• Microburst !!!


    Single cell Thunderstorm moves in line with medium level winds (generally 10,000 feet). Active period is less than one hour. 

    Developing Stage: 

    – Updrafts 3-4000 fpm 

    – Turbulence mixes rising air with environmental cold air and limits its development. 

    – Moderate to sever turbulence 

    – No precipitation 

    – Last for 15-20 mins  Continue reading  Post ID 35

  • Fog, Dew and Frost !!!

    Fog, Dew and Frost

    Evaporation or Mixing Fog
    This type of fog forms when sufficient water vapor is added to the air by evaporation and the moist air mixes with cooler, relatively drier air. The two common types are steam fog and frontal fog.

    Steam fog forms when cold air moves over warm water. When the cool air mixes with the warm moist air over the water, the moist air cools until its humidity reaches 100% and fog forms. This type of fog takes on the appearance of wisps of smoke rising off the surface of the water.

    Frontal fog is the other type of evaporation fog. This type of fog forms when warm raindrops evaporate into a cooler drier layer of air near the ground. Once enough rain has evaporated into the layer of cool surface, the humidity of this air reaches 100% and fog forms.
    Continue reading  Post ID 35

  • Metar !!!

    A METAR message is valid AT the time of observation (not for any specific time period). It is an actual observation at a specific time, normally made at hourly or half hourly intervals. If weather changes by a significant degree a special observation “SPECI”, will be issued. 

    Arrow A ceiling is defined as, height above ground or water of the lowest layer of cloud below 20000 ft covering more than half of the sky. 

    Arrow In a METAR message, the wind group is 23010 MPS means wind from 230 deg true at 20 knots. Multiply MPS (metrs per second) by 2 to get the answer in Knots. Knots is double the MPS value. Met reports have wind direction in degrees “true”. ATC provides (like in ATIS) wind direction in degrees magnetic. 

    Arrow In the METAR code the abbreviation VC indicates “Vicinity” i.e. present weather within a range of 8 km, but not at the airport. 

    Arrow The visibility transmitted in a METAR is the lowest observed in a 360 deg scan from the meteorological station. 

    Arrow DLLO 121550Z 31018G30KT 9999 FEW060TCU BKN070 14/08 Q1016 TEMPO 4000 TS= 

    Above METAR cannot be abbreviated to CAVOK because the cloud base is below the highest minimum sector altitude. CAVOK only refers to ceiling and visibility. 

    The FEW060TCU gives cloud below the MSA and TCU, both of these preclude CAVOK. 

    The ICAO definition of CAVOK changed and now mentions TCU which it didn’t before. In other words, before the change you could have TCU present and it could still be CAVOK but now TCU would preclude it being CAVOK. (http://www.atpforum.eu/showthread.php?t=10747) 

    The definition of CAVOK will change, such that Towering Cumulus Clouds (TCU) are now regarded as significant clouds. If there are Towering Cumulus Clouds (TCU) present, CAVOK will no longer be permitted to be reported. This change will be applied on 5 Nov 2008. (http://www.caa.co.uk/docs/1382/UK%20Met%20Consultation.pdf) 

    Arrow When gusts are at least 10 knots above the mean wind speed then the surface wind in a METAR records a gust factor? 

    Arrow VV is vertical visibility in hundreds of feet and not in meters. 

    Arrow Trend forecast is a landing forecast appended to METAR/SPECI, valid for 2 hours. 

    Arrow If CAVOK is reported then there cannot be low drifting snow. 

    Arrow The cloud base, reported in the METAR, is the height above airfield level (i.e. AAl not AGL). 

    Arrow LSZH VRB02KT 5000 MIFG 02/02 Q1015 NOSIG 

    The report is possible, because shallow fog (MIFG) is defined as a thin layer of fog below eye level. Shallow fog is low-lying fog that does not obstruct horizontal visibility at a level 2 m (6 ft) or more above the surface of the earth (i.e. the fog layer is not deeper than 2 meters). This is, almost invariably, a form of radiation fog. 

    Arrow In METAR messages, the pressure group represents the QNH rounded “Down” (not Up) to the nearest hPa. 

