Thursday, March 1, 2007
Severe Weather Flying
Dennis Newton’s book reviewed
All understanding comes from knowledge of the basics. Newton begins his book by detailing the elements that make up severe weather: water, temperature, lifting action and stability. These “Four Fundamentals” act together and in unison to create all weather phenomena—and the effects are often cumulative. In particular, from a pilot’s standpoint, “water is the enemy”. So knowing where the water is (in the atmosphere), a pilot can begin to predict where severe weather may form. “When you look at any sort of weather chart,” Newton teaches, “ask yourself where the water is. What are the dew points? Are the winds coming from dry land, or from a source of moisture?”
The final form that moisture takes in the air depends on three additional factors:
- Temperature. Temperature affects the air’s ability to sustain more or less moisture that will contribute to severe weather, and what form (rain or snow, water or ice) it will take.
- Lifting. Rising air generally cools, so if lifting action is available (from fronts, terrain, solar convection or local wind patterns) any moisture is more likely to condense into clouds to contribute the severe weather development.
- Stability. “One of the most important factors in weather, and one of the least understood,” stability is subject to an entire chapter in Newton’s work. Stability is simply “how [something] acts to being disturbed.” If cold air overrides warm (all temperatures being relative), warm air below will rise and accelerate upward. The sky is considered “unstable” because lifting action is enhanced.
|OBSERVATION||PROBABILITY OF WIND SHEAR|
|Localized strong winds (with nearby convective activity)||High|
|Heavy precipitation||High |
|Nearby convective activity with lightning||Medium |
|Temperature/dew point spread between |
30 and 50 degrees F
How do we take this elementary knowledge and use it to be safer pilots? How can we avoid canceling flights just because of the threat of storms or ice, and instead intelligently plan a flight to avoid severe weather hazards? Newton steps readers through discussions of various types of severe flying weather, from the standpoint of development, growth and movement of each.
Air-mass thunderstorms. Air-mass thunderstorms are those that pop up away from fronts, on warm days when the air mass is moist and unstable. The lives of individual air-mass storms are brief, sometimes lasting as little as half an hour from beginning to end, although if conditions are widespread and enough moisture is present the general area may be visited by storms for some time. Specific air-mass storms may be difficult to detect in aviation weather reports. “The distance between primary weather stations is about 100 miles, on the average” says Newton, “and the distance between upper-air [observation] stations is more like 200 miles. Surface observations are made hourly, but usually only every 12 hours at the upper-air stations. This essentially limits [air-mass thunderstorm] forecasting to a statement that storms are probable in an area much larger than the storms themselves.” In other words, air-mass storms can slip through the weather-reporting cracks. “If you don’t have radar or a lightning detector” on board your aircraft, “you must use your eyes, and what you know about moisture, stability, temperature, and lifting, to fill in the gaps.”
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