Original: However, the summers are short; no more than three months of the year (but at least one month) and must have a 24-hour average temperature of at least  to fall into this category of climate.
Simplified: However, the summers are short; no more than three months of the year (but at least one month) and must have a 24-hour average temperature of at least 10°C (50°F) to fall into this category of climate.