• Hoar Frost !!!

    Hoar Frost 

    Cause: Water vapor freezing without first forming a liquid. 

    Frost, like fog, tends to occur on clear nights when the absence of cloud allows heat to rapidly radiate from the ground, resulting in a significant drop in temperature. For frost to form, the temperature must fall to below freezing (i.e. below 32° F or 0° C). 

    True frost, known as hoar frost, occurs when a thin layer of moist air near the ground cools to below freezing and immediately forms ice crystals, without first condensing as liquid (dew). These crystals will coat any cold surface including stone, grass, leaves, berries, and even spiders’ webs. Sometimes, hoar frost is so thick and white that it is mistaken for snow. 

    The ice crystals that result from hoar frost have exquisite, jewel-like patterns that branch outward from the edges of leaves and grass stems. These intricate structures are easy to see when hoar frost forms on window panes. This normally happens on the windows of an unheated house, when the exterior temperature falls to below freezing. Because moisture levels inside the house are higher than those outside, hoar frost crystals readily form on the inside of the cold window pane, coating the glass with delightful columns, plates, and feathers of frost. 

    If condensation takes place and dew forms before the air temperature falls below 32° F (or 0° C), the water or dew simply freezes, forming solid droplets rather than delicate ice crystals. These droplets are a form of ice rather than frost, and they occur in the same way as the ice on puddles, ponds, and lakes. 

    When temperatures fall below freezing, the water within the leaves and stems of plants will freeze. This can cause cell damage in the plants and produce a blackening of the leaves. Although this phenomenon is known in some parts of the world as black frost, it is not always accompanied by a frost. Air with a low dew point may cool to below 32° F (0° C) without reaching saturation point, which means that no water vapor is released by the air and no real frost formation can occur. Continue reading  Post ID 41


  • Geostrophic Force (Coriolis Effect) and Geostrophic Wind !!!

    Geostrophic Force (Coriolis Effect) and Geostrophic Wind 

    Under normal circumstances (i.e. if the Earth were not spinning) air would just move from high to low pressure, across the isobars (due to the Pressure Gradient Force, or PGF). The PGF acts at right angles to the isobars, from high to low pressure. Its size depends on the spacing of the isobars and air density. 

    However, this is only true around the Equator. In the Northern Hemisphere, air actually moves clockwise round a high pressure area and anticlockwise round a low, because the Earth is spinning, and deflects normal air movement (over the ground), until eventually the wind blows along the isobars (instead of across) at around 2,000 feet. Thus, an imaginary force appears to act at right angles to the rotating Earth, causing a moving body to follow a curved path opposite to the direction of the Earth’s rotation. 

    Not only that, the Earth moves faster at the equator than it does at the Poles (based on a cosine relationship), so, if you fire an artillery shell from the North Pole to the Equator, progressively more of the Earth’s surface would pass under its track, giving the illusion of the object curving to the right (or West of A) as it lags behind – the Earth is moving slower towards the North. If you threw whatever it was the other way, it would “move” to the East of B, because you are adding the Earth’s movement at both latitudes. That is, B will be moving slower relative to A. In other words, a bullet might fly in a straight line, but its target will move to the right. 

    This apparent movement (East or West) is like extra centrifugal force, which is called in some places the Coriolis Effect, but actually is Geostrophic Force when it refers to air movement, although no “force” is involved, hence the use of the word “effect”. That is, the wind at 2,000 feet is assigned a geostrophic property, which is only true when the isobars are straight and parallel. They are actually mostly curved, so the geostrophic wind becomes the gradient wind. The extra energy to keep the air curving comes from the cyclostrophic force, which is similar to centripetal force, as it operates inward, at 90° to the instantaneous motion, to the right in the Northern Hemisphere and the left in the Southern Hemisphere, until it balances the PGF and the wind follows the isobars. Around a low, it is the difference between PGF and GF – around a high, between GF and PGF. 

