Low-level clouds

Low-level clouds include cumulus, stratocumulus, stratus, nimbostratus, cumulonimbus and fog. Low-level clouds consist mostly of water droplets however at very cold temperatures these clouds can also contain ice particles and snow.

(image embedded from NASA on 30 May 2009)
Stratus (ST)
Stratus is a uniform but usually shallow layer of cloud resembling fog but with its base above ground level. The sun, when visible, is without halo but its outline is a little blurry. Turbulence is not usually associated with stratus. In situations where the freezing level is at ground level, very light rime ice may be experienced. Precipitation may involve light drizzle which accentuates the reduced visibility which is sometimes associated with stratus.

Thick, overcast Stratus cloud (image embedded from NASA on 05 April 2009)


Nice, flat, uniform lighting Stratus clouds (image embedded from NASA on 05 April 2009)


Nice, flat, uniform Stratus (image embedded from NASA on 05 April 2009)

Nimbostratus (NS)
Nimbostratus is a low amorphous (shapeless) layer of dark grey cloud of nearly uniform appearance. This sun is blotted out by nimbostratus. Low ragged clouds often appear below the layer. This cloud is normally associated with steady and persistent rain, snow, or sleet. Generally no turbulence will be encountered with nimbostratus, apart from the occasional embedded Cb which is an indicator of severe turbulence. The most common type of ice found in nimbostratus is rime ice. However, clear ice may also be experienced. The rate of ice accretion can be substantial due to the high concentration of water.

Typical Nimbostratus (image embedded from NASA on 07 April 2009)


Low Nimbostratus with rain shafts visible in the background (image embedded from NASA on 07 April 2009)


Snowing Nimbostratus (image embedded from NASA on 07 April 2009)

Cumulus (CU)
Cumulus is a detached cloud type with sharp outlines and vertical development. The upper surface is dome shaped and the base nearly flat. Cumulus is a bright, white cloud when viewed from the sunny side but it can appear dark and menacing on the shadow side and at the base. Cumulus can be caused by either or both of surface heating and/ or orographic ascent. The former cause is referred to as fair weather cumulus and is a typical occurrence on a fine summer’s day. Precipitation is unusual with cumulus, however when the development is substantial, brief showers may occur when the clouds reach their maximum extent. Turbulence is likely both within and outside of the cloud. Depending on the wind strength, bumpy and turbulent flight conditions may result. Icing is not a problem due to the fragmented nature of this cloud. However, if aircraft is being operated within continuous cumulus cloud, ics accretion may be a problem.

Low Cumulus and high Cirrus multi- layer tropical cloud (image embedded from NASA on 07 April 2009)


Fair weather Cumulus (image embedded from NASA on 07 April 2009)


Cumulus clouds at sunset over the ocean (image embedded from MedioImages/Corbis on 10 April 2009)

Stratocumulus (SC)
Stratocumulus appears as a layer or patch composed of globular masses or rolls. The elements are fairly large. Often arranged in groups, lines, or waves aligned in one or two directions. The lines may be so close that their edges join. Stratocumulus may cover large parts of the sky. Turbulence is light, unless when a turbulence inversion is involved, turbulence can be moderate. Icing is not a problem except in latitudes where the freezing level is at or near ground level. Precipitation may be light showers or drizzle patches but will rarely be moderate or heavy.

Light Stratocumulus clouds (image embedded from NASA on 11 April 2009)


Opaque Stratocumulus clouds (image embedded from NASA on 11 April 2009)


Stratocumulus at Sunset (image embedded from Windows to the Universe on 11 April 2009)

Cumulonimbus (CB)
Cumulonimbus involves heavy masses of cloud with great vertical development whose summits rise in the form of mountains or towers. Cb clouds can be positively identified by a smooth or fibrous anvil- like structure formed by ice crystals spreading away from the cloud at the highest levels. Turbulence in Cb clouds can be moderate to severe depending on its stage of development. The massive up and down draughts in the mature stage can produce severe flight conditions, while in the decaying stage strong down draughts and its associated turbulence can occur below the base of the cloud. Icing can be a serious problem and can take the form of both glaze and rime ice. Associated heavy shower activity may include hail. In some cases this cloud is associated with a squall situation where the rapidly descending down draught flows outward and up near the surface, producing a wave- like action in the direction of CB travel.

Cumulonimbus clouds (image embedded from ARIC on 11 April 2009)


Side View of Cumulonimbus Anvil (image embedded from NASA on 11 April 2009)


Underneath a cumulonimbus (thunderstorm) cloud (image embedded from NASA on 11 April 2009)

By definition, fog is cloud on the surface, whatever that surface may be. Since formation of most clouds requires the initial presence of water vapour, a cooling mechanism and hygroscopic particles, it follows that when these factors are found near the surface fog is likely to form. When fog is present, the visibility can be greatly reduced.

Early morning fog (image embedded from SunSentinel.com on 14 April 2009)


Fog with trees


Fog with lake

1. NATIONAL AERONAUTICS AND SPACE ADMINISTRATION (2008).On-Line Cloud Chart. Retrieved from National Aeronautics and Space Administration on 20 March 2009.
2. UNIVERSITY OF ILLINOIS AT URBABA- CHAMPAIGN (2009).Cloud Types. Retrieved from University of Illinois at Urbana-Champaign on 22 March 2009.
3. WIKIPEDIA (2009). List of cloud types. Retrieved from Wikipedia on 20 March 2009.

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