Weathersymbols on forecast maps are intended to describe the expected average weather over a certain period of time - and on a weather map for a certain area. Therefore there are not as exact as a written forecast summary, rather they will help to get a quick situational overlook.

For example, the pictogram 'partly cloudy' doesn't say whether the clouds are thickening or clearing, but it tells you that there will be sunshine at some point of the day.