    The GF increases with the speed of the air, and it is dependent on the sine of the latitude, being maximum at the Poles (sin 90° = 1) and zero at the Equator. 

    So, the geostrophic wind is the imaginary wind that would result if the Coriolis and Pressure Gradient forces are balanced. When the air starts to move faster, the geostrophic force is increased and deflection starts again. Coriolis force is directly proportional to wind speed, in that it is zero when the wind is still and at its maximum when the wind is at maximum speed. It is also zero at the Equator and at its maximum at the Poles (meaning that the above relationships break down near the Equator, and isobars cannot be used to represent weather patterns. Streamlines are used instead). 

    As always, there is a mathematical solution: 

    GF = 2wrVsinq 

    where w = the Earth’s rotational velocity, r is density, V is the wind speed and q is the latitude. You can see that, as latitude increases, so will the geostrophic force, or that the wind speed will decrease. To get windspeed, at 2,000 feet, the wind is parallel to the isobars (when they are straight and parallel), meaning that the PGF must be balanced by another force, which we shall call GF. Now all you need to do is swap GF for PGF and play with the formula: 

    V = PGF 
    2wrsinq 

    It also shows that the windspeed increases with height as density reduces, but it all breaks down within about 15° of the Equator, or you would have an infinite windspeed. Given the same pressure gradient at 40°N, 50°N and 60°N, the geostrophic wind speed will be greatest at 40°N. 

    As you descend, friction with trees, rocks, etc. will slow the wind down by just over 50%, which lessens the geostrophic effect and gives you an effective change of wind direction to the left, so there are two forces acting on air moving from high to low pressure – Coriolis effect which deflects it to the right and frictional effect which brings it back to the left slightly. Over the sea, the geostrophic effect will be less, giving about 10° difference in direction, as opposed to the 30° you can expect over land (the speed reduces to about 70% over water, and 50% over land). If the winds are high, you could get into a stall on landing as you encounter windshear, described later. 

    The Coriolis effect depends directly on latitude and wind speed. It is greater for stronger winds, ranging from zero at the Equator to a maximum at the Poles. 

    In any case, wind in a low would be lower than the equivalent geostrophic wind, and higher round a high. In the case of a low in the Northern Hemisphere, the centrifugal force goes in the same direction as the Coriolis force. Since the forces must remain in balance, the Coriolis force weakens to compensate and reduce the overall wind speed (the PGF doesn’t change), so the wind will back, tend to go inwards and contribute towards the lifting effect, since it is forced up, to cause adiabatic cooling, and precipitation. 

    Inside a high, air movement (winds), will tend to increase with the help of centrifugal force, other things being equal, contributing towards the subsidence and adiabatic warming from compression. However, this is offset by the pressure gradient in a low being much steeper, creating stronger winds anyway. This is known as the isallobaric effect, since lines joining places with an equal rate of change of pressure are isallobars. Centrifugal force helps a low by preventing it being filled, and causes a high to decay by removing mass from it. 

    According to Professor Buys Ballot’s Law (a Dutch meteorologist), if you stand with your back to the wind in the Northern hemisphere, the low pressure will be on your left (on the right in the Southern hemisphere). The implication of this is that, if you fly towards lower pressure, you will drift to starboard as the wind is coming from the left (a common exam question). It’s the opposite way round in an anticyclone. Buys Ballot’s Law, by the way, had already been deduced by US meteorologists William Ferrel and James Coffin, but they didn’t get to be famous. Note that it does not always apply to winds that are deflected by local terrain, or local winds such as sea breezes or those that flow down mountains. 

    Source: 
    (http://www.pprune.org/archive/index.php/t-396698.html) Continue reading  Post ID 41


  • Thunderstorm !!!

    Thunderstorm 

    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 

    Mature stage: 

    – Updrafts and downdrafts. 

    – Rain begins to fall. 

    – Downdraft 3-4000 fpm may reach 6,000 fpm in severe storms. 

    – First gust 

    – Severe wind shear under thunderstorm. 

