Precipitation refers to any type of water particle that forms in the atmosphere and falls to the ground. It has a profound impact on flight safety. Depending on the form and strength of precipitation, it can reduce visibility, create icing conditions and negatively affect landing and takeoff performance of an aircraft. It can occur in several forms as it falls toward the Earth, including drizzle, rain, ice pellets, hail, snow, and ice.
Coalescence
Raindrops form via coalescence. As droplets are carried by the updrafts and downdrafts in a cloud, they collide and coalesce to form larger droplets. When the droplets become too large to be sustained by the air currents, they begin to fall as rain.
Types
The following are common types of precipitation. Some definitions may be asked on the written exam.
Drizzle: Drizzle is a light liquid precipitation consisting of liquid water drops smaller than those of rain – generally smaller than 0.5 mm in diameter. Drizzle is normally produced by low stratiform clouds and stratocumulus clouds. Water droplets of larger sizes are referred to as rain.
Virga: Rain that falls through the atmosphere but evaporates prior to striking the ground is known as virga.
Freezing Rain: Very often, precipitation first falls from a cloud as snow (when it is cold enough high up where the cloud is). If it falls through warmer air before reaching the ground, it can melt and turn to rain droplets. On rare occasions, if it then falls through cold air again just before hitting the ground, the droplets can become ‘supercooled’ and this means that they are still falling in liquid form, even though their temperature has fallen below zero. When this ‘supercooled’ droplet hits the ground (which is below zero too) it almost instantly freezes, encasing the surface in a layer of clear ice. This is why it is called freezing rain.
Ice Pellets: If rain falls through a temperature inversion, it may freeze as it passes through the underlying cold air and fall to the ground in the form of ice pellets. Ice pellets are an indication of a temperature inversion and that freezing rain exists at a higher altitude.
Hail: In the case of hail, freezing water droplets are carried up and down by drafts inside cumulonimbus clouds, growing larger in size as they come in contact with more moisture. Once the updrafts can no longer hold the freezing water, it falls to the Earth in the form of hail. Hail can be pea sized, or it can grow as large as five inches in diameter, larger than a softball.
Snow: Snow is precipitation in the form of ice crystals that falls at a steady rate or in snow showers that begin, change in intensity, and end rapidly. Snow also varies in size, from very small grains to large flakes.
Precipitation in any form poses a threat to safety of flight. Often, precipitation is accompanied by low ceilings and reduced visibility. Aircraft that have ice, snow, or frost on their surfaces must be carefully cleaned prior to beginning a flight because of the possible airflow disruption and loss of lift. Rain can seep into the fuel tanks causing fuel contamination or create visual illusions on the windshield making it difficult to perceive the outside environment. Precipitation can create hazards on the runway surface itself, making takeoffs and landings difficult, if not impossible, due to snow, ice, or pooling water and very slick surfaces.
Scroll to Top
Login
Accessing this course requires a login. Please enter your credentials below!