Severe Weather Flying

Increase your knowledge and skill to avoid thunderstorms, icing and severe weather

Dennis Newton

• Publisher: Aviation Supplies & Academics, Inc.
• Year: 2018
• ISBN:
  9781619544154
• Topic: Technology & Engineering
• Subtopic: Aviation
• Index: Technology & Engineering

At the outset of Severe Weather Flying, author Dennis Newton reminds readers that this book is not about flying in severe weather, but rather how to detect and therefore avoid it, with advice on how to escape it if you become caught in it accidentally. Newton is a meteorologist, weather research pilot, engineering test pilot, ATP, and flight instructor. He speaks pilot-to-pilot in this valuable guide on how not to fly severe weather. He believes that given the knowledge, pilots can truly lessen their chances of being caught in thunderstorms and other extreme weather conditions.The emphasis is on types of weather that are potentially hazardous to flight; with each type of weather discussed, the author provides rational answers to a pilot's very sensible question, "And then what? How does this affect me?" He also discusses the capabilities and limitations of airplanes and equipment in avoiding and in dealing with severe weather.Meteorology can be a tough "language" and not always clear to the lay person. The author translates and brings across the most crucial principles pilots can use to fly more wisely in weather. Covering weather fundamentals, the atmosphere, and the stability of the air, he then digs deeper into the individual aspects of severe weather situations: air mass and nocturnal thunderstorms, downbursts, lightning, icing, turbulence and wind shear.In print for more than 30 years, this book in its Fourth Edition blends in good coverage of detection equipment for the cockpit, and the weather briefing information available to the pilot for decision-making in flight planning — even the enroute phase. Details on aircraft icing certification, critical aircraft icing information, and high altitude ice crystals are also included.

The Four Fundamentals

Four basic ingredients go into the recipe for severe weather, and essentially all other weather for that matter. They are as follows:

1. Water. All weather (except some of the winds which serve to move the water around) is made of water. Icing clouds are made of water. Thunderstorms are made of lots of water. Water, of course, is everywhere. About two-thirds of the planet is covered with it. Water to make weather out of enters the air from oceans, lakes and rivers. It is no coincidence that some of the most treacherous weather in the world occurs in the area of the Great Lakes. Enough water to make a lot of weather can even come right off the ground, particularly after a rain. Everyone has seen a day begin to dawn bright and clear after a night rain, only to sock in tight as the morning heating lifts water into the air. When you look at any sort of weather chart, ask yourself where the water is. What are the dew points? Are the winds coming from dry land, or from a source of moisture? Water is the enemy.
2. Temperature. Various types of weather require temperatures in various ranges. Icing, for example, requires temperatures somewhat, but not too much, below freezing. Fog requires temperatures near the dew point. Thunderstorms require relatively warm temperatures in the lower layers of air in which they form for the simple reason that warm air can hold more water. There is even some correlation between temperature and lightning strikes to aircraft.
3. Lifting. Very early on, you are likely to find in most basic weather texts for pilots some statements to the effect that low-pressure areas are associated with obnoxious weather. Why should this be true, you might ask yourself. It’s really quite simple. Air is drawn into the low near the surface. It comes in from all directions, so it has no horizontal way out. Air is very nearly incompressible at natural wind speeds, so it can’t just pile up in the low-pressure center. Where does it go? The only way left: Up. Voilá! If moisture is present in sufficient quantity, the result is weather. It is a small oversimplification indeed to say that all a front does is lift air. There are such things as sea breeze fronts and dew point fronts, in addition to the commonly known cold, warm and occluded fronts, which are in the business of lifting air. The jet stream, and other smaller-scale upper-air wind flows, create “holes” in the upper air that result in lifting of low-level air to fill them. Hills lift air. A thermal over a hot parking lot surface lifts air.
4. Stability. This is one of the most important factors in weather, and one of the least understood. If you want to understand thunderstorms, icing, or any other kind of weather for that matter, you have to understand stability. Fear not. Stability is very simple and even fun—it is only the explanations that are weird and mysterious. More on the subject is forthcoming.

As a large-scale example of the effects of the Four Fundamentals, wouldn’t we expect to find that more thunderstorms occur in areas with lots of warm water and something to lift it than in other places? Take a look at the southeastern United States (see Figure 1-1).

Figure 1-1. Mean annual number of thunderstorms per year. Data is based on 30 years of observations, 1955–1985. (Storm Data, NOAA)

The Gulf of Mexico is one of the best warm water sources in the world. We have hot land and sea breeze fronts to create lifting in Florida—an area surrounded by warm water, and apparently which has the most thunderstorm days (days in which one or more thunderstorms are observed by the National Weather Service reporting network) in the country. I say “apparently” because there are also lots of stations in Florida, and that may have something to do with it. Look at the mountainous area in Colorado and New Mexico. Warm, moist air from the Gulf flows in there, too. Not as much as Florida or the rest of the southeast, probably, but there is much lifting due to the terrain that will continue to try to squeeze the water out. The area to the west, into Utah, could easily be more active than is indicated—the low numbers shown could be due to the low density of reporting stations in the area. I’ve certainly seen enough storms (see Figure 1-2) while flying in that area. Two things apparent from Figure 1-1 are the deep penetration of Gulf air into the United States, and the effect of lifting by terrain.

