Weather Theory

barometric pressure. Approaching Indianapolis, the weather Fast-Moving Cold Front deteriorates to broken clouds at 2,000 feet with three miles Fast-moving cold fronts are pushed by intense pressure visibility and rain. With the temperature and dew point the systems far behind the actual front. The friction between same, fog is likely to develop. At St. Louis, the sky is overcast the ground and the cold front retards the movement of the with low clouds and drizzle and the visibility is one mile. front and creates a steeper frontal surface. This results in a Beyond Indianapolis, the ceiling and visibility are too low very narrow band of weather, concentrated along the leading to continue VFR. Therefore, it would be wise to remain in edge of the front. If the warm air being overtaken by the Indianapolis until the warm front passes, which may take cold front is relatively stable, overcast skies and rain may up to two days. occur for some distance behind the front. If the warm air is unstable, scattered thunderstorms and rain showers may Cold Front form. A continuous line of thunderstorms, or squall line, A cold front occurs when a mass of cold, dense, and stable may form along or ahead of the front. Squall lines present air advances and replaces a body of warmer air. a serious hazard to pilots as squall-type thunderstorms are intense and move quickly. Behind a fast-moving cold front, Cold fronts move more rapidly than warm fronts, progressing the skies usually clear rapidly, and the front leaves behind at a rate of 25 to 30 mph. However, extreme cold fronts gusty, turbulent winds and colder temperatures. have been recorded moving at speeds of up to 60 mph. A typical cold front moves in a manner opposite that of a Flight Toward an Approaching Cold Front warm front. It is so dense, it stays close to the ground and Like warm fronts, not all cold fronts are the same. Examining acts like a snowplow, sliding under the warmer air and a flight toward an approaching cold front, pilots can get a forcing the less dense air aloft. The rapidly ascending air better understanding of the type of conditions that can be causes the temperature to decrease suddenly, forcing the encountered in flight. Figure 12-26 depicts a flight from creation of clouds. The type of clouds that form depends Pittsburgh, Pennsylvania, toward St. Louis, Missouri. on the stability of the warmer air mass. A cold front in the Northern Hemisphere is normally oriented in a northeast to At the time of departure from Pittsburgh, the weather is VFR southwest manner and can be several hundred miles long, with three miles visibility in smoke and a scattered layer of encompassing a large area of land. clouds at 3,500 feet. As the flight progresses westward to Columbus and closer to the oncoming cold front, the clouds Prior to the passage of a typical cold front, cirriform or show signs of vertical development with a broken layer at towering cumulus clouds are present, and cumulonimbus 2,500 feet. The visibility is six miles in haze with a falling clouds may develop. Rain showers may also develop due barometric pressure. Approaching Indianapolis, the weather to the rapid development of clouds. A high dew point and has deteriorated to overcast clouds at 1,000 feet and three falling barometric pressure are indicative of imminent cold miles visibility with thunderstorms and heavy rain showers. front passage. At St. Louis, the weather gets better with scattered clouds at 1,000 feet and a ten mile visibility. As the cold front passes, towering cumulus or cumulonimbus clouds continue to dominate the sky. Depending on the A pilot using sound judgment based on the knowledge of intensity of the cold front, heavy rain showers form and may frontal conditions will likely remain in Indianapolis until the be accompanied by lightning, thunder, and/or hail. More front has passed. Trying to fly below a line of thunderstorms severe cold fronts can also produce tornadoes. During cold or a squall line is hazardous, and flight over the top of or front passage, the visibility is poor with winds variable and around the storm is not an option. Thunderstorms can extend gusty, and the temperature and dew point drop rapidly. A up to well over the capability of small airplanes and can quickly falling barometric pressure bottoms out during frontal extend in a line for 300 to 500 miles. passage, then begins a gradual increase. Comparison of Cold and Warm Fronts After frontal passage, the towering cumulus and Warm fronts and cold fronts are very different in nature as are cumulonimbus clouds begin to dissipate to cumulus clouds the hazards associated with each front. They vary in speed, with a corresponding decrease in the precipitation. Good composition, weather phenomenon, and prediction. Cold fronts, visibility eventually prevails with the winds from the west- which move at 20 to 35 mph, travel faster than warm fronts, northwest. Temperatures remain cooler and the barometric which move at only 10 to 25 mph. Cold fronts also possess a pressure continues to rise. 12-20