In the incoming April total eclipse in North America, if a city's sun is said to be 99.5% blocked, What darkness can it reach compared to the period of dawn or dusk or evening?
I ask this question because I live in a city where the sun is said to be 99.5% blocked during the eclipse. So if the darkness is similar to a predawn level (dark enough), there is no need to travel to a place (50-100 kilometers away) where the sun is 100% blocked, especially on a cloudy day.
This misses the point.
During totality the sun's corona becomes visible. If any part of the photosphere is exposed, then the corona isn't visible.
The experience of a total eclipse is not just "darkness" (you get about 12 hours of that every day!) It is the ring of the solar corona.
To answer your question, 99.5% of the sun is obscured a few seconds before sunset. But this isn't a fair comparison since at sunset the sun is obscured by the Earth which is inside the atmosphere, and during an eclipse the sun is obscured by the moon, which is beyond the atmosphere.
My granddad used to say "a miss is as good as a mile" If the sun is not covered by the moon, it is a partial eclipse and those are pretty common. You will only experience totality if the sun is covered by the moon. 99.5 % is not totality and is a completely different thing.
I agree with James K's observation that the main point of a total eclipse is not the darkness but the visibility of the corona.
However, to answer your original question, blocking 99.5% of the Sun* would reduce the daytime illuminance by a factor of 200, roughly from 20,000 lux (full daylight) to 100 lux. Even so, this would not feel very dark, because our eyes have a very wide dynamic range.
At dawn or dusk (in the absence of an eclipse), natural illuminance of 100 lux occurs during civil twilight shortly before sunrise or shortly after sunset. You can still see well. For additional context, this is comparable to a day with very heavy overcast clouds, or to the indoor lighting in an office building.
So the change in general brightness during a 99.5% eclipse is noticeable, but not dramatic. Even if the eclipse is covered by clouds, there would be a much bigger perceived change in brightness where the eclipse is total. And if the eclipse can be viewed clearly, then of course there is no comparison.
* A technicality: The most commonly quoted measure of a partial eclipse, the "magnitude", is the maximum fraction of the Sun's apparent diameter (not area) that is covered by the Moon. So for a precise answer, you would need to ensure which number you read. I will assume here that 99.5% is the fraction of area, known as the "obscuration". The issue does not affect the qualitative conclusions.
There's no equivalent. At 99.5% coverage, the overall light level is comparable to that of twilight a few minutes after sunset. However, the majority of the light is still coming from a point source, so you get phenomena such as sharp shadows that aren't present after sunset. The overall result is an impression of "wrongness" to the light, but it's nothing like the experience of totality.
During the 2017 eclipse, I recorded a video of the final 30 seconds before totality in an attempt to catch the Moon's shadow sweeping across the ground. It mostly didn't work, but I did catch a phenomenon nobody had mentioned before: the crowd cheering the moment totality started. Below is a frame from 25 seconds before the cheering, showing how a camera sees 99.5% coverage.
My experience with the last total eclipse was that it got noticeably dimmer and instantly cooler. It was not like night at all. I would say the level of light was probably closer to dawn, or like when a heavy cloud goes over the sun.
This webpage answers my question completely. Even a 99.9% block of the sun belongs to a partial eclipse, you can not experience any of the following:
So a 99.9% eclipse will not be enough at all. You need a 100% eclipse to enjoy those experiences.
In addition to @Mark's answer that everything looks "wrong" with a near point-like dim source (basically this is what daylight looks like at 15 AU, (between Saturn and Uranus)) there are other effects you can think about. I am not sure 99.5% coverage is enough, but when the Sun is covered except for a very, very thin sliver, atmospheric effects on Earth called "shadow snakes" or what Wikipedia calls shadow bands become visible.
Slightly related: Would an extremely bright star produce same kind of shadow band effect as seen in solar eclipses just before totality?
The eclipse as seen in Toronto, Ontario, Canada reached a maximum magnitude of 0.998, reported on a Toronto and Region Conservation Authority website as "a magnitude of 99.56%" (they presumably actually meant obscuration; this is from a vantage point that is probably about as close to the path of totality as it's physically possible to get in Toronto while still being on land).
This was enough, per CBC News coverage, to have noticeable effect on animal behaviour and cause a large drop in temperature - presumably substantively the same drop as observed within totality (since, although the human ocular response to light is highly non-linear, the same can't really be said about the Earth's temperature response to incident infrared radiation). A temperature drop is detectable with even a fairly small amount of obscuration, although it isn't very impressive. But from my own experience within totality, it was not noticeably colder at totality than several minutes before. (It did seem to warm up more quickly afterward, but that may have been psychological.)
I was also able to find some independent local footage (timestamped at or near the moment of maximum eclipse). It's dark enough to impress viewers and commentators, and quite noticeable on film, but still bright enough to see clearly in an area with tons of light pollution. It looks like moderately deep twilight rather than late night. I know there is some other footage I saw by the CBC taken from a rooftop of their building, which looked somewhat darker than this (away from street-level lights); but I can't seem to find it now.
Here's another timestamp from the same CBC News coverage, allowing us to see the contrast between the darkness of totality (as seen in Kingston) and the near-totality in Toronto.