Figure 1-2. Orographic thunderstorms building in Colorado.

So much for an introduction to the Four Fundamentals. They are basic to everything from here on, and they make the understanding of weather much easier. Two of them, lifting and stability, are covered in more detail in the next two chapters.

The Ups and Downs of Air

The atmosphere can be thought of as a heat engine, driven by the sun, with water as the working fluid that transports energy. Lifting of air that contains water is a prerequisite to the creation of most of the weather potentially hazardous to flight. It is therefore worth some time to think about the mechanisms in the atmosphere, both large- and small-scale, that do this lifting.

On a non-rotating earth, there would be no large-scale lifting mechanisms in the mid-latitudes, e.g., fronts. Heated air would rise in the vicinity of the equator. Cold air would descend near the poles. The result would be a single large-scale cell of circulation in each hemisphere. In the northern hemisphere, which is all I’ll consider here for simplicity, this would be southerly winds aloft transporting air toward the pole and northerly winds near the surface carrying it back toward the equator. In the real world, things are not that simple. There are actually three cells of circulation in each hemisphere instead of one.

Air near the equator, a little north or south depending on the season, rises as you would expect. There is also a belt of weather associated with this lifting, called the Intertropical Convergence Zone. However, this air begins a tropical cell of circulation that ends with descending air in the subtropical latitudes. This descending air produces the well-known consistently fine weather of the subtropical high pressure regions.

Cold air descends near the pole, as would be expected on the nonrotating earth. However, this rises again in the mid-latitudes, to complete a polar cell of circulation. The third cell fits between these two, as shown in Figure 2-1.

Figure 2-1. A simplified sketch of atmosphere circulation. Note the lifting at the Polar Front.

At the southern end of this mid-latitude cell, its air joins air from the northern end of the tropical cell in descending, contributing to the general subsidence of air in this region. However, at the northern end of the middle cell, the opposite effect happens. Its low-level air trying to move north runs into the southward-moving airflow of the polar cell and guess what? “Aha,” you say, looking at the area where these two cells meet, “lifted air!” Right you are.

This area of lift, where the polar air meets the mid-latitude air, is the Polar Front. If nothing else were going on, this front would more or less sit there, sort of like a bathtub ring of crummy weather around the earth: maybe moving a little south as a cold front and a little north as a warm front now and then. Naturally, though, something else is going on.

First there’s the Coriolis force. Without belaboring the physics, the Coriolis force is a phenomenon caused by the rotation of the earth which makes everything in motion want to turn to the right (in the northern hemisphere, that is—but I already promised not to confuse things by going south). It’s pretty small, but then molecules of air are pretty light. Since air in the middle cell of circulation is northbound at the surface, the right-turning effect results in a belt of mid-latitude westerly winds. Low-level easterly winds, of course, are the result in the northern and southern latitudes. (By the way, for the benefit of any skeptics to whom the Coriolis force sounds like Black Magic, it can be observed in much larger things than air given enough time. Rivers erode the right bank. The right rail wears out first in tracks traveled only in one direction.)

Second, there’s the jet stream. A complicated result of the atmospheric circulation pattern is the appearance of the westerly jet stream in the upper air above the Polar Front. What is the jet? By arbitrary definition, a narrow band of upper winds is called a “jet stream” when its speed exceeds 50 knots. However, it’s common knowledge that the wind speeds in the core of the jet are much higher than 50 knots, easily triple that at times. Consider, for a moment, what that means: At those speeds, airflow is basically incompressible. The high speed in the core of the jet then either means there has to be a much lower pressure there than at its slower-moving west end, or that a lot more air is moving in the middle than at the end, or both. Either way, the conclusion is that th...

Table of contents

• Copyright
• Preface
• Introduction
• Abbreviations
• 1: The Four Fundamentals
• 2: The Ups and Downs of Air
• 3: Stability
• 4: Air-Mass Thunderstorms
• 5: In This Corner, Mama Bear
• 6: Will the Real Papa Bear Please Stand Up?
• 7: The Downburst
• 8: Lightning
• 9: Thunderstorm Weather Briefing
• 10: Nocturnal Thunderstorms
• 11: Thunderstorm Detection Equipment
• 12: Hows and Whys of Icing
• 13: The FAA Aircraft Icing Plan
• 14: Icing Weather Briefing
• 15: In-Flight Icing Strategies
• 16: Ice Protection Equipment
• 17: Icing Certification
• 18: Supercooled Large Drop (SLD) Icing
• 19: High Altitude Ice Crystals
• 20: Nonconvective Turbulence and Wind Shear
• Appendix
• About the Author
• Index