Weather Theory

warm front is colder than the air of the cold front. When this and instability are present, the clouds continue to increase is the case, the cold front rides up and over the warm front. If in vertical height. Continuous, strong updrafts prohibit the air forced aloft by the warm front occlusion is unstable, moisture from falling. Within approximately 15 minutes, the weather is more severe than the weather found in a cold the thunderstorm reaches the mature stage, which is the most front occlusion. Embedded thunderstorms, rain, and fog are violent time period of the thunderstorm’s life cycle. At this likely to occur. point, drops of moisture, whether rain or ice, are too heavy for the cloud to support and begin falling in the form of rain Figure 12-27 depicts a cross-section of a typical cold or hail. This creates a downward motion of the air. Warm, front occlusion. The warm front slopes over the prevailing rising air; cool, precipitation-induced descending air; and cooler air and produces the warm front type weather. Prior violent turbulence all exist within and near the cloud. Below to the passage of the typical occluded front, cirriform and the cloud, the down-rushing air increases surface winds and stratiform clouds prevail, light to heavy precipitation falls, decreases the temperature. Once the vertical motion near the visibility is poor, dew point is steady, and barometric pressure top of the cloud slows down, the top of the cloud spreads drops. During the passage of the front, nimbostratus and out and takes on an anvil-like shape. At this point, the storm cumulonimbus clouds predominate, and towering cumulus enters the dissipating stage. This is when the downdrafts clouds may also form. Light to heavy precipitation falls, spread out and replace the updrafts needed to sustain the visibility is poor, winds are variable, and the barometric storm. [Figure 12-28] pressure levels off. After the passage of the front, nimbostratus and altostratus clouds are visible, precipitation decreases, and It is impossible to fly over thunderstorms in light aircraft. visibility improves. Severe thunderstorms can punch through the tropopause and reach staggering heights of 50,000 to 60,000 feet depending Thunderstorms on latitude. Flying under thunderstorms can subject aircraft A thunderstorm makes its way through three distinct stages to rain, hail, damaging lightning, and violent turbulence. before dissipating. It begins with the cumulus stage, in A good rule of thumb is to circumnavigate thunderstorms which lifting action of the air begins. If sufficient moisture identified as severe or giving an extreme radar echo by at CIRRUS WARM AIR CUMULONIMBUS CIRROSTRATUS ALTOSTRATUS C NIMBOSTRATUS O L D A COLD AIR IR Indianapolis Columbus Pittsburgh St. Louis 200 miles 400 miles 600 miles 10061005 1002 999 999100210051006 1011 1014 1017 1020 METAR KSTL 1950Z 31023G40KT 8SM SCT035 05/M03 A2976 METAR KIND 1950Z 29028G45KT 1/2SM TSRAGR 1023 VV005 18/16 A2970 2 8 6 42 0 3 7 2 6 6 6 16 22 2 05 8 8 5 5 2 2 1 1 3 4 4 2 4 4 7 7 0 2 2 0 0 0 METAR KCMH 1950Z 16017KT 2SM BR St. Louis Indianapolis Columbus Pittsburgh OVC080 11/10 A2970 METAR KPIT 1950Z 13012KT 75SM BKN130 08/04 A3012 1011 1014 1017 1020 1023 Figure 12-27. Occluded front cross-section with a weather chart depiction and associated METAR. 12-22