ATPL Meterology Flashcards
Briefly explain the purpose of and the types of met info available to international aviation through the world area forecast system (WAFS) ???
The world area forecast system (WAFS) is a worldwide system by which world area forecast centres (WAFCs) provide aeronautical met en route forecasts in uniform standardised formats.
This includes forecasts of upper winds, temperatures , humidity, max wind, icing, turbulence and CB cloud.
The two WAFCs also provide forecasts of significant weather (SIGWX) including tropical
Convert between Celsius, Fahrenheit and absolute temperature
To convert temperatures in degrees Fahrenheit to Celsius, subtract 32 and multiply by .5556
To convert Celsius to Fahrenheit multiply by 1.8 and add 32
Absolute temperatures are expressed in degrees kelvin. 0°C is equal to 273 Kelvin (100°C equals 373 Kelvin)
Explain what is meant by the terms solar radiation
Solar radiation: virtually all the atmospheric energy, including that required for heating of air, is derived from the sun, which is essentially a huge nuclear reactor. The solar energy produced by the sun falls within a wavelength range of 0.5u to 4.0u (u is a micrometer also known as a micron). Because of the very high source temperatures involved, solar energy is short wave radiation. Solar energy is made up of ultraviolet, visible light, infrared
What percent of ultraviolet makes up solar energy
Approximately 9%
Ultraviolet radiation is largely absorbed by the ozone contained in the stratosphere while a large part of the visible radiation is absorbed by water vapour. Also CO2 absorbs a very small percentage of incoming radiation
What percent of visible light is solar energy made up of
45%
What percent of infrared makes up solar energy
46%
How much solar radiation does the earth receive
Because of the large distance from the sun, earth receives only about 0.0005% of the total solar radiation emitted into space. Of this minuscule amount, some radiation is reflected back into space by cloud tops while particles within the atmosphere prevent some solar radiation reaching the earth surface due to the effect of scattering. Although this scattering process expels some radiation into space again, some is redirected to earth. The latter is known as sky radiation, thus the total solar radiation received by earth is the sum of sky radiation and direct radiation, called cloak solar radiation
What’s the basis of the huge convective cell (Hadley cell)
The rising air spreads pole ward at high levels but because of the coriolis force it does not travel all the way to the poles and as a consequence it sinks towards the surface in the belt of sub tropical highs in each hemisphere. The surface air then moves towards the equator as the trade winds, blowing from the southeast and the northeast in the southern and northern hemisphere respectively
What are the doldrums in the Hadley cell
A zone of ascending air within the regions at or near the meteorological equator caused by convective action from warm surfaces. These regions contain the equatorial trough and are referred to as the doldrums
What are the horse latitudes in the Hadley cell
Subsiding air within the regions covered by the semi stationary anticyclones centred at average latitudes 30°N and 30°S. These regions are referred to as the horse latitudes
When is the worst month for tropical cyclones in the Southern Hemisphere
February
What is the airflow over the equatorial regions in the Hadley cell
High level divergence over equatorial regions and high level convergence over the anticyclonic regions
What are the trade winds in the Hadley cell
Low level air flows (winds) from the anti cyclonic regions towards the equatorial trough. These winds are known as the trade winds
What is the equatorial trough
The heated surfaces at or near the thermal equator produce a high degree of instability and convection so that the main thrust of air movement within the affected regions is upward. This results in the establishment of the equatorial trough, a system of comparative low pressure and surface convergence
What cloud development is found in the equatorial trough
Since the surfaces which the equatorial trough move have vast amounts of oceanic areas, considerable cloud formations are involved. These formations (which can at time penetrate into the stratosphere) reflect a certain percentage of solar radiation, which would have an adverse effect on the degree of surface heating if the main cloud concentration were positioned at the thermal equator. Maximum convergence, convection and cloud development are normally located a few degrees of latitude away from the equatorial trough on the geostrophic equator side
What do the trade winds produce in the equatorial trough
