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    Ah, give someone a calculator.......
    From: Gary LaPook
    Date: 2010 Aug 14, 15:52 +0200
    It's raining right now in Paris. I went online to the Weather Underground site to get a forecast at :

    http://www.wunderground.com/cgi-bin/findweather/getForecast?query=paris%20fr&wuSelect=WEATHER&MR=1

    According to Weather Underground:

    "Clouds:
    Scattered Clouds 853 m
    Few 1005 m
    (Above Ground Level)"

    How do you measure the base of clouds to the precision on just one meter? Isn't even the idea of a cloud something vaporous and indistinct?

    Of course the answer is that you can't measure the base of clouds to such a level of precision, it is merely the work of someone with a calculator who can punch in numbers but who has conception of what the numbers actually mean.

    The actual weather measured at Charles de Gaulle Airport is :

    METAR LFPG 141200Z 08004KT 020V120 9999 SCT028 FEW033TCU 18/10 Q1016 NOSIG

    This is the METAR format familiar to every pilot, and encodes the weather in a standard format.

    METAR = regular hourly meteorological observation.

    LFGP = code for Charles de Gaulle Airport, Paris.

    141200Z= observation taken on the 14th day of the month at 1200Z.

    08004KT 020V120 = wind is from 080° at 4 knots and the wind direction is variable from 020° to 120°.

    9999 = visibility is at least 10 km.( 9999 meters)

    SCT028 FEW033TCU = scattered clouds at 2800 feet and a few clouds at 3300 feet above ground level that are of the 'Towering Cumulus" type.

    18/10 = temperature and dew point in degrees Celsius.

    Q1016 = atmospheric pressure (altimeter setting) is 1016 mbar.

    NOSIG = no significant weather.

    The standard practice for measurring cloud heights worldwide is that they are measured to the nearest 100 feet as shown in the METAR, 2800 and 3300 foot clouds. Somebody then punched these numbers into his calculator and divided by the "feet to meters" conversion factor of 3.28084 which gets you to 853 meters and 1005 meters (mercifully they rounded to whole meters!)

    Since the actual measurement was to the nearest 100 feet (meaning plus or minus 50 feet ) the lowest clouds could have been anywhere between 2750 and 2850 feet, 838 to 869 meters. A pilot reading 2800 feet understands the precision is to the nearest 100 feet but what is the obvious precision of 853 meters, plus or minus 1 meter? If someone had a brain who set up this conversion then he would have set it to give the heights to the nearest 30 meters which is consistent with the precision of the original data, say 860 meters and 1000 meters.


    gl




       
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