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  • 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 18


  • 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 18