The first reason is that it is a prefect place even during summer, because the temperatures are lower than in Tokyo - the average high in July is 28°C and in August 30°C. In the evening when I took this photo, the wind was blowing from the ocean so, after a hot summer day, it was a little chilly...
The second reason is obviously because of the view. The Yokohama skyline looks amazing from here: form left to right, we have the Yokohama Landmark Tower, the tallest building in Japan and the 3 buildings of the Queen's Square.
And the circle of light in front of them is the "Super Planet" Ride:
EXIF info:
Nikon D90
Lens: VR 18-55mm F/3.5-5.6G
Focal Length: 18mm
Aperture: F/8
Shutter Speed: 2s
ISO Sensitivity: ISO 200