[Attempt to quantify phenomena spawn rate]

That are great insights which i sadly missed upon creating this thread. Here is the updated calculation:

 

 

Here is the LaTeX Source Code:

Spoiler

\begin{align*}
k &= 2 & \text{Concurrent phenonema spawns} \\
n &= 177 & \text{Known phenomena locations} \\
t_{up} &= 10\text{ min} & \text{Time of a single phenonema spawn being active} \\
t_{down} &= [5, 15]\text{ min} & \text{Cooldown time before a phenomena respawns after being up} \\
\tilde{t}_{down} &= 10\text{ min} & \text{Let's take the average downtime for simplicity} \\ \\
p(\text{phenomena}) &= \frac{k}{n} * \frac{60}{t_{up} + \tilde{t}_{down}} & \text{Probability for a phenomena to appear at a given location per hour} \\
&= \frac{2}{177} * \frac{60}{20} = 0.0339 \\
&= 3.39\%
\end{align*}

 

 

[Attempt to quantify phenomena spawn rate]

A phenomena is a special encounter which occurs randomly and cyclic in all areas of Unova and has 4 appearances: Rustling Grass, Rippling Water, Flying Shadow and Dust Clouds. There are exclusive species to phenomena, encounters have a >50% chance to have at least one 31 IV, and it may drop Mysterious Gems or Fossils.¹

 

EDIT: Please see the second answer for updated calculation.

 

Original post with presumably incorrect assumptions:

It is believed only a single phenomena at a time spawns for t = 15min for the whole server², and afterwards it "jumps" to another location³. There are n = 177 known phenomena locations⁴.

So we can conclude, that for a single location there is a chance of per hour for a phenomena to appear.

Q.E.D.

 

¹ https://forums.pokemmo.eu/index.php?/topic/76118-changelog-27022018/
² http://boards.4channel.org/vp/thread/38677228/pokemmo-general-v-06-pmmog-tripfag-edition#p38688626
³ http://boards.4channel.org/vp/thread/38677228/pokemmo-general-v-06-pmmog-tripfag-edition#p38688523
⁴ https://forums.pokemmo.eu/index.php?/topic/77087-phenomenon-locations-guide/