Tuesday, July 1, 2008
Training: Safety Watch
Severe Weather: Always Respect It
Summer is upon those of us in the Northern Hemisphere and that means severe weather is imminent. It is critical that crewmembers are familiar with the inherent dangers it poses.
Often called nature’s heat engine, thunderstorms are born of cumulus clouds that grow into towering cumulus and ultimately reach adulthood as cumulonimbus. While they can arise any time during the year, they often occur in the late afternoon to early evening of hot summer days. Thunderstorms can form by themselves (as single-cell, super-cell and air-mass storms) or in clusters (frontal squall lines).
Turbulence is the greatest meteorological danger to aviation. It is caused by tremendous updrafts and downdrafts within a thunderstorm. The most severe occurs 8,000 to 25,000 ft agl within the updraft. Updraft speeds can exceed 65 fps. Downdrafts also produce turbulence. Usually it is less severe and occurs below 10,000 ft agl, but it has been known to slam landing aircraft into the ground.
Microbursts are small-scale, intense downdrafts that spread out in all directions once they hit the surface. They pose the greatest threat in front of or at the leading edge of a thunderstorm. They are small (less than 2.5 mi dia) and short-lived (usually less than 15 min), and most often without surface precipitation. Microbursts are not easily detectable using conventional weather radar or wind-shear alert systems. One’s downdraft can reach 100 fps, generating horizontal winds near the surface of 45 kt. Those winds can go from a direct headwind to a direct tail wind as you pass through the microburst’s center, resulting in a 90-kt shear change in airspeed. A microburst will intensify for about five min after it strikes the ground.
Ice is another significant hazard associated with thunderstorms. It can occur during all three stages of a thunderstorm-the cumulus (or developing) stage, the mature stage and the dissipating stage. Icing generally happens at the mid levels of a thunderstorm, where temperatures are -15 to 0 C. Super-cooled water (below freezing) will solidify on contact with an aircraft. Clear ice can quickly become extremely hazardous. I investigated many accidents where icing was a contributing cause.
Hail is one of a thunderstorm’s worst hazards; it can seriously damage an aircraft in seconds. Hail is usually found at 10,000 – 15,000 ft agl, most often in a storm’s mature stage. It can be found as far as five nm outside and ahead of an advancing thunderstorm.
Lightning can damage the skin and electronic components. It generally occurs within 5,000 ft of the freezing level, in light precipitation or light to negligible turbulence. Lighting "crawlers" can travel more than 35 mi along the clouds and have been observed out to 75 mi on radar. It can be a beautiful, but dangerous, light show.
Tornadoes are nature’s most violent storms. They happen year-round, but in the United States March – May is their peak season, followed by the month of November. Spawned from powerful thunderstorms, they kill and destroy in seconds, with winds reaching 300 mph. Most tornados travel from southwest to northeast and occur in the afternoon, coinciding with the maximum heating of the day.
At any given time, there are about 2,000 thunderstorms in progress. About 45,000 occur each day. Each year, the United States experiences about 100,000 thunderstorms; about 16 million thunderstorms occur annually around the world. The average thunderstorm lasts 30 min. Lightning from these storms strikes the Earth about 100 times each sec.
Another factor that affects aircrews and aircraft is heat. Excessive temperatures can lead to many heat injuries, such as heat cramps, sun stroke and heat exhaustion. Aircrews, especially hovering electronic newsgathering pilots without air-conditioned aircraft, must be aware of these hazards and plan their flights accordingly.
Finally, there is density altitude and its effects on aviation during the summer heat. Density altitude is the pressure altitude corrected for temperatures deviations from the standard atmosphere.
Variations in atmospheric pressure, temperature and humidity change the density of air, which in turn affect the lift or thrust that an airfoil moving through it can generate. Moving cold, dense air produces more lift or thrust.
An increase in temperature or humidity will reduce air density. In hot and humid conditions, density altitude may be significantly higher than the physical altitude. Too often, pilots associate density altitude only with high elevation airports. Hot temperatures also affect aircraft performance.
The spring and summer seasons pose many hazards. Vital steps in mitigating these hazards include taking preventative measures, employing risk management procedures and being fully aware of the operational weather. Always respect the weather.
I would like to thank the U.S. Army Combat Readiness Center and U.S. Air Force Weather Detachment at Fort Rucker, Ala. for information contained in this article.