When warm and cold air masses collide, precipitation can occur. Cold fronts are depicted as blue lines with teeth pointing in the direction of motion on weather maps. Warm fronts are shown as red lines with bumps pointing in the direction of motion. High pressure systems usually bring nice weather while low pressure is often associated with precipitation. As a cold front moves across a region over multiple days, it can cause weather changes. Thunderstorms produce lightning and rain, and sometimes spawn tornadoes or funnel clouds. Hurricanes form over warm ocean waters and cause damage when making landfall.