I visited Jogashima in early March, 2007. It was pleasantly warm, and I was able to wear a t-shirt most of the day.

The following description was blatantly plagarized from the City of Yokohama's website, http://www.city.yokohama.jp/me/yoke/echo/98.1/echof98.1.html

At only about an hour from Yokohama, Jogashima Island offers a pleasant getaway from the crowds and pollution of the city. It is both convenient and easily accessible for those only wanting to take a short day-trip. ... (more)