1. Aviation Weather
Understand basic facts and general principles of aviation weather.
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.
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.

2. Overview 1. Weather Hazards 2. Severe Weather
3. Arctic and Tropic Weather
1. Weather Hazards
2. Severe Weather
3. Arctic and Tropic Weather

3. Weather Hazards Reduced Visibility
Three miles lateral visibility is acceptable for safe flight under visual flight rules (VFR).
Possibility for accidents is greatest when visibility is reduced and the pilot is not trained to fly according to instrument flight rules (IFR).
Clouds, rain, snow, fog, and obstructions.
Haze and smoke can reduce visibility when the wind is calm.
Reduced Visibility
Generally 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.

4. Weather Hazards Reduced Visibility Blowing dust, sand, and snow
Whiteouts
Blowing dust, sand, and snow
Blowing dust of significant proportions is found in the relatively dry areas of the country. This develops when the air is unstable and there are strong winds.
The "horizontal wind" picks up dust, while strong convective currents ("vertical winds") carry it upward into the atmosphere.
These winds can spread dust over hundreds of miles and upward to 15,000 feet reducing visibility in all directions to very low levels.
Aviators should turn toward a clear airport and land before approaching blowing dust.
Blowing sand is much more localized than dust. It occurs only in desert regions and only when the wind is strong enough to lift loose sand.
Being heavier than dust, blowing sand is seldom lifted higher than 50 feet above the surface.
This condition would prohibit an aircraft from taking off or landing.
Blowing snow causes the same visibility problems as fog. For this condition to exist, the snow must be lifted to a height of at least 6 ft. The blowing snow can reach up to 1,000 feet above the surface.
Whiteouts
This condition is more frequent in the arctic areas, but it can occur anywhere there is snow-covered ground and a low-level cloud deck of uniform thickness.
This is not a physical obstruction to visibility; it is an optical phenomenon.
When the Sun is at an angle of about 20o above the horizon, its light rays are diffused as they penetrate the cloud layer. This causes them to strike the snow-covered surface at many angles; thus, no shadows are produced.
This causes the loss of any reference to the horizon and of any other references needed for depth perception.
People, buildings, trees and all other dark objects appear to float in space, which is not conducive to landing an airplane.

5. Weather Hazards
Icing
Ice is present in the atmosphere at all times-15,000 feet in summer and as low as 1,000 feet in winter.
Glaze and Rime ice 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.
Icing
Icing 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.

6. Weather Hazards
Icing
Rime ice forms when an airplane is flying through super-cooled cloud condensation.
Frost disturbs airflow to reduce lift efficiency.
Larger, more sophisticated aircraft are equipped to break or melt ice as it is formed.
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.

7. Severe Weather

8. Severe Weather
The NWS severe weather classifications are based upon destructive effects with regard to surface cultural features.
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.

9. Severe Weather Thunderstorms
A storm accompanied by thunder and lightning.
A Thunderstorm is local in nature and is always produced by the growth of a cumulus cloud into a cumulonimbus cloud.
Three stages
Cumulus
Mature
Dissipating
Thunderstorms
A 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.

10. Severe Weather Thunderstorms - Cumulus Stage The Cumulus Stage
Most 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.

11. Severe Weather Thunderstorms - Mature Stage The Mature Stage
The 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.

12. Severe Weather Thunderstorms - Dissipating Stage The Dissipating Stage
The 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.

13. Severe Weather Tornadoes
Local storm that focuses destructive forces on a small area.
Occurs with severe thunderstorms.
When it touches the ground its path may be very erratic.
Tornadoes
A 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.

14. Severe Weather Tornadoes Destructiveness is caused by high winds.
Very low pressure gives tornadoes great suction.
The great destructiveness of a tornado is caused by high wind speeds and the low pressure inside the funnel. Wind speeds have been measured as high as 318 mph.
The very low pressure gives tornadoes great suction. They can lift objects as heavy as automobiles and rooftops of houses. They have been known to suck water from creeks and small rivers.

15. Severe Weather Tornadoes
Occur most often in the spring months and in the afternoon hours.
Very difficult to forecast.
Tornadoes occur most often in the spring months and in the afternoon hours.
Tornadoes are very difficult to forecast. The storm only lasts a short time and is a local condition. Even today, forecasters do not know for certain that a given combination of weather elements will always produce a tornado.