    Arrow Runway report 01650428 appended to a METAR means you should consider the friction co-efficient which is 0.28 when making performance calculations. see the decode here (http://www.atpforum.eu/showthread.php?t=941) 

    Arrow Cloud base is reported in steps of 100 ft up to 10,000 ft and in steps of 1,000 ft above 10,000 ft in a METAR 

    Arrow If a large number of reports are sent as a block bulletin (in bulk) they are prefixed by the code SA (Station Actual) for METARS and SP for SPECIs. If TAF is issued in a bulletin then report type is coded as FC (9-12 hrs) or FT (12-24 hrs). 

    Arrow NOSIG means No Significant Change Continue reading  Post ID 35

  • Tropopause


    Average Height and Temperature of Tropopause 

    – Poles: 8 km and -45 deg C. 

    – Mid Latitudes: 11 km and -56 deg C. 

    – Equator: 16 km and -75 deg C. 

    Typical Tropopause Heights 

    Latitude 30 deg: 16 km in summers and winters. 

    Latitude 50 deg: 12 km in summers and 9 km in winters. 

    Latitude 70 deg: 9 km in summers and 8 km in winters. 
    Continue reading  Post ID 35

  • Atmospheric Humidity !!!


    The term humidity describes the fact that the atmosphere can contain water vapor. The amount of humidity found in air varies because of a number of factors. Two important factors are evaporation and condensation. At the water/atmosphere interface over our planet’s oceans large amounts of liquid water are evaporated into atmospheric water vapor. This process is mainly caused by absorption of solar radiation and the subsequent generation of heat at the ocean’s surface. In our atmosphere, water vapor is converted back into liquid form when air masses lose heat energy and cool. This process is responsible for the development of most clouds and also produces the rain that falls to the Earth’s surface.

    Scientists have developed a number of different measures of atmospheric humidity. We are primarily interested in three of these measures:mixing ratio, saturation mixing ratio, and relative humidity. Mixing ratio is a measure that refers to the mass of a specific gas component relative to the mass of the remaining gaseous components for a sample of air. When used to measure humidity mixing ratio would measure the mass of water vapor relative to the mass of all of the other gases. In meteorological measurements, mixing ratio is usually expressed in grams of water vapor per kilogram of dry air. Saturation mixing ratio refers to the mass of water vapor that can be held in a kilogram of dry air at saturation. Saturation can be generally defined as the condition where any addition of water vapor to a mass of air leads to the condensation of liquid water or the deposition of ice at a given temperature and pressure. The data in Table 8c-1 indicates that warmer air has a higher saturation mixing ratio than cooler air at a constant atmospheric pressure. It is important to note that this relationship between temperature and water vapor content in the air is not linear but exponential. In other words, for each 10° increase in temperature, saturation mixing ratio increases by a larger quantity.

    Homework-Desk.com provides students with professional online physics homework help and physics assignment assistance. Continue reading  Post ID 35

  • Visibility and RVR !!!

    Prevailing Visibility

    The prevailing visibility roughly represents the average visibility.

    It is the greatest distance that can be seen throughout at least half the horizon circle.

    The areas could comprise contiguous or non-contiguous sectors.

    The lowest visibility observed will also be reported if the visibility in any direction is either:

    a) Less than 1500 metres


    b) Less than 50% of the prevailing visibility.

    If the lowest visibility is observed in more than one direction then the most operationally significant direction will be reported.

    When visibility is fluctuating rapidly and the prevailing visibility cannot be determined then only the lowest visibility will be reported without direction.

    e.g. If visibility near the airport is 900 meters in the North East quadrant, 5 km in South East, 3 km in the South West and 4 km in the North West quadrant then what would the prevailing visibility be reported as?

    The maximum is 5 and the second highest is 4, so prevailing visibility reported will be the more restrictive of the two i.e. 4 km.

    The visibility (900m) in one particular direction i.e. NE is less than 1500 and less than half the prevailing visibility so it will be reported together with its direction.

    So for the above example, the reported visibility format will be 4000 0900NE.

    RVR – Runway Visual Range

    The maximum distance in the direction of takeoff or landing at which the runway, or specified lights delineating the runway, can be seen from a position on the centreline corresponding to the average eye level of a pilot at touchdown.

    – RVR is not normally reported if it is 1500m or more.

    – Between 1500 and 800m it is reported in steps of 100m.

    – Between 800 and 200m it is reported in steps of 50m.

    – Between 0 and 200m it is reported in steps of 25m.

