Cloud
Free PNG images in the category “Cloud”: 3 files with transparent background, no sign-up.
Clouds for weather and for data: plain outlined clouds, a cloud half hiding a spiky sun, a blue sun-shower icon with rain strokes, cloud symbols with upload and download arrows, and hearts drifting past a hot-air balloon.
Filters
Cloud iridescence, Colorful clouds, ink PNG
Sun rays, cloud, orange, cartoon PNG
Computer Icons Symbol Cloud computing PNG
Sky or server
In the nature section this category sits beside sky and trees, yet half of it has nothing to do with weather. The cloud has become the standard sign for online storage, so the icons with arrows belong to apps and help pages rather than forecasts.
Outline versus filled
Thick black outlines scale down to a toolbar and can be coloured to match an interface. The filled blue and yellow weather icons are ready for a forecast widget as they are. An empty outline doubles as a thought bubble or a caption box.
Sets that match
Weather icons look best in a family: same stroke width, same corner rounding. Before mixing a sun-and-cloud from one file with a rain cloud from another, compare the line weight at the size you will actually use.