A ranking that will not show its criteria is an opinion. These are ours.
Warm days mean a mean daily maximum of 25–33 °C, scoring nothing below 20 °C or above 36 °C. The upper bound is not a formality: Dubai in July is 41 °C, and a list that treats hotter as better puts it first.
Swimmable means a sea of at least 22 °C. There is no meteorological standard for this — it is the convention northern-European travel publishers use, and we are telling you so rather than implying otherwise.
The criteria multiply rather than add, so a perfect air temperature cannot compensate for a cold sea. That is the whole reason the warm list and the swimming list are two separate lists.
Snow reliability is classified against the 100-day rule (Abegg 1996, after Föhn 1990): snow-reliable means at least 30 cm of cover on at least 100 days between 1 December and 15 April, in seven winters out of ten, with the line at about 1,200 m in the Swiss Alps. We classify against that rule and cite it; we do not compute it, because no free dataset carries per-resort snow depth.
Aurora is ranked on corrected geomagnetic latitude, not geographic latitude, and on median winter cloud. The oval sits at roughly 65–70° geomagnetic and moves about 2° equatorward per unit of Kp.
Tropical-cyclone seasons are applied separately from rainfall. A peak month is excluded outright; a shoulder month is capped no matter how dry it is.
Temperature and rainfall are 1991–2020 monthly normals computed from NOAA's Global Historical Climatology Network station records — the station used for each destination is named on its card, along with how far away it is. Sea-surface temperature comes from NOAA OISST v2.1 over the same period. Rain days count days with at least 2.5 mm.
These are thirty-year averages, not a forecast. They tell you what a month is usually like, never what next Tuesday will be.