Isidoro Orlanski's Home Page

From "Atmospheric Fronts" (see publication list below)

Atmospheric fronts are perhaps the most common phenomena appearing on weather maps. Fronts are a crossroad for interaction between the lower and upper levels of the atmosphere. They serves as a major link for the interaction between cloud-scale phenomena and the larger planetary circulation, and act as efficient vehicles for the transport of chemical tracers into the upper atmosphere and over long distances.

email: io@GFDL.gov
phone: 609-452-6546
FAX: 609-987-5063

Research Interests

Manuscript

• Isidoro Orlanski 1997:
  On the Poleward Deflection of Storm Tracks,
  Submitted to the Journal of Atmospheric Sciences

Some Recent Publications

• Orlanski and Sheldon 1995:
  Stages in the Energetics of Baroclinic Systems,
  Tellus 47A , 605-628
• Chang and Orlanski, 1994:
  On Energy Flux and Group Velocity of Waves in Baroclinic Flows,
  Journal of Atmospheric Sciences 51 , 3823-3828.
• Orlanski and Gross,1994:
  Orographic Modification of Cyclone Development,
  Journal of the Atmospheric Sciences 51 , 589-611.
• Orlanski and Sheldon, 1993:
  A Case of Downstream Baroclinic Development over Western North America,
  Monthly Weather Review 121 , 2929-2950.
• Chang and Orlanski, 1993:
  On the Dynamics of a Storm Track,
  Journal of the Atmospheric Sciences 50 , 999-1015.
• Orlanski, 1992:
  Atmospheric Fronts,
  Encyclopedia of Earth System Science, Volume 1 Academic Press, Inc. , 201-216.

Preprint servers

• Atmosphere-Ocean sciences
  

Other home pages

• Stephen Griffies
  
• Kevin Hamilton
  
• Isaac Held
  
• Gareth Williams
  

Cloud Classification

Clouds are generally classified based on characteristics, such as, altitude, appearance, or origin. Altitude distinctions apply to those clouds that fit in various layers of the atmosphere as follows:

• high clouds - have bases above 18,000 feet
• middle clouds - have bases between 7,000 and 18,000 feet
• low clouds - have bases below 7,000 feet
• fog - cloud in contact with the ground
• multi-level clouds... vertically thick spanning multiple layers
• orographic clouds - distinct clouds that form via interaction between wind and mountainous terrain features

In appearance, clouds may be thick or thin, have well defined edges or be very diffuse, appear hairlike, cellular, towering, or in sheets, and be associated with fair weather or precipitation. Most clouds owe their existence to upward vertical motion of air, hence they are often associated with weather producing phenomena, such as fronts, troughs, and low pressure systems. However, topography can also help move air upwards and produce clouds.

Cloud Descriptions and Pictures

This section provides verbal descriptions and pictures of clouds that have been observed in this area. Because of their size, these images have been stored in JPG format. To view a picture, click on the appropriate cloud name or other highlighted text.

High Clouds are primarily composed of ice crystals and include the following:

• Cirrus are high altitude wispy clouds. They are usually quite thin and often have a hairlike or filament type of appearance. The curled up ends as depicted in this picture are very common features.
• Cirrocumulus are high clouds that have a distinct patchy and/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. These features are common to all types of cumuloform clouds.
• Cirrostratus are high clouds that usually blanket the sky in ill-defined sheets. These clouds are usually optically thin and the sun and moon can usually shine some light through. Like other stratiform clouds, one usually can't detect distinct cells or sharp features. This picture shows the sun shining through a gray, diffuse cirrostratus overcast.

• Altocumulus have distinct cloud elements and are either in a patchy, scattered distribution or can appear in linear bands. The altocumulus in this photo consists of a number of individual cloud elements.
• Altostratus have a more uniform and diffuse coverage where it is difficult to detect individual elements or features. In this picture, a few altocumulus clouds in the foreground precede a more uniform deck (see arrow) of altostratus.

• Cumulus are usually puffy and often have very distinct edges and usually a noticeable vertical development. They often have a poporn-like appearance. Cells can 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. However, since the sun is on the other side of the cloud, its thickness is evident from the negligible amount of light passing through its center.
• Stratocumulus are usually concentrated closer together in clusters and have very little vertical development. These relatively flat clouds usually lack the sharp edges and "popcorn" appearance of most normal cumulus clouds. This photo shows a grouping of stratocumulus.
• 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 ill-defined 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 over the Pemigewasset River basin with clear skies elsewhere.

• Nimbostratus are often included in many texts as low clouds, but here they are considered multi-layer clouds because their vertical extent often goes well into the middle cloud region and 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. If it's a gray and rainy day as shown in this photo, the sky most will most likely be filled with nimbostratus clouds.
• 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. The "cb" depicted in the photo had a base at around 3,000 feet and it extended upward to around 30,000 feet - small compared to most thunderstorms which are associated with really severe weather. Sometimes, these strong cumulonimbus clouds can have appendages protruding 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. The picture of mammatus clouds, shown here, was taken by Mark Gibbas, a PSC meteorology alumnus, at Acadia National Park.

• Cap clouds [no photo yet] 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.
• Lenticular clouds are lens-shaped clouds that can result from strong wind flow over rugged terrain. At the time of this photo, the winds were blowing around 30-40 mph from right to left, forming several lenticular clouds. Sometimes they stack up like pancakes in multiple layers as are several depicted in this first photo. The strong flow produces a distinct up and down wavelike pattern on the lee side of the mountain or large hill and the lenticular clouds tend to form at the peaks of these waves. They sometimes are very round and the edges are so well defined that they resemble flying saucers. This close up view shows one larger lenticular cloud and a smaller one forming directly beneath it. Lenticular clouds are often placed into the middle cloud category since they are most common at those altitudes.