The trade winds which converge within the equatorial trough, will frequently produce the intertropical convergence zone (ITCZ)
What is the equatorial trough a permanent feature of
The equatorial trough is a more or less permanent feature of convergence and convection whereas the ITCZ is an erratic feature, the presence and intensity of which depends on the variable degree of trade wind convergence. Put differently, the equatorial trough is a general term for the belt of comparatively low pressure around the globe in the tropics. The ITCZ is a synoptic or weather feature inside this trough
What are the latitudes covered by the equatorial trough called
The latitudes covered by the equatorial trough are known as the doldrums. Within these areas, winds are generally light and variable mainly due to the fact that the predominant movement of air is upward rather than horizontal. The convergence associated with the doldrums is not of a constant nature
Where is the equatorial trough located during the northern summer (May to October)
The entire trough is located in the northern hemisphere but, due to the strongly differing nature of the surface, the latitudinal change in position and intensity of the trough varies greatly. Over the major oceanic regions, the equatorial trough is fairly constant in terms of latitude but the influence of continental land masses, their differing mountainous natures and local pressure systems cause the trough to shift considerably and even fracture in a few localities
What can cause the fractures in the ITCZ (in the northern summer)
The influences of heat lows, monsoons and orographic factors in the India/into china region causes an almost complete fracture while the nature of the trough alters dramatically over North Africa
Where is the equatorial trough in the Southern summer (November to April)
A large deflection takes the trough south through South Africa while a smaller wave brushes over the northern parts of Australia for similar reasons as stated for the northern hemisphere i.e influence of variations in surface characteristics. The major difference however, is the position of the trough in the central Pacific and Atlantic oceans where the equatorial trough remains in the northern hemisphere. The main reasons for this are firstly, stronger trade wind strength in the northern compared to the Southern Hemisphere and, secondly, the influence of oceanic surface temperatures
What speed to changes occur with weather in the ITCZ
Changes in activity take place suddenly and dramatically
What does the zone of an active ITCZ become covered with
Considering the high moisture content of air especially when the ITCZ is positioned over oceanic regions, any lifting will almost immediately involve the SALR when air saturates at low levels. Thus the zone becomes associated with massive cloud developments, which can involve a horizontal width of some 5 to 600km and a depth of some 50,000 to 60,000ft
What does the cloud consist of in an active ITCZ
Cloud consists mainly of the cumulus type such as CU, CB and AC but also some stratiform cloud at high levels (CS) where tops of CB clouds reach a balance with the surrounding cold air. It is not uncommon for tops of CBs to exceed the height of the equatorial tropopause through simple momentum
What are the main dangers of an active ITCZ
Turbulence, icing and other factors associated with CBs
How is turbulence a danger in an active ITCZ
The presence of large numbers of CB clouds in their various stages of development means that turbulence will be moderate to severe. The width of the ITCZ when active can be up to 600km, thus penetration will involve prolonged exposure to turbulence. It will be difficult to avoid this phenomenon unless the flight path can be maintained above the tops of CBs
How is attempting to avoid turbulence by flying below the base of CBs potentially dangerous because most often flight will have to be below 1000ft where the following problems are encountered
- heavy precipitation will seriously reduce visibility
- depending on the stage of development, strong up or downdraughts can be expected below the cloud base
- strong convergence below growing cells will reduce ambient pressure with the result that the altimeter will overhead
How is icing a danger in a active ITCZ
The high water content experienced in ITCZ cloud developments poses a risk of moderate to severe icing above the freezing level and especially within the first 10,000ft to 15,000ft above it
What are other CB factors that are dangers of an active ITCZ
Exposure to dangers of thunderstorms. When the ITCZ extends over continental areas, its characteristics will be influenced by the nature of the land, the temperature differences of the air masses either side of the zone and the presence of major airflows.
