Figure 12-26.  Cold front cross-section with surface weather chart depiction and associated METAR.  Stationary Front  Occluded Front  Wind Shifts  steeper frontal slope. Violent weather activity is associated with  cold fronts, and the weather usually occurs along the frontal  boundary, not in advance. However, squall lines can form  during the summer months as far as 200 miles in advance of  a strong cold front. Whereas warm fronts bring low ceilings,  poor visibility, and rain, cold fronts bring sudden storms, gusty  winds, turbulence, and sometimes hail or tornadoes.  Cold fronts are fast approaching with little or no warning,  and they bring about a complete weather change in just a  few hours. The weather clears rapidly after passage and drier  air with unlimited visibilities prevail. Warm fronts, on the  other hand, provide advance warning of their approach and  can take days to pass through a region.  Wind around a high-pressure system rotates clockwise, while  low-pressure winds rotate counter-clockwise. When two  high pressure systems are adjacent, the winds are almost in  direct opposition to each other at the point of contact. Fronts  are the boundaries between two areas of high pressure, and  therefore, wind shifts are continually occurring within a front.  Shifting wind direction is most pronounced in conjunction  with cold fronts.  When the forces of two air masses are relatively equal, the  boundary or front that separates them remains stationary and  influences the local weather for days. This front is called a  stationary front. The weather associated with a stationary  front is typically a mixture that can be found in both warm  and cold fronts.  An occluded front occurs when a fast-moving cold front  catches up with a slow-moving warm front. As the occluded  front approaches, warm front weather prevails but is  immediately followed by cold front weather. There are two  types of occluded fronts that can occur, and the temperatures  of the colliding frontal systems play a large part in defining  the type of front and the resulting weather. A cold front  occlusion occurs when a fast moving cold front is colder  than the air ahead of the slow moving warm front. When  this occurs, the cold air replaces the cool air and forces the  warm front aloft into the atmosphere. Typically, the cold  front occlusion creates a mixture of weather found in both  warm and cold fronts, providing the air is relatively stable.  A warm front occlusion occurs when the air ahead of the  12-21  St. Louis  Indianapolis 200 miles  Columbus  400 miles  Pittsburgh 600 miles  METAR KSTL 1950Z 30018KT 10SM SCT010 08/02 A2979 METAR KIND 1950Z 20024KT 3SM +TSRA OVC010  24/23  A2974  METAR KCMH 1950Z 20012KT 6SM HZ BKN025 25/24 A2983 METAR KPIT 1950Z 20012KT 3SM FU SCT035  24/22  A2989  1011  1008 1005 1005 1008  1011  1011  1011  1014  1014  St. Louis  Indianapolis  Columbus  Pittsburgh  46  42  10  066 10  33  74  071  3  71  77  6  73  75  3  70  35  4  8  12  102  122  10  25  WARM AIR  C O L D A I R  CUMULONIMBUS 