Yellow filters, such as the Hoya K2, are particularly effective in black-and-white photography for increasing contrast between a blue sky, scattered cloud and foreground detail. Tonal rendition is natural.
They block blue light and therefore render a blue sky darker, making white clouds more pronounced. Yellows are lightened because yellow light is passed readily. They also tend to remove slight haze by blocking ultra-violet rays, invisible to the human eye, to which a film or sensor may be sensitive. Yellow filters are arguably most effective at intermediate distances. They generally improve the contrast of buildings against the sky, improve shadow contrast and enhance texture in snow scenes.
The filter factor is normally 2 x or 3 x.