16. Severe Weather Hurricanes
A strong tropical cyclone with winds that surpass mph.
A large revolving storm with a calm center (eye).
Hurricanes
This 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 winds around the low-pressure core. The tropical cyclone is an area of convection action, with the air moving upward in spirals around its eye.

17. Severe Weather Hurricanes
Born over the tropical water of the Atlantic and Pacific Oceans, the Gulf of Mexico and the Caribbean Sea.
Energy gained from heat given off by warm air.
Tropical cyclones that affect the United States are born over the warm tropical waters of the Atlantic and Pacific Oceans, the Gulf of Mexico, and the Caribbean Sea.
They get their energy from the heat given off by the condensation of moisture in the warm air. Heat lowers the atmospheric pressure, and wind begins to flow into the heated (low-pressure) area. The Coriolis force deflects that wind, setting up the typical cyclonic movement of winds at less than 39 mph. At this point the storm is call a tropical depression.

18. Severe Weather Hurricanes Tropical depression Tropical storm Hurricane
The next level is tropical storm. In this stage the winds range between mph. When wind speeds reach or exceed 75 mph, the tropical cyclone is called a hurricane.
A typical hurricane will move westward or northwestward in the Northern Hemisphere at a speed of about mph. When it reaches 25 to 30o north latitude, it curves to the north and east. It then increases in speed to a movement that may exceed 50 mph.
Heavy rainfall accompanies the hurricane, with individual convective showers and thunderstorms lining up in a circular pattern around the eye of the hurricane. Heavy rainfall often continues after the hurricane moves inland, even after the wind circulation decreases.
The National Weather Service keeps a constant watch for hurricane development, especially during the "hurricane season"-1 June to 30 November.

19. Severe Weather
Hail
May be within a strong cumulus cloud before any type of precipitation falls to the surface.
The combined velocity of an aircraft and hail gives these small pellets a tremendous amount of energy.
If the pellets are “hard” because of exceptionally cold temperatures at their level, aircraft will experience damage.
Hail
Hail may be within a strong cumulus cloud before any type of precipitation falls to the surface. At this developing stage, the hail pellets may not be very large because strong air currents are still able to carry them about within the clouds.
The combined velocity of an aircraft and the velocity of the hail give these small pellets a tremendous amount of energy. Under these conditions and especially if the pellets are "hard" because of exceptionally cold temperatures at their level, aircraft will experience damage.

20. Hail
Orlando, Florida…1992

21. Severe Weather
Hail
Encounters with larger hail are even more damaging. Hail having the size, weight, (baseball size), and velocities produced by thunderstorms in the western and mountain areas could rip a small plane apart.
Hail and rain pulled into an aircraft engine can cause the engines to stall and flame out, as well as cause internal damage.
Encounters with larger hail are even more damaging. Hail having the sizes, weight, (baseball size), and velocities produced by thunderstorms in the western and mountain areas could literally rip a small plane apart.
Hail and rain pulled into an aircraft engine can cause the engines to stall and flame out, as well as cause internal damage.

22. Arctic and Tropic Weather
Arctic Weather
Mountains around Arctic Circle holds in cold air
Precipitation is light
Strong winds occur in fall and winter
Visibility is a major problem
Arctic Weather
Mountains around the Arctic Circle hold Arctic air masses in the area. Little evaporation takes place, because temperatures are so cold and the air is dry.
Precipitation in the arctic is light, ranging from 3 to 7 inches per year in the coastal areas to 5 to 15 in the interior.
Strong winds occur frequently during the fall and winter. Winds stronger than 100 mph are not uncommon along the Greenland coast during the winter. Arctic winters are characterized by frequent wind storms and well-defined frontal passages.
Visibility is a major problem at low levels in the Arctic due to fog, haze, blowing and drifting snow, and peculiar light conditions.

23. Arctic and Tropic Weather
Tropic Weather-fronts are rare and there is more moisture in the air, especially in the oceanic and coastal regions.
Oceanic Weather
Clouds cover about half the sky over the tropical oceans.
Northeast and southeast trade winds converge.
When the wind convergence gets strong, thunderstorms develop that may produce winds up to 70 mph.
Oceanic Weather
Clouds cover about half the sky over the tropical oceans outside the equatorial trough. The equatorial trough is an area of low pressure and light winds near the equator, varying from north to south of the line with the changing seasons.
It is also the area in which the northeast and southeast trade winds converge and it is sometimes referred to as the intertropical convergence zone. Some amount of unsettled convective weather is found in this area at all times.
When the wind convergence gets strong, thunderstorms develop that may produce winds of up to 70 mph. Normal cloudiness includes a line of cumulonimbus clouds with sheets of cirrus clouds spread from north to south.

