Hanko, where it's located

The climate of Hanko is Baltic, that is, moderately continental, with cold winters, during which the average temperature is a few degrees below freezing, and mild to pleasantly warm summers.
Hanko (Hangö in Swedish) is located in the south-western tip of Finland, in the area where the climate is (relatively) milder. The white nights occur from May 13th to July 31st.
The average temperature of the coldest month (February) is of -4.1 °C (25 °F), that of the warmest month (July) is of 17.3 °C (63 °F). The cold record is -34 °C (29 °F), the heat record is 31 °C (88 °F). Here are the average temperatures.
Hanko - Average temperatures
Min (°C)-5-7-406101413950-3
Max (°C)-1-2161317211914941
Min (°F)231925324350575548413227
Max (°F)302834435563706657483934

Precipitation amounts to 635 millimeters (25 inches) per year: so, it is at an intermediate level. It ranges from 30 mm (1.2 in) in the driest month (April) to 80 mm (3.1 in) in the wettest (August). Here is the average precipitation.
Hanko - Average precipitation

The sea temperature ranges between 1 °C (34 °F) and 17 °C (63 °F). So, it never becomes warm for swimming. Here are the average sea temperatures.
Hanko - Sea temperature
Temp (°C)32136111617151185
Temp (°F)373634374352616359524641


Best Time

The best time to visit Hanko is the summer, from June to August, since it is the mildest season. However, you must take into account some rainy days and a bit of cold at night.