Is there a minimum density a cloud must attain before it is, legally speaking, a cloud?

Question

I was doing a simulated tour of local untowered airfields the other day, operating VFR on VATSIM in Class B airspace (Logan).

Whenever I'm in Class B as a VFR flight I like to give ATC as boring a radar target as I possibly can. Obviously, as a VFR flight, I'm instructed to maintain "clear of clouds."

It was scattered clouds just above me at the altitude ATC put me at, but as I was getting near the coast they were getting lower - but nothing near me...until suddenly there was.

The cloud I encountered was about eight times the size of the Cessna 152 I was flying. I elected to plow through it, justifying the choice by the fact that I could easily see the terrain and other clouds on the far side of it - I could see right through it (it was more of a mist, really). But even at the time I recognized that it was entirely possible for me to miss a steel-grey-against-grey-clouds aircraft because of the visual noise. I wound up clipping through the top quarter of it; the transit lasted about three seconds. I never lost visual reference to the ground.

Is that legally VFR? Should I have diverted around it to maintain fully clear of anything that might possibly be construed as a cloud?

And relatedly, if I do have to divert... do I need to inform a busy Class B approach controller that I'm about to depart from the altitude he put me at in order to maintain clear of clouds?

Answer 1

To answer the question in the title,

A cloud is a visible aggregate of minute water droplets and/or ice particles

per FAA AC-006B, page 105. If you can see it, it's a cloud.

More practically (not a direct FAA quote; I didn't find chapter and verse),
VFR visibility minimums

...state a specific visibility distance requirement and a specific distance separation from clouds. Visibility through clouds varies by the angle of the view... while you may be able to see through a specific cross section of a cloud you're approaching, IFR traffic elsewhere in that cloud might not see you.

(Page 139 of the AC distinguishes clouds from rain:

Precipitation is any of the forms of water particles, whether liquid or solid, that fall from the atmosphere and reach the ground. The precipitation types are: drizzle, rain, snow, ...

Although the AC doesn't specifically define that a cloud does not "fall from the atmosphere," it draws a distinction between clouds and rain even more strongly by putting them in separate chapters.)

Answer 2

No - there is no minimum density needed to meet the definition of a cloud, only that it be visible and consist of liquid or frozen water.

Meteorologists may care about actual cloud density as it relates forecasting rainfall, but pilots only care about the effect of density on visibility.

The minimum visibility for flight under VFR (in most US airspace) is three statute miles. Three statute miles is actually very poor compared to a day when visibility is unlimited. It is possible to fly in conditions where an area of reduced visibility exists due to “a visible aggregate of minute water droplets and/or ice particles”, (The FAA definition of a cloud) but that visibility within this aggregation could exceed 3 statute miles.

The purpose of cloud clearance requirements is to provide safe separation between VFR traffic and IFR traffic operating in IMC. While the requirements only mention “clouds”, clearly the intent is to avoid cloud formations within which the visibility is below VMC, not that you “avoid anything that might possibly be construed as a cloud”.

The rules state to avoid clouds because 90+% of the time visibility is reduced because of… clouds. However, whether an area of IMC is due to clouds, smoke, dust, haze, volcanic ash, or any other atmospheric phenomenon, if visibility is less than 3 miles you should avoid it if you are VFR.

Conversely, if visibility is greater than 3 miles there is nothing restricting you from transiting under VFR, regardless of the composition.

In short, the adage “if you can see through it you can fly through it” is generally accurate enough.