What is the weather associated with an inactive ITCZ
When inactive, the zone is associated with various oktas of cumulus and some shower activity from the more developed clouds. Winds are light and variable
What is the South Pacific convergence zone (SPCZ)
A zone of intense cloudiness due to convergence of two major airflows is frequently identified from the eastern tip of Papua New Guinea to slightly southeast of French Polynesia. Like the ITCZ, it varies in intensity
Where do the two converging airflows originate from in the SPCZ
- the semi stationary anticyclonic system in the eastern Pacific. The easterlies on the equator side of the system turn north easterly on approaching the western pacific; and
- the anticyclones originating and travelling eastwards from the Australia/ New Zealand region the south-easterlies preceding these systems extend to the region where the SPCZ forms
When is the SPCZ more active
When the north easterlies and the south easterlies meet they form a convergence zone with weather characteristics similar to the ITCZ. The zone is more active in summer than winter
What strengthens or re activates the SPCZ
The SPCZ appears to be associated with the northeast passage of mid latitude cold fronts when the cloud band of the latter merges with that of the convergence zone. This cold fronts appear to strengthen or re activate the SPCZ
Where is the northern limit of the SPCZ
The northern limit of the SPCZ is often taken to lie over Papua New Guinea and fluctuations in its intensity may bring cloudiness and rainfall down into Australia’s Northern Territory and northern Queensland from time to time. It is this occurrence that may seem to indicate incorrectly that the ITCZ extends to Australia
Where are the semi permanent anticyclones preferred to reside most of the year
There are a number of preferred localities at the western sides of major continents where almost permanent anticyclones reside most of the year. These systems are centred around latitudes 30°N and 30°S. In addition to these and involving the same latitudes are anticyclones that slowly travel eastwards (horse latitudes)
What are the horse latitudes
The centres of the large semi permanent anticyclones are invariably associated with calm conditions, which are referred to as the horse latitude
What do the semi permanent anticyclones have a strong influence on
Tropical weather due to their continuous surface divergence
How high are the trade winds restricted to
The general outflow from the high pressure belts at average latitudes 30°N and 30°S towards the equatorial trough produce steady wind in the lower troposphere, generally restricted to approx 8,000ft AMSL. Even though the coriolis factor is weak in subtropical latitudes, it is sufficiently effective to produce bending to these winds
What way do the Tradewinds go in both hemispheres
In the Southern Hemisphere equator bound airflow becomes southeasterly.
In the northern hemisphere the equator bound airflow becomes a north easterly.
These predominant winds are known as the trade winds and these are the winds that converge at the equatorial trough and are instrumental in producing the weather associated with the trough
What is the strength of trade winds
Trade winds are quite steady and persistent in strength. In the pacific region they vary throughout the year from 10kts to 20kts with the occasional increase to 30kts. In global terms, the trade winds are generally slightly stronger in winter than summer. The wind above the trade winds are usually from the west, often light
What are the variations in trade winds caused by
Although their direction in the Southern Hemisphere is mainly from the southeast there are minor variations in direction due to underlying terrain (islands versus oceans for instance) and pressure gradient fluctuations
How is a subsidence inversion created from trade winds
Since these winds are fundamentally associated with the anticyclonic systems it is to be expected that subsidence, normally present in these systems, will influence the weather characteristics within the trade wind zone. However, as the winds travel further from the highs towards the equatorial trough and penetrate increasingly warm surface conditions, the effect of this subsidence lessens. Thus the weather, especially at the source of the trade wind zone, is for a large part influenced by subsidence inversions, which vary in height from 6,000ft to 8,000ft above sea level
What are the conditions like below the trade wind subsidence inversion
Below the inversion, visibility is usually fair to good with varying amounts of cumulus with bases between 2,000 and 3,000ft and tops at some 8,000ft. These amounts and vertical limits are prone to increase when trade winds cross the windward side of islands so that increased wind speed and shower activity are likely in those localities
What is the moisture content like below the trade wind subsidence inversion
The moisture content below the subsidence inversion is much greater than that above. Thus altitudes above some 8,000ft are characterised by good visibility and generally dry air and clear skies. However, as the trade winds approach the vicinity of the equatorial trough, the subsidence inversion weakens and rises in height.
