Weather (cont.)
Air masses
Air masses are large bodies of air that take on the characteristics of the surrounding area they are located in or its source region.
Source Regions
A source region is typically an area in which the air remains relatively stagnant for a period of days or longer. During this time of stagnation, the air mass takes on the temperature and moisture characteristics of the source region. Areas of stagnation can be found in polar regions, tropical oceans, and dry deserts.
Classification and characteristics of air masses
Air masses are generally identified as arctic (A), polar (P) or tropical (T) based on temperature characteristics and maritime (m) or continental (c) based on moisture content.
| Type of air mass | Characteristics |
|---|---|
| cA | Extremely cold and dry |
| cP | Cold and dry |
| mP | Cold and moist |
| cT | Hot and dry |
| mT | Warm and moist |
Air mass modification
A continental polar air mass forms over a polar region and brings cool, dry air with it. Maritime tropical air masses form over warm tropical waters like the Caribbean Sea and bring warm, moist air. As the air mass moves from its source region and passes over land or water, the air mass is subject to the varying conditions of the land or water which modify the nature of the air mass.
Fronts
Definition
As an air mass moves across bodies of water and land, it eventually comes in contact with another air mass with different characteristics. The boundary layer between two types of air masses is known as a front. With a front approaching, changes in weather are to be expected.
Types
Warm Front, occur when a warm advancing mass of air replaces a body of cold air. A warm front typically moves at a rate of 10 to 25 miles per hour.
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Cold Front, occur when a cold advancing mass of air replaces a body of warmer air. A cold front typically moves at a rate of 25 to 30 miles per hour.
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Warm Occluded Front, occur when a fast, cool front catches up to a warm front and they replace a colder mass of air.
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Cold Occluded Front , occur when a fast, cold front catches up to a warm front and they replace a cool mass of air.
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Stationary Front, occur when two fronts of equal strength collide head on, and neither front is able to overcome the other. Such a front can stay in one place for days.
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Turbulence
Turbulence is air movement that normally cannot be seen and often occurs unexpectedly. It can be created by many different conditions.
Types of turbulence
Mountain waves are turbulent undulating waves of air which can form above and downwind of a mountain range. They can extend 50 to 100 NM downwind and upwards to the troposphere. Beneath the top of the 'waves' of wind we find rotors, which are rapid changes of wind direction, that can be very dangerous.
Mechanical turbulence is caused by air flowing over irregular terrain, or man-made structures, creading eddies and vertical, choppy air motion in the lower atmosphere.
Microbursts are intense, localized downdraft (sinking air) from a thunderstorm, less than 2.5 miles (4 km) across, causing damaging horizontal winds of up to 150 mph. Lasting only 2-5 minutes, they resemble a downburst but are smaller and more intense. They are caused by evaporation cooling and precipitation.
Wind shear is layers or columns of air, flowwing with different velocities (i.e. speed and/or direction) to adjacent layers or columns. Wind shear is a major hazard for aviation especially when operating at low levels.
Wake turbulence is formed by wing-tip vortices from airplanes or helicopters, and is characterized by rotating, downwards and outwards motions of air. Wake turbulence is particularly intense during high-lift phases like takeoff and landing.
Convective turbulence is caused by rapidly rising, warm air, often referred to as thermals. Such turbulence often occurs over parking lots during the summer and causes bumpy rides, sudden altitude changes and potential damage to aircraft and persons.
Clear Air Turbulence (CAT) is invisible, high-altitude atmospheric instability occurring in cloud-free air, usually above 15 000 feet near jet streams. Caused by strong wind shear, it causes sudden, intense shaking for the aircraft, is difficult to detect via radar, and can lead to severe in-flight passenger injuries and structural damage.
Categories of turbulence intensity
Turbulence can be categorized by intensity and duration. AIM 7-1-11 states that turbulence should be reported according to the following criteria:
| Intensity | Aircraft Reaction |
|---|---|
| Light | Turbulence that momentarily causes slight, erratic changes in altitude and/or attitude. Reported as Light Turbulence; or Turbulence that causes slight, rapid and somewhat rhytmic bumpiness without appreciable changes in altitude or attitude. Reported as Light Chop. |
| Moderate | Similar to Light, but of greater intensity. Aircraft remains in positive control at all times. Usually causes variations in indicated airspeed. Report as Moderate Turbulence. or Similar to Light Chop, but of greater intensity. Rapid bumps and jolts. Report as Moderate Chop. |
| Severe | Turbulence that causes large, abrput changes in altitude and/or attitude. Causes large variations in indicated airspeed. Aircraft may be momentarily out of control. Report as Severe Turbulence. |
| Extreme | Turbulence in which the air is violently tossed about and is practically impossible to control. It may causes structural damage. Report as Extreme Turbulence. |
Structural icing
Icing can occur before or during a flight, and it is important to stay vigilant and keep an eye out for any indications of icing during flight as icing can have a severe effect on performance and lift.
