Summary1. Weather Hazards2. Severe Weather
*Lesson Objective: Understand basic facts and general principles about aviation weather. Samples of Behavior/Main Points:1. Explain the weather hazards associated with aviation. 2. List the types of severe weather that affect aviation 3. Describe arctic and tropic weather characteristics.
*Reduced VisibilityGenerally speaking, the FAA considers three miles lateral visibility to be acceptable for safe flight under visual flight rules (VFR). This includes a possible six-tenths or more of cloud cover with a base at least 1,000 feet above ground level (AGL). The possibilities for accidents are increased when visibility is reduced and is greatest for those pilots who are not trained to fly according to instrument flight rules (IFR).Clouds, rain, snow, fog, and obstructions can cause reduced visibility so that mid-air collisions between aircraft and collisions with tall stationary objects (up to 15,000 feet above ground level) are increased. The possibilities for landing accidents are also increased when visibility is decreased.Haze and Smoke can cause reduced visibility when the wind is calm, so the "haze and smoke" cannot move laterally out of the area. If this condition persists for several days, the visibility will become progressively less. Especially common in the heavily populated and heavily industrialized areas of the country.Especially noticeable in the early morning because radiation cooling of the surface during the nighttime has cooled the air near the ground level and lowered the ceiling of the haze layer.During the summertime haze and smoke within a stable high-pressure cell may extend upward more than 10,000 feet during the heat of the day. What appears to be ample visibility at takeoff suddenly appears to be complete obstructions to lateral visibility.
*IcingIcing is a definite hazard to aircraft. Even a thin film of ice on the runway can cause loss of directional and braking control. In flight, including takeoff, the threat of ice hazards is increased.Ice is present, or potentially present, somewhere in the atmosphere at all times, no matter what the seasons or outside air temperature. The freezing level may be around 15,000 feet during the summer and as low as 1,000 feet above ground level (AGL) on winter days.When temperature and dew points are close, water vapor is condensing within the carburetor of an aircraft engine. If the engine is run at low speed, the condensation turns to ice.Glaze and Rime ice are icing types that form on an airplane's windshield, its propeller, and other aerodynamic surfaces.Glaze ice is formed and builds quickly as an airplane flies through super-cooled rain droplets. These droplets instantaneously turn to ice as they strike the airplane.This rapid increase of weight on an airplane in flight is a dangerous factor, but the greatest problem is the changing shape of the airfoil.The shape of the wings and tail surfaces (airfoils) contribute to lift, and the propeller's airfoil provides thrust.
*Rime ice has a frosty appearance (like that seen on the walls of frozen food lockers). It forms when the airplane is flying through super-cooled cloud condensation. It normally is no problem as far as weight is concerned, but it will reduce lift and if allowed to accumulate, will become a danger to flight.Frost is another "ice factor". It disturbs the airflow enough to reduce the efficiency of aerodynamic surfaces. The increased drag caused by frost makes a much longer takeoff run necessary and reduces the rate of climb.Larger, more sophisticated aircraft are equipped to break or melt ice as it is formed. This includes the propellers of reciprocating-engine aircraft.
**The National Weather Service's severe weather classifications are based upon destructive effects with regard to surface cultural (manmade) features.A 40-knot (46mph) wind has enough force to damage lightweight structures.Hail of one-fourth inch diameter can shred certain crops.
*ThunderstormsA thunderstorm may be defined as any storm accompanied by thunder and lightning. It is attended by some form of precipitation and can cause trouble for aircraft in the form of turbulence, icing, and poor visibility.A thunderstorm is local in nature and is always produced by the growth of a cumulus cloud into a cumulonimbus cloud. A thunderstorm may be studied by dividing it into three separate stages: the cumulus, or building stage; the mature stage; and the dissipating stage.
*The Cumulus StageMost cumulus clouds do not become thunderstorms, but all thunderstorms are born in cumulus clouds.The main features of this first stage of the thunderstorm are the updraft, a large air current flowing upward from the ground through the chimney-like cloud and the downdraft, the same air current returning downward toward the ground through the chimney-like cloud.The drafts may reach speeds of several thousand feet per minute and altitudes of 40,000 feet or more.During this period, small cloud droplets grow into raindrops as the cloud builds upward into a cumulonimbus cloud. It could lead to dangerous situations relative to airspeeds.
*The Mature StageThe mature stage of a thunderstorm is marked by rain at the Earth's surface. The raindrops (or ice particles) have become so large that the cloud's updraft can no longer support them, and they begin to fall.As the raindrops fall they drag air behind them, causing the characteristic strong downdrafts of mature thunderstorm. These downdrafts spread out horizontally when they reach the surface, producing strong, gusty winds, sharp drops in temperature, and a sharp rise in pressure.As the downdrafts continue to build and spread, the updrafts weaken, and the entire thunderstorm becomes an area of downdrafts.
*The Dissipating StageThe downdrafts produce heating and drying, causing the rainfall to gradually cease and the thunderstorm to dissipate, or weaken.During this stage, the cloud develops the characteristic anvil shape at the top and may take on a stratiform (layered) appearance at the bottom. This is usually the longest of the three stages of a thunderstorm's life.*TornadoesA local storm that focuses nature's most destructive forces on a small area. It is made of violently swirling winds with rapidly rising air at its center. It is small and short-lived.Tornadoes occur with severe thunderstorms. Their circular whirlpools of air take the shape of a funnel or tube hanging from a cumulonimubus cloud. The rotating column of air in a tornado may range in diameter from 100 feet to a half-mile.When a tornado touches the ground, its path may be very erratic. It may touch the ground at some points along its path and completely miss other points.
*HurricanesThis storm is a strong tropical cyclone with winds that often surpass 100 mph and have been clocked at more than 200 mph. A hurricane is a large, revolving storm with a calm center, called the eye, resulting from the speed of the whirling wi