Ame Onna

Ame onna are yokai that appear when it rains. They look like crazy, haggared old women, soaking wet and licking rainwater off their arms and hands. Although they are sometimes connected to minor rain deities, they are quite different. In some places, they are believed to be able to summon the rain and thus can be seen as a beneficial force, particularly if rain is needed for crops. However, they have a much more sinister motive.

'Hyakumonogatari Ameonna' by Utagawa Yoshiiku
The ame onna is a yokai looking for young children to steal away, particularly baby girls. It is on these dark rainy nights that they wander the streets looking for their prey while most people are in their homes. In some accounts, the women how have had their children stolen actually become ame onna as a result of their overwhelming grief, and they too begin wandering from home to home on rainy nights searching for young children to steal.
'Ameonna' from the Konjaku Hyakki Shui by Toriyama Sekien
In modern times, ameonna (or ameotoko if male) is a term used for someone who rain seems to follow wherever they go. Below are some ukiyo-e prints of the kind of dark rainy nights that may see the ame onna wandering around.

'Night Rain in Rakuhoku (Kyoto)' - Kotozuka Eiichi, 1950.
'Night Rain on Shinohashi Bridge' - Kawase Hasui, 1929.

'Tochinoki Hot Springs' - Kawase Hasui.
'Rainy Night at Kiridoshi' - Takahashi Hiroaki, ca. 1936.