Hello guys, it's again me , so I have me more question for you, this also refresh a little bit your METEOROLOOGY knowledgement.

What would be the minimum increase ithe opacity of the surface based layer at CYMO that just occur in order for it to constitute the ceiling?

This is the METAR PRovided

CYMO 120100Z 26010KT 3/4SM -RA BR BKN009 08/07 A2953 RMK FG5SC1 SLP994=

According to the definition of ceiling as per the AIM.
A ceiling is the lesser of the height above ground or water of the base of the lowest layer of cloud covering more than half the sky or the vertical visibility in a surface-based laye which completely obscures the whole sky

Possible answers
A) None, it already constitutes a celiling.
B) 2/8
C)3/8
D)1/8

My first answer was A as the Fog is covering more than half the sky according to the METAR, it's actually covering 5 oktas , but the book's answer is C)

Would somebody oease explain me why is this?

Thank you!

A should be correct. 5/8 to 7/8 is BKN and a ceiling even if its fog. Good luck studying

Answer C is the correct one. The gotcha here is that we are looking at making a ceiling out of the 'surface-based layer'. In order for that to occur (which would lead to a VV field in the METAR) we require more than half of the sky to be obscured.

gnieto wrote:According to the definition of ceiling as per the AIM.
A ceiling is the lesser of the height above ground or water of the base of the lowest layer of cloud covering more than half the sky or the vertical visibility in a surface-based laye which completely obscures the whole sky

In the question the fog currently covers 5/8 of the sky. For it to become a ceiling (since it's a surface-based layer) it must cover 8/8 of the sky so the increase in opacity required is 3/8, or answer C.

If it had been a layer of clouds it would already have been a ceiling since 'more than half of the sky' is the requirement for a 'cloud' ceiling. In the case of a 'surface-based' ceiling 8/8 is required.

Nice one! Whats up with the SC1 layer? Would you not see something like BKN 009 BKN 012, the second layer being the SC octa?

T-roper wrote:Nice one! Whats up with the SC1 layer? Would you not see something like BKN 009 BKN 012, the second layer being the SC octa?

Summation is from the ground looking up. So the FOG is covering 5/8th of the sky, and even though the layer at 900 feet is covering 1/8... the total coverage as seen from the ground is 6/8. This makes it a BKN ceiling.

Gnieto the answer is C due to the reason already posted.

Thank you all very much for your answers. VERY informative

Thanks for the help guys!!