A 20.5°C difference in mean temperature
The January mean is −3.2°C in Sapporo and 17.3°C in Naha for 1991–2020: a gap of 20.5°C. These are readings for weather stations in the two cities, not averages across Hokkaido and Okinawa prefectures.
Daily highs and lows differ too
Sapporo's mean daily high is −0.4°C and its mean daily low is −6.4°C in January. Naha's corresponding figures are 19.8°C and 14.9°C. Even Sapporo's average high sits below freezing, while Naha's average low remains above 10°C. A mean daily low is not the coldest temperature recorded in the period.
Using a normal to plan a trip
A normal describes the climate over many years; it cannot forecast the temperature on your travel date. Check a current forecast for clothing and travel decisions.
How to read the evidence. January 1991–2020 normals for the Sapporo and Naha stations, not prefecture-wide averages.