Increased instability from warm surfaces and, especially over ocean areas, great abundance of moisture, cause the formation and development of instability cloud which ultimately includes towering cumulus with associated thunder, squall and shower activity
What happens when Southern Hemisphere trade winds cross the equator
Trade winds converging at the equatorial trough are subject to directional change depending on season. For instance, in mid Pacific, the trough never enters the Southern Hemisphere, which means that the south east trade winds (of the Southern Hemisphere) cross the equator to flow to the trough (in the northern hemisphere)
What happens when the northern hemisphere trade winds cross the equator
During the Southern winter, the equatorial trough is in the northern hemisphere north of Australia so that trade winds in that region also cross the equator. However, in summer time in Australia, the trough is south of the equator so that the northern hemisphere trade winds will have to do the crossing
How does the coriolis force affect the moving trade winds en route to the equatorial trough in the northern hemisphere
When the south east trade winds cross the equator en route to the equatorial trough in the northern hemisphere, they will be subjected to the coriolis force applicable to the northern hemisphere i.e veer to the right. The strength of the force is weak but still present. Therefore, after crossing the equator, the trade winds blow initially towards the trough from the south and ultimately from the southwest
How does coriolis force affect the trade winds when its crossing the equator towards the trough near Australia
The north east trade winds from the northern hemisphere crossing the equator towards the trough near northern Australia in summer will initially become northerly and ultimately north west
What are the influences of islands on the trade winds
In summertime, the land of major islands warms to a greater degree than the surrounding seas so that sea breeze effects develop over the land. Air pressure will be slightly lower over the land than over the sea and instability increased. Thus coastal regions exposed to the southeast experience an increase in cloud development and possibly heavy precipitation depending on the nature of terrain. In some situations this effect will be felt even at the lee side of islands
What is the influence of islands on Tradewinds in the winter
In wintertime, the land is colder than the surrounding seas so that pressures are somewhat higher inland and stability is enhanced. Although some cloudiness may still occur at the southeast coasts from exposure to the Tradewinds, generally conditions are much drier and more stable
What are the main disturbances in the tropical circulation
Individual cumulus, mesoscale convective areas (MCAs), wave disturbances, monsoons and tropical cyclones
How is individual cumulus a tropical disturbance
These are the smallest disturbances, measure in a life span of only a few hours and a dimension of some 5km across, resulting from turbulence experienced in the trade wind friction layer. These individual clouds often align themselves in cloud streets more or less parallel to the wind direction, or they are arranged in honeycomb patterns
How is mesoscale convective areas (MCAs) a tropical disturbance
These are systems comprising of grouped clusters of cumulus and CB developments within areas from 100km to 1,000km across. MCAs are characterised by instability through deep layers - they can often be part of the equatorial trough. Although many MCAs last no longer than one or two days, others can persist for extended periods of time
What are wave disturbances
Apart from tropical cyclones, it is rare for closed circulation disturbances to appear in tropical latitudes. Instead, disturbances demonstrates themselves in wave form in the equatorial easterlies where waves travel from the east to the west but not necessarily at the same speed as the trade winds
How do wave disturbance generally originate
The formation of a wave generally originates some two to four degrees of latitude on the pole ward side of the equatorial trough. Of all the wave disturbances that from in the tropical throughout a year, approx 25% develop to the tropical depression stage while some 10% become named tropical cyclones. Easterly waves are more frequent in the northern hemisphere than in the South Pacific
What’s the life span of easterly waves
Easterly waves normally have a life span of about 10 days, have an average wavelength of some 3,000km, travel approx 6 degrees in longitude per day and slope eastwards with height
What will happen in wave disturbances when the easterly wind is greater than the speed of the wave
A region of convergence lies behind the wave while a region of low level divergence lies in front of the wave. Thus within the convergent zone behind the wave it is common to see well developed CB clouds with heavy showers and thunderstorm activity while ahead of the air is relatively clear. The system is almost identical to the mid latitude cold front -the sequence of events are the same
What will happen to the wave disturbances when the speed of the easterly wind is slower than the speed of the wave
The region of convergence and cloudiness is found ahead of the wave and surface divergence coupled with relatively clear skies behind the wave
What causes monsoons
When large land masses of the world heat up during their summer season, intense instability and convection cause air to rise so that replacement air from the oceans travel towards the land and inland. The great water content of this air produces large cumulus developments so that thunderstorms, heavy precipitation and squalls result. Mountain barriers accentuate the development. This is the basis of the large scale wind regime of the wet monsoon, which may well involve winds contradicting the prevailing wind
What is the dry monsoon
When land masses cool in winter, subsidence on a grand scale over the land causes air to flow outward towards the oceans, which have retained a great deal of their stored heat. Thus the weather over the land is generally clear due to adiabatic warming. This reflects the dry monsoon
Where are the major monsoon regions found
With the exception of perhaps the north Australian continent, the major monsoon regions are found in the northern hemisphere. The most developed monsoon winds are concentrated along the South Asian continent when in summer time the transport of moist air from the Indian Ocean causes widespread precipitation and cloudiness in India, Pakistan, Myanmar etc.
