Rainbands ahead of warm occluded fronts and warm fronts have little upward motion. They tend to be wide and stratiform in nature. In an atmosphere with rich low level moisture and vertical wind shear, narrow, convective rainbands form. These are known as squall lines, and are usually in the cyclone's warm sector, ahead of strong cold fronts associated with extratropical cyclones. Wider rain bands can form behind cold fronts, which usually have more stratiform, and less convective, precipitation. In colder cyclones, small bands of heavy snow can occur with a width of to . These bands are linked with areas of frontogensis, or zones of strengthening temperature contrast. The curved air flow from extratropical cyclones bringing cold air across the relatively warm Great Lakes can lead to narrow lake effect snow bands. These can bring heavy localized snowfalls.