Anonymous
Anonymous asked in Science & MathematicsWeather · 9 years ago

Is it legal to fly under these clouds?

Airplane pilots flying by Instrument Flight Rules rely on Air Traffic Control to tell them where to fly so that they don't run into other aircraft. If a pilot wants to fly with some freedom of navigation (say, to sightsee), they must fly by Visual Flight Rules (VFR), which requires them to fly at least 500 feet (about 150 meters) above the ground and, simultaneously, at least 500 feet below the base (bottom) of clouds, so that they can see other aircraft and still have time and space to avoid them.

You, like pilots, can estimate the height of the base of cumulus clouds by looking at the air temperature and dew point temperature at the ground as given by a current weather report. Then, the temperature spread (difference of air temperature and dew point temperature) has a typical lapse rate of 8.2°C/km. The lowest point in the cloud is where the temperature spread is zero (where the air temperature decreases to match the dew point temperature, and water vapor saturation occurs). For example, when the air temperature is 26.4°C and the dew point temperature is 10.0°C, the spread is 26.4-10.0=16.4°C. The spread decreases to zero at 16.4°C/(8.2°C/km)=2km. Thus, the bottom of the clouds are expected to be about 2km above the ground. The pilot can fly at any altitude from 150m to 1850m above the ground under VFR.

The current weather report at the Ann Arbor airport reveals an air temperature of 16.5°C and a dew point temperature of 15.0°C. There are cumulus clouds overhead. True or False: It is legal to fly under these clouds using Visual Flight Rules.

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  • 9 years ago
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    Hello Andrew,

    I am a private pilot and I own a plane that I fly as much as possible, each week-end, and I also teach meteorology to students at my club. What you describe is, indeed, what I do to estimate the ceiling from the spread that I can read from a METAR. Capercorn has a point by writing that you can listen to the ATIS to find the ceiling or simply ask by radio. Flight Information Region control will always help you, even when flying VFR without flight plan - if they have the time.

    But, back to your question: I use the average adiabatic lapse rate as stated by the Standard Atmosphere (SA) which is then 6.5 C per km, and not 8.2 C. I don't know where you got that value from and it is possible that it is more correct for the dry air under the cumulus but ... my rule of thumb is: 6.5 C per km, which is the same as 2 C per 1,000 feet. I use the latter because, even in Europe, altitude for aviators are always in feet.

    According to my rule of thumb and since your spread is 1.5 C, then the ceiling is under 1,000 ft AGL and therefore VFR flight is not legal. Since your lapse rate is higher than mine, yours won't be legal either.

    Incidentally, if you have a text book saying that we should use a lapse rate of 8.2 / km, I'd appreciate if you could send me an email with a link. I am on Yahoo!Answers to help when I can, but also to learn from others. Thanks in advance.

  • My gut pilot instincts are telling me no. Granted, my ASOS data does the thinking for me; I can get the ceilings just by dialing up the ATIS frequency.

    Source(s): Lol, Ann Arbor...
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