Weather boost means better catches
In-game weather boosts certain types. A boosted Pokémon always has a guaranteed minimum of 4 in each stat, in attack, defense, and HP, instead of the usual minimum of 0. This makes a 0 percent (nundo) impossible to find, and raises the chance of finding a perfect 100 percent (hundo). Boosted Pokémon also have higher CP, both in the wild and in raids.
Check the weather before you go out. It pays off to hunt the right type in the right weather.
How much better are the odds?
Without weather boost, each stat can be anywhere from 0 to 15, so there are 16 possible values. The chance that one single stat lands on exactly 15 is 1 in 16. All three stats must land on 15 for a hundo, so you multiply them together: 1/16 × 1/16 × 1/16 is 1 in 4096.
With weather boost, the floor is 4 instead of 0. Each stat can only be between 4 and 15, so there are 12 possible values. The chance for one stat becomes 1 in 12, and all three at once becomes 1/12 × 1/12 × 1/12, which is 1 in 1728.
4096 divided by 1728 is about 2.4. So the chance of getting a hundo is about 2.4 times higher during weather boost.
Picture three raffles in a row, one for each stat. Normally each raffle has 16 tickets, and you need to draw the best ticket in all three raffles to win. With weather boost, the game removes the 4 worst tickets from each raffle before you draw. With fewer bad tickets left, the best ticket is easier to find.