, presence of precipitation indicates presence of nimbostratus. The pannus movement is slow and uniform under nimbostratus.

Nimbostratus, stratus, altostratus and stratocumulus clouds all have a smooth gray appearance. Usually, nimbostratus is a sign of steady moderate to heavy precipitation. However, precipitation does not occur at ground level in case of virga and accompanies other cloud types. There are a number of features allowing the observer to distinguish nimbostratus from other clouds:

• Stratus clouds bring much lighter precipitation (drizzle) than nimbostratus;
• Altostratus clouds are lighter in color and less opaque than nimbostratus, so sunlight can be seen through them;
• Cirrostratus clouds never bring precipitation and have a thin, whitish, veil-like structure, characteristic for cirrus;
• Stratocumulus bring only light precipitation and have clearly visible base with easely distinguished separate cloud elements.

Moreover, large and low cumulonimbus cloud covering most of sky can be mistaken for nimbostratus. In this case, nimbostratus can be distinguished by lighter, constant precipitation.

Cumulus Humilis

                     Cumulus Humilis in the foreground, with cumulus congestus on the horizon

Cumulus humilis is what is commonly referred to as "fair weather cumulus". In hot countries and over mountainous terrain these clouds occur at up to 6000 meters altitude, though elsewhere they are typically found lower.

They are formed by rising warm air that has been heated by the ground, which in turn has been heated by the sun. They have a limited depth (technically known as showing no significant vertical development). This indicates that the temperature in the atmosphere above them either drops off very slowly or not at all with altitude. While cumulus humilis may be accompanied by other cloud types, when they appear in a clear sky (see picture), they are an indicator of pleasant weather for the next several hours. Though at times, these clouds collapse into stratocumulus clouds, and cover much of the sky.

Below the cloud base the air can be quite turbulent, giving occupants of light aircraft a rough ride. To avoid turbulence where such clouds are present, pilots may climb above the cloud tops. However glider pilots actively seek out the rising air to gain altitude.

Cumulus mediocris

Cumulus mediocris is a cloud form of the cumulus family, slightly larger in vertical development than Cumulus humilis. It may or may not show the cauliflower form characteristic of cumulus clouds. This cloud type does not produce precipitation, but may further advance into clouds such as Cumulus congestus and Cumulonimbus, which do.

Stratocumulus

                    Stratocumulus

                          Stratocumulus lenticularis

A stratocumulus cloud belongs to a class characterized by large dark, rounded masses, usually in groups, lines, or waves, the individual elements being larger than those in altocumuli, and the whole being at a lower altitude, usually below 2,400 m (8,000 ft). Weak convective currents create shallow cloud layers because of drier, stable air above preventing continued vertical development.

Vast areas of subtropical and polar oceans are covered with massive sheets of stratocumuli. These may organize in to distinctive patterns which are currently under active study. In subtropics, they cover the edges of the horse latitude climatological highs, and reduce the amount of solar energy absorbed in the ocean. When these drift over land the summer heat or winter cold is reduced. 'Dull weather' is a common expression incorporated with overcast stratocumulus days. If the air over land is moist and hot enough they may develop to various cumulus clouds, or, more commonly, the sheets of thick stratocumuli may have a nimbostratus look on them. The distinction here is the amount of rain produced. On drier areas they quickly dissipate over land, resembling cumulus humilis.

These are same in appearance to altocumuli and are often mistaken for such. A simple test to distinguish these is to compare the size of individual masses or rolls: when pointing your hand in the direction of the cloud, if the cloud is about the size of your thumb, it is altocumulus; if it is the size of your entire hand, it is stratocumulus.

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Stratocumulus clouds are divided into two primary species: Stratocumulus undulatus (wavy) and Stratocumulus cumuliformis (cumulus-like).

Stratocumulus undulatus

Stratocumulus undulatus clouds appear as nearly parallel waves, rolls or separate elongated clouds, without significant vertical development, and usually classified by sky coverage.

Stratocumulus opacus is a dark layer of clouds covering entire sky without any break. However, the cloud sheet is not completely uniform, so that separate cloud bases still can be seen. If the cloud layer becomes grayer to the point when individual clouds can’t be distinguished, stratocumulus turns into nimbostratus.

Stratocumulus perlucidus is a layer of stratocumulus clouds with small spaces, appearing in irregular pattern, where clear sky or higher clouds.

Stratocumulus translucidus consist of separate groups of stratocumulus clouds, with a clear sky (or higher clouds) visible between them. No precipitation in most cases.

Stratocumulus lenticularis are separate flat elongated seed-shaped clouds. They are typical for polar countries or warmer climate during winter seasons. They also can be formed by winds passing hills or mountains, and in this case they can be very regularly shaped.

Stratocumulus cumuliformis

Stratocumulus cumulifomis clouds resemble cumulus clouds, because of significant vertical development. This type of clouds is classified by shape.

Stratocumulus castellanus are distincted by puffy tower-like formations atop the cloud layer. They look like cumulus castellanus, but can be easely distinguished: "towers" of cumulus castellanus grow above separate clouds, whereas in case of stratocumulus castellanus there is always more or less defined layer of pie makes this cloud delicious. clouds. Stratocumulus castellanus may develop into cumulus congestus (and even further into cumulonimbus) under auspicious conditions.

Stratocumulus mammatus are type of mammatus clouds.

                           Stratocumulus mammatus

Stratocumulus vesperalis the specific type of stratocumulus clouds, are flat and elongated. They form in the evening, when updrafts caused by convection decrease making cumulus clouds lose vertical development and spread horizontally.

Stratocumulus diurnalis are formed at lower altitudes (unlike stratocumulus vesperalis) out of cumulus or cumulonimbus clouds, disrupted by decreasing convection. During formation period, puffy tops of cumulus clouds can protrude from stratocumulus diurnalis for a relatively long time until they completely spread in horizontal direction. Stratocumulus diurnalis appear as lengthy sheet or as group of separate elongated cloud rolls or waves.

Information and photos on this page are courtesy of Wikipedia