    – e.g. R36L/P1500: Runway 36 Left touch-down RVR is more than 1500m.

    – If RVR is more than the maximum that the equipment is calibrated, then that maximum is given preceded by P (plus).

    – If it is less than the minimum, the minimum is given preceded by M (minus).

    – If the RVR has been steady the group can be followed by N (No change).

    – If it has been changing rapidly then the group is followed by “U” for up or “D” for down.

    – If it has been very variable over the 10 minute observation period, the maximum and minimum is given separated by a “V”.

    – RVR is not normally recorded or reported if it is more than 1500m.

    – METAR reports only touchdown RVR.

    – ATIS and ATC voice warnings reports mid-point and stop-end RVR. Continue reading  Post ID 35

  • Altitude và Flight level là gì ???

    – Flight levels use QNE or pressure altitude, while altitude references QNH or local pressure adjusted to sea level pressure. Altitudes are used at low levels and flight levels at higher levels. The transition between altitudes and flight levels differs by country and is generally just above the highest obstacle in that country. In the US the transition altitude/level is 18,000′ / FL180. Some countries transition as low as 5000′ / FL050 and the transition altitude/level may vary from airport to airport.

    – In the altitudes knowing accurate elevations relative to the ground and obstacles is important for collision avoidance and this is the reason QNH is used here. Each airport will report QNH and controllers will issue the current QNH as needed. You need to know the QNH for obstacle / terrain avoidance but you need to be using the same QNH as those around you for aircraft vertical separation.

    – Above all terrain/obstacles the only thing we care about is vertical separation, so we no longer need to know about the actual pressure and instead use a standard reference pressure, QNE / 1013.25 hPa / 29.92″ Hg.
    Note that flight levels drop the last two zeros of the corresponding altitude and so 30,000 is FL300, not FL30000. When checking in with a controller, FL300 would be pronounced flight level tree zero zero.
    It is also worth noting that an altimeter cannot actually determine altitude. It can only determine pressure (technically local static pressure compared to a reference pressure). It converts this pressure to an altitude using a calibrated non-linear scale.

    – FL180 and FL300 stand for Flight Level 180 and Fight Level 300.
    Flight levels are spaced 100ft apart on an altimeter that is set to the standard sea level pressure (QNE) of 1013.25 hectopascals or 29.92 inches of Mercury. So indeed, FL300 means 30,000 ft.
    Altitude 18000 means that the altimeter indicates 18,000 feet and that the altimeter is set to the QNH, which is the pressure reading on the ground corrected to sea level pressure using the standard atmosphere.

    When there is a low pressure area, the QNH will be lower then 1013.25 hPa. When you compare two altimeters; the first set to QNH < 1013.25 hPa and the second one is set to the standard setting of 1013.25, the first altimeter will indicate a lower value than the second altimeter.

    QNH setting is used at lower altitude where obstacle and terrain clearance are important. But for long distance traffic it is a nuisance to change the altimeter setting as the aircraft flies through different pressure areas on the ground. Therefor the Flight Level concept was introduced, allowing everybody on higher altitude to use the same setting. This also reduces the chance that aircraft have a different altimeter setting in the same airspace, which would cause vertical separation problems.


    The vertical distance of an object measured from mean sea level.
    Flight Level

    To understand a flight level, we should understand how altitude is measured in an altimeter, which is essentially a calibrated barometer – it measures air pressure, which decreases with increasing altitude. To display correct altitude, a pilot re-calibrates1 the altimeter from time to time, according to local air pressure.

    Flight levels solve this problem by defining altitudes based on a standard pressure of 1013.2 mb (29.92 inches Hg). All aircraft operating on flight levels calibrate to this same standard setting regardless of the actual sea level pressure. Flight levels are then assigned a number which is the apparent altitude (“pressure altitude”) to the nearest thousand feet, divided by one hundred. Therefore an apparent altitude of 18,000 feet is referred to as Flight Level 180. Note that aircraft may be at some other actual height than 18,000 feet, but since they all agree on a standard pressure, no collision risk arises.

    Flight levels are not used close to the ground, for perhaps obvious reasons – obstacles are fixed to the ground and so their absolute height needs to be known. The altitude of the lowest flight level varies from country.
    1: Re-calibration of altimeter is done to avoid airplanes flying at the same height, though their altimeters show different altitudes. This is safety issue.