     Microburst 

    – Activity ends by two factors: 

    a) Mixing of rising air with dry cold environmental air – reducing instability. 

    b) Downdrafts suppressing the updrafts. 

    – Mature stage lasts about 30-40 minutes 

    Dissipating Stage: 

    – Cessation of continous rain. 

    – Beginning of sporadic showers 

    – Air subsides, vertical currents weaken 

    – Higher clouds have anvil appearance 

    – Virga 

    Gust Fronts 

    The cold air descending from a thunderstorm can also run ahead of the storm producing a mini cold front with windshear, turbulence and squally conditions. This is called a gust front. Gust fronts can extend as far as 15 to 20nm ahead of the thunderstorm and up to 6,000ft. When there is a line or there are group of thunderstorms gust fronts can extend to twice this distance. Windshear around a gust front has been measured with speed changes up to 80KT and direction changes of 90 deg. 

    The Super-Cell Thunderstorm 

    – Good supply of warm moist air at low level. Held down by a thin stable layer above so that the energy supply is not dissipated by turbulence or small-scale convection. 

    – A change of direction and strengthening of the winds aloft which will tilt the towering CB. 

    – The updraft is no longer on the same axis as the downdraft and the two exist side by side enabling the convection to continue. 

    – The increased size and intensity of the storm isolates the central core of the convection from the cold dry environmental air allowing it to reach high vertical speeds. 

    – This carries hail aloft and takes the top of the storm through the tropopause. 

    – Occur most frequently at the boundary between sub-tropical and polar air. 

    – The main area for super-cells is in the central area of the American Mid-West. 

    – Can last for several hours in their fully active state. 

    – Tend to move at an angle to the medium level wind and at a different speed, either 20 deg to the right and slower or 20 deg to the left and faster. The 20 deg right and slower option being more common in the Northern Hemisphere. 

    Thunderstorm Avoidance: 

    – Flying restriction under the anvil relates to hail rather than to CAT. 

    The recommended turbulence avoidance limits are: 

    – In visual flight – Avoid by 10 NM 

    – With weather radar – Avoid the echoes by: 

    Between 0 to FL250 by 10 NM 

    Between FL 250 to 300 by 15 NM 

    Over FL 300 by 20 NM 

    Note: Wx radar echoes show the core of the cloud not the edge so the avoidance limits are greater. 

    Exam Question Tips 

    Arrow At temperate latitudes, hail may be expected in connection with a CB from ground up to a maximum of FL 450. 

    Arrow The building stage of a thunderstorm last for approximately 20 mins. 

    Arrow The mature stage of a thunderstorm lasts for approximately 20 to 30 mins. 

    Arrow Airmass thunderstorms are the most difficult to forecast and detect. 

    Arrow If you cannot avoid penetrating a thunderstorm, the best area to penetrate is the “Sides”. 

    Arrow Icing is possible in temperatures lower than -23 deg C in a CB with thunderstorm in its mature stage. 

    Arrow The highest probability for severe thunderstorms is when there is advection of maritime cold air over a warm sea surface. 

    Arrow Thunderstorms are often preceded by Altocumulus Castellanus. 

    Arrow A cold front approaching a mountain range in the evening favors the formation of heavy thunderstorms? 

    Arrow Aircraft struck by lightning may sometimes get considerable damage and at least temporarily the manoeuvring of the aircraft will be made more difficult. Aircraft made by composite material may get severely damaged, the crew may be blinded and temporarily lose the hearing. 

    Arrow The aircraft is temporarily part of the lightning trajectory. 

    Arrow In North America tornadoes are most likely to occur in Spring and Summer. 

    Arrow The diameter of a typical tornado is 100-150 meters. Continue reading  Post ID 41


  • Microburst !!!

    Thunderstorm 

    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 41


  • 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 41


  • 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 41


  • Tropopause

    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 41


  • Atmospheric Humidity !!!

    Introduction

    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 41


  • 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

    or

    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 41