24. Arctic and Tropic Weather
Continental Weather
Subject to extreme variations because of land features.
Weather ranges from hot and humid in the lower Congo River to arid Libyan Desert to snow-capped mountains of Kenya.
Day time temperatures may exceed 100o F.
Precipitation often evaporates before it reaches the ground.
Thick early morning steam fog often forms in the jungles.
Continental Weather
Continental tropical weather and climate are quite different from oceanic tropical weather. It is subject to extreme variations because of the different features of the land: coastal mountain ranges, altitudes, wind flow patterns, and rate of evaporation vary from surrounding ocean surfaces.
Various combinations of these factors produce tropical weather ranging from the hot, humid climate of the lower Congo River to the arid Libyan Desert to the snow-capped mountains of Kenya in Africa. Snow is not uncommon on the higher mountain peaks in the Hawaiian Islands.
In the arid areas behind coastal mountains, daytime temperatures may exceed 100o F, but nighttime temperatures in the desert regions may go below freezing.
Precipitation often evaporates before it reaches the ground. "Dry" thunderstorms produce sudden, violent squall winds and may cause severe dust or sandstorms.
When mountains or high terrain are not present to obstruct the flow of maritime air onshore, the warm moist air influences wide continental areas in the tropics. Cloudiness and precipitation are at a maximum over these jungles regions and tropical rain forests.
Thick early morning steam fog often forms in the jungles. There is also cloudiness over these areas. Occasional heavy rains with descending cold air currents can drive nighttime temperatures down to about 60o F, about 30o below normal daytime ranges.

25. Arctic and Tropic Weather
Island and Coastal Weather
Weather similar along coastal areas and mountainous islands.
During the day, large cumuliform clouds develop.
Clouds seen from long distances indicating presence of islands.
Trade winds result in onshore winds on one side of island and offshore winds on the other.
Island and Coastal Weather
Weather conditions are similar along coastal areas and over the various mountainous islands of the tropic. During the day, as warm, moist air moves inland and is lifted over the terrain, large cumuliform clouds (vertical development clouds with dome-shaped upper surfaces) develop.
While these clouds are common in coastal areas, the lifting of moist air on the windward side of mountainous islands also produces towering cumulus clouds. These clouds frequently may be seen from long distances, indicating the presence of islands.
These islands, especially when mountainous, also have a very interesting effect on rainfall. The almost constant trade winds result in a predominantly onshore wind on one side of the island, while the opposite side has an offshore wind most of the time.

26. Arctic and Tropic Weather
Island and Coastal Weather
When approaching a coastal area or an island, the aviator should be prepared for instrument conditions.
Precipitation and cloudiness are heavier on windward side.
One of the major factors influencing tropical weather is the easterly wave.
As the wave line approaches, the pressure falls and the wind shifts to the east as the line passes.
Island and Coastal Weather
When approaching a coastal area or an island, the aviator should be prepared for instrument conditions.
Precipitation and cloudiness are considerably heavier on the windward side than the leeward. An example of this is Kauai, Hawaii, where Mount Waialeale receives the highest average annual rainfall of all rain gauges in the world, 460 inches. Only 10 miles away, on the leeward side of Kauai, sugar cane plantations must be irrigated.
One of the major factors influencing tropical weather is the easterly wave. This is a common tropical weather disturbance that normally occurs in the trade wind belt. Easterly waves of the Northern Hemisphere have advance winds blowing somewhat more northerly than the usual trade wind direction. As the wave line approaches, the pressure falls.
The wind shifts to the east as the line passes. The typical wave is preceded by fair weather, but followed by cloudiness, low cloud ceilings, rain, and usually thunderstorms.

27. Summary 1. Weather Hazards 2. Severe Weather
3. Arctic and Tropic Weather
In this lesson we discussed:
1. Weather Hazards
2. Severe Weather
3. Arctic and Tropic Weather