HOME HEATING Few things in nature are more counter-intuitive than the way our global home is heated. Although the source of energy is the sun above, it is the earth below that does the heating. In essence, incoming short wave solar energy in the form of light is absorbed by the earth and then re-radiated back to the atmosphere at a longer wavelength in the form of heat.
ATMOSPHERIC MEASUREMENT While doing its business, the airliner must swim in a thin three-dimensional gaseous soup, which clings to the earth. This soup, which is called the atmosphere, is very thin indeed being only about 100 miles thick compared to the earth’s radius of 4000 miles. To put a handle on these things, a baseline model called the International Standard Atmosphere or ISA has been established. At any given moment, the actual condition of the atmosphere may be different than the model but it is used as a standard, upon which performance calculations can be based. The unit of measure for air pressure is the millibar (MB) and the pressure at any point is the weight of the column of the air above it.
About half the weight of the atmosphere is below 18,000-foot level, with the other half petering out to zero at about 100 miles out in space. The standard sea-level pressure is a 1013.25 millibars and the standard temperature is 15° C.
Pressure declines with altitude on a smooth curve, however, temperature behaves quite differently. The standard temperature decreases with altitude at about 2° C per 1,000 feet up to an altitude of 36,000 feet and then quite suddenly, the temperature remains constant at – 56° C. This break point is called the tropopause and marks the boundary between the lowest level of the atmosphere, the troposphere, and the level above, which is called the stratosphere. The troposphere is the home of the airliner for jet powered airliners cruise in the vicinity of the tropopause whereas the propeller-driven airline aircraft fly in the mid troposphere.
STABILITY An important concept in the understanding of weather is vertical stability. The three conditions of stability for air are:
Absolute Stability is the condition where the parcel of air is denser than surrounding air and tends to fall from its original level.
Absolute Instability is the opposite condition where, if the same parcel of air starts to rise, it will remain less dense than the surrounding air and will continue to rise.
Neutral Stability represents the middle ground where the parcel remains at the same level.
CLOUDS and PRECIPITATION All air contains water vapour, and warm air is capable of containing much more water vapour than cold air. When air is cooled to the saturation level, or its dew point, water condenses into tiny droplets. This creates cloud (or fog when it clings to the ground). When drops grow large enough, they will fall from cloud as rain. At sufficiently low temperatures, cloud is composed of ice crystals and precipitation will occur in the form of snow.
Turbulence Cloud results where there is a strong wind at the surface and the air above the surface is subject to turbulence because of surface friction. If the air is sufficiently moist, a layer of cloud is formed with its base at the dew point level. The cloud type is called stratocumulus .
Orographic Cloud ,when wind blows against hills and mountains it is forced to rise, cooling will result. Clouds form on the windward side of the barrier.
Convective Cloud is created by air heated from below. If the air is stable, the vertical development of the cloud is restricted, and the result will be cumulus cloud. However, if the air is unstable, the vertical development of the cloud will continue, in many cases up to the tropopause. In the tropics, vertical development up to 50,000 feet is not uncommon. The velocity in upward currents may reach 2000 feet per minute, and very violent weather phenomena can result including heavy rain, lightning and, even, hail. The formal name given to this type of cloud is cumulonimbus, and it is nature’s great sound and light show, the thunderstorm. The hazards to aircraft flying in thunderstorms are;
- Lightning; dangerous but only if the aircraft is not properly grounded.
- Turbulence; The gusts and vertical currents in cumulonimbus are always disagreeable to passengers and crew. In extreme cases and they have caused the loss of aircraft either because of structural failure or loss of control.
- Icing; above freezing level, clouds contain a dense conglomeration of supercooled water droplets, which freeze on contact with the aircraft.
- Hail. Hailstorms can grow to the size of baseballs and, obviously, may cause serious structural damage.
FRONTS and CLOUDS 
When air over a large area has the same characteristics of temperature and humidity, it is termed an air mass. Thus, maritime tropical air mass is warm and humid whereas continental polar air mass is cold and dry, and so on. In mid latitudes, weather is dominated by the interaction of polar and tropical air masses and where the two meet it is called the polar front. The polar front has a warm front , where the warm air is advancing towards the cold air and a cold front , where the cold air is advancing towards the warm air. Typically, the cold front will move faster than the warm front.
The main cause of the frontal cloud and rain is the gradual lifting and cooling of warm, moist air. When one front overtakes the other front, the result is called an occluded front. Typically, it is the cold front, which has overtaken the warm front, which will force the warm air to rise creating a trough of warm air aloft or a trowal. In many cases, the trowal is often the cause of one of nature’s real villains, freezing rain.
An important genus of cloud, that is not associated with fronts, is radiation fog. This is a clear weather happening and results in circumstances where the land cools quickly at night forcing the air closest to the ground below its dew point. Flying above radiation fog, you may see objects penetrating above the top of the very thin fog layer but the pilot cannot land the aircraft because horizontal visibility at the airport is too limited; doubtless this is one of those situations where airline passengers can become a touch grumpy.
Another sort of fog that can seriously mar airline operations is advection fog. This type occurs in coastal locations where moist air is cooled from below by cold ocean currents. This fog will blow, or “advect”, ashore, the end result being poor flying weather along the coast, which may last for days.
