Weather Theory

Cloud classification can be further broken down into specific Drizzle is classified as very small water droplets, smaller cloud types according to the outward appearance and cloud than 0.02 inches in diameter. Drizzle usually accompanies composition. Knowing these terms can help a pilot identify fog or low stratus clouds. Water droplets of larger size are visible clouds. referred to as rain. Rain that falls through the atmosphere but evaporates prior to striking the ground is known as virga. The following is a list of cloud classifications: Freezing rain and freezing drizzle occur when the temperature of the surface is below freezing; the rain freezes on contact • Cumulus—heaped or piled clouds with the cooler surface. • Stratus—formed in layers • Cirrus—ringlets, fibrous clouds, also high level clouds If rain falls through a temperature inversion, it may freeze above 20,000 feet as it passes through the underlying cold air and fall to the ground in the form of ice pellets. Ice pellets are an indication • Castellanus—common base with separate vertical of a temperature inversion and that freezing rain exists at a development, castle-like higher altitude. In the case of hail, freezing water droplets are • Lenticularus—lens-shaped, formed over mountains carried up and down by drafts inside cumulonimbus clouds, in strong winds growing larger in size as they come in contact with more • Nimbus—rain-bearing clouds moisture. Once the updrafts can no longer hold the freezing water, it falls to the Earth in the form of hail. Hail can be • Fracto—ragged or broken pea sized, or it can grow as large as five inches in diameter, • Alto—middle level clouds existing at 5,000 to 20,000 larger than a softball. feet Snow is precipitation in the form of ice crystals that falls Ceiling at a steady rate or in snow showers that begin, change in For aviation purposes, a ceiling is the lowest layer of clouds intensity, and end rapidly. Snow also varies in size, from very reported as being broken or overcast, or the vertical visibility small grains to large flakes. Snow grains are the equivalent into an obscuration like fog or haze. Clouds are reported of drizzle in size. as broken when five-eighths to seven-eighths of the sky is covered with clouds. Overcast means the entire sky is covered Precipitation in any form poses a threat to safety of flight. with clouds. Current ceiling information is reported by the Often, precipitation is accompanied by low ceilings and aviation routine weather report (METAR) and automated reduced visibility. Aircraft that have ice, snow, or frost on weather stations of various types. their surfaces must be carefully cleaned prior to beginning a flight because of the possible airflow disruption and Visibility loss of lift. Rain can contribute to water in the fuel tanks. Closely related to cloud cover and reported ceilings is Precipitation can create hazards on the runway surface itself, visibility information. Visibility refers to the greatest making takeoffs and landings difficult, if not impossible, horizontal distance at which prominent objects can be due to snow, ice, or pooling water and very slick surfaces. viewed with the naked eye. Current visibility is also reported in METAR and other aviation weather reports, as well as Air Masses by automated weather systems. Visibility information, as Air masses are classified according to the regions where predicted by meteorologists, is available for a pilot during a they originate. They are large bodies of air that take on the preflight weather briefing. characteristics of the surrounding area or source region. A source region is typically an area in which the air remains Precipitation relatively stagnant for a period of days or longer. During Precipitation refers to any type of water particles that this time of stagnation, the air mass takes on the temperature form in the atmosphere and fall to the ground. It has a and moisture characteristics of the source region. Areas of profound impact on flight safety. Depending on the form of stagnation can be found in polar regions, tropical oceans, and precipitation, it can reduce visibility, create icing situations, dry deserts. Air masses are generally identified as polar or and affect landing and takeoff performance of an aircraft. tropical based on temperature characteristics and maritime or continental based on moisture content. Precipitation occurs because water or ice particles in clouds grow in size until the atmosphere can no longer support A continental polar air mass forms over a polar region and them. It can occur in several forms as it falls toward the brings cool, dry air with it. Maritime tropical air masses form Earth, including drizzle, rain, ice pellets, hail, snow, and ice. 12-17