Types
Clear ice is ice formed by large supercooled droplets that hit the leading edge of an airfoil, or part of the airframe, and freeze as they flow across the surface. This type of icing is clear and can be difficult to spot from the cockpit. Clear ice can cause a reduction in lift, due to the change in shape of an airfoil, and decrease in performance, due to added weight and uneven breaking off ice across a rotor disk.
Rime ice is ice formed by smaller supercooled droplets that, upon impact with an airfoil or airframe, splatters and rapidly freezes on impact. Rime ice is milky in color due to the air that gets trapped within the ice as it freezes, and has a jagged, vertical shape. Rime ice also causes a reduction in lift due to the separation in airflow happening earlier than normal because of the ice buildup at the front of the airfoil.
Mixed ice is a combination of clear and rime ice. It features both the glossy, see-through parts of clear ice and the milky, jagged shape of rime ice.
Intensity and reporting (AIM 7-1-11)
| Intensity | Description |
|---|---|
| Trace | Ice becomes perceptible. The rate of accumulation is slightly greater than the rate of sublimation. It is not hazardous even though deicing/anti-icing equipment is not utilized, unless encountered for an extended period of time - over 1 hour. |
| Light | The rate of accumulation may create a problem if flight is prolonged in this environment (over 1 hour). Occasional use of deicing/anti-icing equipment removes/prevents accumulation. It does not present a problem if the deicing/anti-icing equipment is used. |
| Moderate | The rate of accumulation is such that even short encounters become potentially hazardous and the use of deicing/anti-icing equipment or flight diversion is necessary. |
| Severe | The rate of accumulation is such that deicing/anti-icing equipment fails to reduce or control the hazard. Immediate flight diversion is necessary. |
Prevention and elimination
The simplest way to prevent icing is to not fly into icing conditions. Icing conditions can be defined as visible moisture and below freezing temperatures. Icing can also be prevented by spraying anti-icing fluid on an aircraft before it flies. This anti-icing fluid does not eliminate icing, but should prevent it for a period of time. Icing can be eliminated during a flight using de-icing equipment. This equipment can come in different varieties, like deicing boots.
Thunderstorms
Thunderstorms are bad. Avoid 'em.
Conditions necessary for formation
There are three conditions necessary for the formation of a thunderstorm:
- Moisture
- Unstable air
- Lifting factor
- Condensation nuclei
Formation and life cycle
There are the three stages to the life cycle of a thunderstorm. With each stage different meteorological conditions follow and there are certain characteristics that define the beginning of each stage.
Cumulus stage
The Cumulus stage is the initial stage of a thunderstorm and is identified by the formation of cumulus clouds, that are beginning to show higher than normal vertical extent, and a lot of updrafts are present. This stage usually lasts for about 15 - 20 minutes.
Mature stage
The Mature stage is the middle stage of a thunderstorms life cycle and is what we most commonly associate with thunderstorms. The cumulus stages from earlier have developed great vertical extent and have formed into cumulonimbus, often with an anvil shape at the top of the cloud. The anvil forms due to rising air meeting an inversion layer in the atmosphere. The air that was warm and rising is now colder or the same temperature as the air above and it can no longer rise, instead spilling out to all sides. The Mature stage's beginning is characterized by the beginning of rain fall. In and near the thunderstorm we have up- and downdrafts, lightning, hail and wind shear. Hail can be thrown from the thunderstorm up to 20 nautical miles, which is why the FAA recommends pilots to maintain at least 20 nautical mile separation from any thunderstorm. This stage also lasts around 15 - 20 minutes.
Dissipating stage
The Dissipating stage is the final stage of a thunderstorm. All the energy in the thunderstorm is spent and the clouds are beginning to dissipate. This stage is associated with strong microbursts, which can exceed 6000 feet per minute, and reduced rain. This stage usually lasts for about 1 hour.
whiteboard here, either one with all stages or one for each with more detail.
Hazards
Hazards associated with thunderstorms are:
- Severe turbulence
- Wind shear
- Microbursts (around 3000 - 6000 feet per minute)
- Lightning
- Hail
This lesson was last edited 1 month, 1 week ago.
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