Moisture in the Atmosphere
Moisture in the air is something we all experience daily. It causes clouds to form, fog to cover the landscape, and in higher concentrations, it leads to rain, snow, hail, and sleet. Water vapor is one of the most crucial elements of our planet’s atmosphere.
Impact on Visibility
Moisture in the air can significantly impact visibility. For safety reasons, it’s essential to maintain visual contact with the aircraft you are flying at all times.
Temperature and Dewpoint
The atmosphere contains moisture in the form of water vapor. Air temperature is the most influential factor in determining the amount of moisture the air can hold. A 20°F change can double or halve the atmosphere’s moisture capacity.
Dewpoint is the temperature at which the air can hold no more moisture. When the air temperature drops to the dewpoint, the air becomes saturated, and moisture begins to condense, forming fog, dew, frost, clouds, rain, hail, or snow. As moist, unstable air rises, clouds often form at the altitude where temperature and dewpoint are equal, known as the cloud base.
If the air reaches saturation with temperature and dewpoint close together, fog, low clouds, and precipitation are likely to form. Understanding these moisture types and their effects on flight operations is crucial for planning and executing a flight.
Dew and Frost
On cool, calm nights, the ground and surface objects can cause surrounding air temperatures to drop below the dewpoint. This results in moisture condensing and depositing as dew on the ground, buildings, cars, and aircraft. If the temperature is below freezing, this moisture forms frost. While dew poses no threat to aircraft, frost can disrupt airflow over the wings, increasing drag and decreasing lift. Frost must be thoroughly removed from an aircraft before flight to ensure safety.
Fog
Fog is essentially a cloud that forms near the ground, typically when air near the ground cools to its dewpoint. Fog forms when water vapor condenses into tiny liquid droplets. It is a stable cloud deck that forms when cool, stable air is trapped under a warmer air mass.
Fog is the most frequent cause of surface visibility being less than 3 miles and is a common and persistent weather hazard in aviation. It can form rapidly, reducing visibility from VFR to less than a mile in minutes, posing a hazard during takeoff and landing, and for VFR pilots who need to maintain visual ground reference.
Clouds
Clouds are visible masses of condensed water vapor in the atmosphere. They are categorized into four types or “families”:
- High Clouds: Composed mostly of ice crystals, with bases ranging from 16,500 to 45,000 feet in middle latitudes.
- Middle Clouds: Primarily water, often supercooled, with bases from 6,500 to 23,000 feet.
- Low Clouds: Almost entirely water, sometimes supercooled, with bases from near the surface to 6,500 feet.
- Vertically Developed Clouds: Contain supercooled water above the freezing level, with bases from 1,000 feet to above 10,000 feet.
Thunderstorms
Thunderstorms can produce large hailstones, damaging lightning, tornadoes, and heavy rain, all hazardous to aircraft. Avoid flying in conditions that may lead to thunderstorms.
Precipitation
Precipitation includes any water particles that form in the atmosphere and fall to the ground, such as drizzle, rain, snow, hail, and ice pellets. It can reduce visibility, create icing conditions, and affect aircraft performance during landing and takeoff.
- Drizzle: Water droplets smaller than 0.02 inches, usually accompanying fog or low stratus clouds.
- Rain: Larger water droplets.
- Freezing Rain/Drizzle: Occurs when surface temperatures are below freezing, causing rain to freeze on contact.
- Sleet: Rain that freezes as it passes through cold air, indicating a temperature inversion.
- Hail: Formed by freezing water droplets carried up and down by drafts inside clouds, growing larger until they fall to the ground.
- Snow: Ice crystals falling steadily or in showers, varying in size from small grains to large flakes.
Precipitation in any form poses a threat to flight safety. Aircraft must be free of ice, snow, or frost before flight to avoid airflow disruption and loss of lift. Snow, ice, and pooling water on runways can also create hazards during takeoff and landing.
Icing
Aircraft icing is a major weather hazard, reducing efficiency by increasing weight and drag, reducing lift and thrust, and interfering with control surfaces, brakes, and landing gear. Icing can also damage avionics, causing false telemetry reports, loss of radio communications, and payload malfunctions.
Icing conditions occur when the air contains supercooled liquid water droplets. These conditions are characterized by droplet size, liquid water content, and air temperature, affecting the extent and speed of ice formation on aircraft.
Model aircraft lack the equipment to combat icing in flight. Do not fly in icing conditions.
