and instability are present, the clouds continue to increase  in vertical height. Continuous, strong updrafts prohibit  moisture from falling. Within approximately 15 minutes,  the thunderstorm reaches the mature stage, which is the most  violent time period of the thunderstorm’s life cycle. At this  point, drops of moisture, whether rain or ice, are too heavy  for the cloud to support and begin falling in the form of rain  or hail. This creates a downward motion of the air. Warm,  rising air; cool, precipitation-induced descending air; and  violent turbulence all exist within and near the cloud. Below  the cloud, the down-rushing air increases surface winds and  decreases the temperature. Once the vertical motion near the  top of the cloud slows down, the top of the cloud spreads  out and takes on an anvil-like shape. At this point, the storm  enters the dissipating stage. This is when the downdrafts  spread out and replace the updrafts needed to sustain the  storm.  [Figure 12-28]  warm front is colder than the air of the cold front. When this  is the case, the cold front rides up and over the warm front. If  the air forced aloft by the warm front occlusion is unstable,  the weather is more severe than the weather found in a cold  front occlusion. Embedded thunderstorms, rain, and fog are  likely to occur.  Figure 12-27  depicts a cross-section of a typical cold  front occlusion. The warm front slopes over the prevailing  cooler air and produces the warm front type weather. Prior  to the passage of the typical occluded front, cirriform and  stratiform clouds prevail, light to heavy precipitation falls,  visibility is poor, dew point is steady, and barometric pressure  drops. During the passage of the front, nimbostratus and  cumulonimbus clouds predominate, and towering cumulus  clouds may also form. Light to heavy precipitation falls,  visibility is poor, winds are variable, and the barometric  pressure levels off. After the passage of the front, nimbostratus  and altostratus clouds are visible, precipitation decreases, and  visibility improves.  A thunderstorm makes its way through three distinct stages  before dissipating. It begins with the cumulus stage, in  which lifting action of the air begins. If sufficient moisture  It is impossible to fly over thunderstorms in light aircraft.  Severe thunderstorms can punch through the tropopause and  reach staggering heights of 50,000 to 60,000 feet depending  on latitude. Flying under thunderstorms can subject aircraft  to rain, hail, damaging lightning, and violent turbulence.  A good rule of thumb is to circumnavigate thunderstorms  identified as severe or giving an extreme radar echo by at  Thunderstorms  Figure 12-27.  Occluded front cross-section with a weather chart depiction and associated METAR.  12-22  St. Louis  Indianapolis 200 miles  Columbus  400 miles  Pittsburgh 600 miles  METAR KSTL 1950Z 31023G40KT 8SM SCT035 05/M03 A2976 METAR KIND 1950Z 29028G45KT 1/2SM TSRAGR VV005  18/16  A2970  METAR KCMH 1950Z 16017KT 2SM BR OVC080  11/10  A2970  METAR KPIT 1950Z 13012KT 75SM BKN130  08/04  A3012  St. Louis  Indianapolis  Columbus  Pittsburgh  1023  1020 1017 1014 1011 1006 1005 1002 999 999 1002 1005 1006  1011  1014  1017  1020  1023  42  8  32  076  26  52  2  51  47  7  40  2  058  142 8  34  200  20 66  62  1 2  WARM AIR  CUMULONIMBUS  C OL D A I R  NIMBOSTRATUS  ALTOSTRATUS  CIRROSTRATUS  CIRRUS  COLD AIR 