I’m in the 40-60 mile range from Philadelphia International Airport. I notice the planes fly under the ceiling on cloudy days…whatever altitude that is. Yet, on clear days, it is not conspicuous. As such, I can only conclude the (subtle) descending altitude on clear days must be > (subtle) descending altitude on cloudy days. Is this correct or my imagination? And if so, why? I WAG I reside at sweet spot where the planes have to drop below the clouds, or such???
When you say ceiling are you taking about the clouds (as opposed to the max altitude the plane can fly at)?
If that’s what you mean, I could see where they may want to come in lower on cloudy days so they can get a visual on the runway, and the control tower can see them, sooner rather than later.
And, yes, if the start their descent sooner, it won’t be as steep, assuming the flight path is the same.
What type of aeroplanes (passenger jet/light piston)?
The goal with passenger jets is to fly a constant descent all the way to the runway with the engines at idle until fairly late in the approach (~5 NM from the runway). Sometimes this can be done, sometimes it can’t, it depends a lot on ATC. At 40 - 60 statute miles from the airport, overflying jets should be at about 10,000 - 15,000’.
I’m guessing that either the jets are more visible against a grey sky on a cloudy day, or you are seeing planes going to somewhere other than Philadelphia.
The spot where planes have to get below the cloud is somewhere between the end of the runway and a couple of miles out, so that’s not it.
If you want some numbers to compare, check out flightaware.com. That website will show you what is flying over you, how high it is, where it is going, etc.
You can also look at the arrival routes (STARS) for PHL and see where that puts the jets coming from different directions and landing on the various runways.
Usually, flying below the cloud ceiling indicates the aircraft are flying VFR (visual flight rules) as opposed to IFR (instrument flight rules). Flying VFR means the pilot needs to see where they are going at all times. Flying IFR means the pilot is being assisted by air traffic controllers and can be below, within or above the ceiling.