I would consider Volcano (the town) to be cold (okay cool) year round. Now the park is lower in elevation and goes down to sea level, so places in the park are considerably warmer.

Keep in mind that I live in Honolulu so cold is relative term. The last time I was there was in August and it was definitely long pants weather for me. Today, for example, the high was expected to be 65 F and the low 47 F. August temps average about 10 F higher. Fifty-something at night is cold!
http://weather.msn.com/monthly_avera...cast:averagesm