To forecast the weather you need multiple information, like movement of the air masses from all directions, their speed, are they cold or warm, do they bring clouds with them, if they bring clouds, what kind of clouds, by knowing what kind of clouds and their proportions you can predict if there will be storms, rain, snow etc... The air pressure at the place you want to forecast the weather, as well as the air pressure from the coming air masses, moisture in the air, the angle of the sun's rays, the longitude of day and night...