Charter23 posted an article about the raid rarity and a 15 minutes spawn break a few weeks ago, which was based on 3k raids. He tracked all of these raids. So he was able to gather more details and he wants to share it with us.
Raid Rarity Tiers
He can confirm the suspected rarity tiers for every raid level. His conclusion is the following: 25% of all raids are Lvl 1, 25% Lvl 2, 20% Lvl 3, 10% Lvl 4 and 20% Lvl 5. It seemed like all of the Pokemon from exactly the same raid Lvl, have almost the same chance of spawning. With the new details, Charter23 said that in Lvl 3 and 4, there is one Pokemon each to break the rules and spawn more often than other Pokemon from their tiers. This is based on following the spawning of Arcanine, which is level 3, and Tyranitar which is Lvl 4.
15 Min Spawn Breaks
We might have some exceptions from this rarity. In his older article, he wrote that there aren’t any raids spawning during the last 15 minutes of every hour.
When asked about the cooldowns, and if there is something like that, he said that definitely this period of cooldown exists. He also said that there most likely is a 30-minute cooldown between selected two raids.
Raids Per Gym Per Day
The undoubted maximum number of the raids that happen at the very same gym is 5. Charter23’s analysis shows that the regular gym is holding 2 raids per day. Less than 10% of all gyms in total, do not hold a single raid on any given day.
He has limited details about when the raid day starts. A raid day, will last for 14 hours and 45 minutes, counting from the start of the first egg countdown, all the way to the end of the last raid.
Charter23 thinks that the start of the raid day, does not match local time zones. His research shows that the raid time zones are split at a specific longitude. The 15th meridian east longitude is splitting up Europe, and the raids in the east will start one hour earlier than the start of the raids in the west. For example, Trainers from Poland, or eastern Austria, can raid until 8:45 PM, even though they are in the same time zone as Germany, where the Trainers are able to raid until 9:45 PM. For the ones wondering, the daylight saving time does not affect the raids.
After the raid day starts, you can find many more raids than usual. This is because there are no gyms that are blocked at that time. This is because, during the busy hours, the gyms are either blocked by raids, or cooldowns that started during the first hours of the raid day.
To conclude everything, Charter23 said that there are definitely periods with moderately more and moderately fewer raid spawns during the day. The observations of “always no raids nearby when I’m about to go outside” and “always lots of raids nearby when I have to work” might be anecdotal.