 Fronts are the principal cause of significant weather . Convective precipitation ( showers , thundershowers , and related unstable weather ) is caused by air being lifted and condensing into clouds by the movement of the cold front or cold occlusion under a mass of warmer , moist air . If the temperature differences of the two air masses involved are large and the turbulence is extreme because of wind shear and the presence of a strong jet stream , " roll clouds " and tornadoes may occur . 
