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As a specific term, “orographic lifting” is defined as an ascending air flow caused by mountains. The mechanisms that produce the orographic lifting fall into two broad categories:
1. The upward deflection of horizontal large-scale air flow by the terrain acting as an obstacle or barrier, or
2. The daytime heating of mountain surfaces to produce an anabatic flow (see below) along the slopes and updrafts in the vicinity of mountain peaks.
This definition, while strictly referring only to lifting by mountains, is sometimes extended to include the effects of hills or long sloping terrain. When sufficient moisture is present in the rising air, Orographic fog or clouds may form. Anabatic Winds
Anabatic winds are those that blow up a steep slope or mountain side. It is sometimes referred to as an upslope flow. These winds typically occur during the daytime in calm,
4-15
International Cloud Classification
Abbreviations and Weather Map SymbolsBase AltitudeCloud TypeAbbreviationSymbolBases of highclouds usuallyabove 18,000feet Cirrus CI Cirrocumulus CC Cirrostratus CS Altocumulus AC Altostratus AS* Cumulus CU* Cumulonimbus Cb Nimbostratus NS Stratocumulus SC Stratus StBases of middleclouds rangefrom 6,500 feetto 18,000 feetBases of lowclouds rangefrom surfaceto 6,500 feet* Cumulus and cumulonimbus are clouds with vertical development. Their bases are usually below 6,500 feet, but may be slightly higher. The tops of the cumulonimbus some times exceed 60,000 feet.18,000 FT6,500 FTSurface
Figure 4-15. Cloud classification per international agreement.
sunny weather. A hill or mountaintop may be warmed by the sun, which in turn heats the air just above it. As that air rises through convection, it creates a low pressure region, into which the air at the bottom of the slope flows, and causes winds. Katabatic Winds
Katabatic winds are the reverse of anabatic winds; that is, they flow down slope, and most frequently at night. They are created by the effect of the air near the ground losing heat thru radiational cooling at a faster rate than air at a similar altitude over the surrounding land mass.Clouds
Clouds are weather signposts in the sky. They provide the balloon pilot with visible evidence of the atmospheric motion, water content, and degree of stability. In this sense, clouds are of significant importance to the aeronaut. However, when they become too numerous or widespread, form at low levels, or show extensive vertical development, they present weather hazards to ballooning.
Clouds are visible condensed moisture, consisting of droplets of water or crystals of ice. They are supported and transported by air movements as slow as one-tenth of a mile per hour. Cloud formation is the direct result of saturation producing processes which take place in the atmosphere. A pilot should be able to identify cloud formations that are associated with weather hazards. Knowledge of cloud types will also assist the pilot in interpreting weather conditions from weather reports and existing weather.Cloud Formation
Clouds are often indicative of future weather. For clouds to form, there must be adequate water vapor and condensation nuclei (miniscule particles of matter like dust, salt, and smoke), as well as a method by which the air can be cooled. When the air cools and reaches its saturation point, the invisible water vapor changes into a visible state. Through the processes of sublimation and condensation, moisture condenses or sublimates onto condensation nuclei. The nuclei are important because they provide a means for the moisture to change from one state to another.
Cloud type is determined by its height, shape, and behavior. They are classified according to the height of their bases as low, middle, or high clouds, as well as clouds with vertical development. The International Cloud Classification is designed to provide a uniform cloud classification system. [Figure 4-15] Within this system, cloud types are usually divided into four major groups and further classified in terms of their forms and appearance.
The four major groups are:
• Low clouds,
• Middle clouds,
• High clouds, and
• Clouds with vertical development.Cloud classification can be further broken down into specific cloud types according to the outward appearance and cloud composition. Knowing these terms can help identify visible clouds. The following is a list of cloud classifications:
• Cumulus—heaped or piled clouds
• Stratus—formed in layers
• Cirrus—ringlets, fibrous clouds, also high-level clouds above 20,000 feet
• Castellanus—common base with separate vertical development, castle-like
• Lenticularus—lens shaped, formed over mountains in strong winds
• Nimbus—rain-bearing clouds
• Fracto—ragged or broken
• Alto—meaning high, also middle-level clouds existing at 5,000 to 20,000 feet
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