Weather is the name given to the processes, occuring within an atmosphere, that dictate whether the sky will be clear or cloudy, wet or dry, hot or cold, or calm or windy. The study of weather is meteorology.
A weather modification network could be used to control the weather on a planet, as was the case on Earth, Caldos II, Hekaras II and Risa in the late-24th century. Without this technology, some planets had harsh climates. For example, after Risa's weather modification network was shut down in 2373, several lightning storms quickly formed with thunder and rain. (TNG: "True Q", "Sub Rosa", "Force of Nature"; DS9: "Let He Who Is Without Sin...")