How can I check before my flight that the cloud separation requirements in VFR flight rules are met? As it is known, in order to fly VFR, the rules of 1000 feet vertical and 1500 meters horizontal cloud separation must be met. Although I can learn the visibility from the METAR weather report or other weather reports, there is no information about cloud separations. The only information given about the clouds is the phrases such as FEW, SCT, BKN, and OVC indicating the amount and altitude of the cloud. Is it possible to ensure that the VFR requirements are met by using the cloud information provided in METAR (FEW, SCT, BKN, and OVC)? If possible, for example: I learn from METAR that there is BKN or SCT clouding at the altitude I will fly How will I interpret VFR conditions? How do we understand this?
How can I check before my flight that the cloud separation requirements in VFR flight rules are met?
3 Answers
Make it simple:
OVC: It's a ceiling. The only thing you're concerned about is the altitude of the base.
BKN: It's also considered a ceiling. The only thing you're concerned about is the altitude of the base.
SCT: SCT is a level of cloud cover that assumes that the gaps are large enough to maintain lateral separation while flying around them, and thus it's not a ceiling, but you are either going to be cruising below the cloud base, or above the could tops (to get to smooth air), and your only concern is maintaining lateral separation on the way up or down. That's something you deal with when the time comes. You aren't going to be cruising at the cloud level, because you will be constantly weaving around them. If the SCT becomes BKN later in a TAF, or somewhere on your route, or it's just late afternoon where the temperatures are going to be dropping and SCT may turn into BKN without warning, you're wise to stay below them.
FEW: Not a concern normally, except where conditions are such that FEW may progress to SCT or BKN. Same as with SCT, you take into account what FEW may progress into later in the day, or at the other end of your trip. But in the short run, FEW can be treated same as SKC, because it's just going to be random clouds here and there.
So really, all you really want to know is the base of the lowest cloud layer for SCT, BKN and OVC. For FEW, not an issue unless conditions are forecast to deteriorate.
For SCT, the question is, if I want to cruise above the cloud base, which will mean above the tops to avoid having to weave around them maintaining lateral separation, what is the risk of SCT closing in on me and turning into BKN, or worse, OVC?
To assess that, you look at the TAFs along your route and consider the overall air mass conditions like humidity, and also the time of day. If you're flying in the morning, cloud conditions will generally improve as the temperature warms up, unless there are convective conditions that cause cumulus clouds to grow. In the afternoon, it's the opposite, anticipate that SCT can turn to BKN.
If in any doubt about what SCT conditions could become on your route, you just stay below the cloud base and live with the bumps. If you do decide you can cruise above the tops of the lowest cloud layer, you still pay careful attention to signs that SCT is turning into BKN while flying, and if it's happening, you get the heck down through it right away.
In stable-air weather conditions where humidity is high and temperatures are dropping, cloud can form under you pretty quickly as the dew point spread closes to zero, and moves lower, so keep that in mind.
As for the maintaining actual lateral separation, you just eyeball it when the time comes. There is nobody with a tape measure to work it out exactly and as I said, SCT conditions represent a level of sky cover that assumes lateral separation can be maintained by maneuvering.
I suspect that the short answer is no, met reports indicate current conditions and possible future conditions but can’t reliably predict separation. You can make your own prediction based on local knowledge but it’s unlikely to be 100% reliable. As cloud base etc will change throughout the day it’s up to you to observe and learn what you can as you go.
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$\begingroup$ Unfortunately, "visibility" is the only data we can evaluate before performing the flight related to VFR conditions. So how can we rationally evaluate cloud separation before flight? If this is not possible, isn't there a situation where a pilot who wants to fly VFR will evaluate cloud separation? $\endgroup$– pilot162Mar 4 at 11:51
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2$\begingroup$ +1 You make the best evaluation with the information you have on the ground and adjust your plans once you're up there, if necessary. I'm VFR only and many times I've had to re-route or cancel the flight. $\endgroup$ Mar 4 at 13:01
1.5km isn't that big in the scheme of things. To put it into context, that's 0.8 nautical miles, and the typical GA aircraft will cruise at about 1nm every 30 seconds.
Weather forecasts/reports can't be that precise. Typically clouds form in uneven clusters, and it's impossible to identify precisely when clusters will appear and how far apart they will be from another. But, even BKN can be enough to provide that 1.5km space.
In any case, the point of visual flight rules is that the primary information source on things like these are the pilots eyes. Weather reports help decision-making, but don't go blindly flying just because a weather report said it was clear.