WIND Typically wind velocities increase up to the tropopause and, consequently, the airliner may take off in calm wind conditions only to encounter a roaring stream of air at altitude.
Wind speeds are greatest near the tropopause and there is a jet stream where wind speeds exceeding 150 mph are not uncommon.
The jet stream has important characteristics including:
- The core of the stream is near the tropopause on the warm airside.
- The strongest winds may be avoided by flying away from core on the colder side.
- Above 5,000 feet, vertical and horizontal wind changes are large.
- Wind speed falls off in the stratosphere above the jet stream.
In addition, Clear Air Turbulence or CAT is sometimes encountered in the vicinity of jet streams as a result of large-scale wind shear, which refers to the change in wind velocity with altitude. Such turbulence is cloudless and, thus, very difficult to detect. It may be severe enough to cause serious injury, consequently airlines must be acutely sensitive to the location of the jet stream.
ICING The undesirable effects of icing include increased drag or reduced lift and in some cases impair the functioning of control surfaces, engines, windscreens and external sensors .In most cloud-types, inflight icing occurs in a narrow range between 0° C and -15° C. At lower temperatures water is normally in the form of ice particles, which do, not stick to aircraft.
Perhaps the most serious form of icing is Freezing Rain. In this case, raindrops from a warmer layer aloft will fall into colder air below forming solid sheets of ice that can accumulate on aircraft very rapidly.
WEATHER REPORTS Although the quality and quantity of weather reporting and meteorological service varies from one part of the world to another, international standards have been developed. Observers broadcast reports at agreed intervals using a set of standard measures. In most countries, the national weather bureau offers a meteorological service including;
- charts showing actual and forecast surface conditions for the route,
- charts showing upper winds and temperatures along the route,
- Airport reports and forecasts, and,
- Reports of significant weather including thunderstorms, icing or turbulence.
A typical aviation weather hourly report is noted below;
YAB SA 1200 5 SCT M8 OVC 2F-ZR 110/-1/-2/0610/001/SF5ST5
CODE DECODE
YAB Station Identification
SA Regular hourly report
1200 “Zulu” time = UCT (Universal Coordinated Time)
5 SCT Scattered Cloud 500ft above the ground
M8 OVC Overcast Cloud Measured at 800ft above ground
2 Visibility 2 statute miles
F-ZR Fog and light freezing rain
110 Sea level pressure =1011.0 milibars
-1/-2 Temperature -1 in degrees C; -2 Dew point in degrees C
0610 Wind from 060 degrees at10 knots
001 Altimeter Setting = 3001 inches of mercury on the barometer
SF5ST5 Strato fractus cloud 5/10 Stratus cloud 5/10
NAVIGATION For most of us this topic can be confusing for our normal earthbound reality finds us in what appears to be a flat place. And yet we have known since Galileo that, in fact, we actually live on a curved surface which is part of a ball which is whirling through space.
LATITUDE To deal with such confusion, navigators long ago developed some conventions, the first being latitude which describes the north –south dimension of any position on or above the earth’s surface.
Latitude is identified in terms of degrees, minutes and seconds, the latter two not be confused with the dots found on your wristwatch. From latitude is derived the standard measure of distance. For centuries, navigators have used one minute or 1/60 of a degree of latitude as that standard. It is called a nautical mile,and, by definition, one degree of latitude equals 60 nautical miles in length. When used to describe speed, nautical miles per hour is reduced to the acronym, knots
However, just to confuse the issue, there is another type of mile namely the statute mile. This linear measure is a legal contrivance, used for centuries by lawyers but not by navigators, to describe distance. It is derived from the ancient Roman term for a thousand paces and is 15% shorter than the nautical mile.
LONGITUDE As for east or west definition, the term longitude is used. To provide an east – west datum, navigators agreed that the Prime Meridian, should be the one passing through Greenwich, England. The longitude of a point is the difference measured around the circumference of the earth between the point and Greenwich.
For the purpose of navigation the earth’s surface has been defined by a lattice of meridians of longitude and parallels of latitude intersecting each other, once and only once, and hence, uniquely defining every position at a point on or above the earth’s surface.
Any position on the surface of the earth may be defined by identifying the meridian of longitude and the parallel of latitude on which a point lies.
An addition in the 3D-airline world and required to complete the full definition of exact location, is altitude, which is the height of the aircraft measured in feet above sea level.
Accordingly, when the airliner reports its’ position, it may do so in terms of:
- Latitude in degrees, minutes and seconds, north or south of the equator;
- Longitude in degrees, minutes and seconds east or west of Greenwich; and,
- Altitude in feet above sea level.
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DIRECTION The term heading is used to describe the direction that the aircraft is moving relative to the north. It is standard to measure direction clockwise from north 0 ° to 359 ° . For centuries the magnetic compass has been the primary direction-sensing device. This is possible because the earth acts like a giant magnet with magnetic poles not far the geographic poles.
A magnetic compass is a pointer, free to rotate and to point towards the magnetic North Pole. The angle between magnetic north and true north is called magnetic variation. Magnetic surveys have been conducted over the earth’s surface, and navigation charts show magnetic variation. The magnetic variation at any point on the earth can be found from these charts.