A word of warning: With some FAA weather questions, even a good understanding of the basics of weather won't always give you the right answer. Some of the questions are poorly written, and figuring out which answer the FAA is looking for requires looking to the source of the questions, which is usually the FAA's advisory circular Aviation Weather. (Note, however, that this is not the only source for FAA weather questions. Most knowledge exam guides list the source for each question.)
A look at a few of the questions on the current FAA private pilot and recreational pilot written examination illustrates how questions can be used to teach weather concepts that go beyond picking the correct answer from the three on the multiple choice exam.
Here's a question that, on the surface, seems to cover anything but practical weather knowledge:
What causes variations in altimeter settings between weather reporting points?
- A - Unequal heating of the Earth's surface.
- B - Variation of terrain elevation.
- C - Coriolis force.
A good way to tackle any multiple-choice exam question that you aren't absolutely sure about is to look at each answer and see if it makes sense. With only a little knowledge you can usually eliminate at least one of the answers and sometimes two of them. If you eliminate all three, you need to study more unless the test writer has managed to give three wrong answers.
In this case, it's hard to find anything dead wrong with answer A, but at first glance it doesn't seem to have much to do with altimeter settings. Let's put it aside for now and look at the other two answers.
A student who isn't clear about the concept of an altimeter setting is likely to go for answer B, especially since he or she is likely to know that the air's pressure decreases with altitude and an altimeter has something to do with air pressure. But, the student who knows that the altimeter setting does not depend on the airport elevation will quickly reject answer B. Altimeter setting is, in effect, what the sea level air pressure would be at the location for which it is reported, if the pressure could be measured at sea level. An airplane's altimeter should read altitude above mean sea level.
Answer C is probably there for students who aren't quite sure what the Coriolis force is, except that it has something to do with the wind and weather. Coriolis force causes winds to follow a curved path across the Earth's surface; it doesn't have anything to do with altimeter setting.
This leaves us with answer A as the correct one, but what does the unequal heating of the Earth's surface have to do with altimeter setting? The answer goes back to the fact that the source of almost all of the energy that drives the weather is the unequal heating of the Earth by the sun. Without large-scale flows of air and ocean currents from the tropics toward the poles and from the poles toward the tropics, year-round, unrelenting solar heating would boil the oceans on either side of the equator while the feeble solar heating of the polar regions would allow them to sink deeper and deeper into lifeless cold.
The imbalance between tropical heat and polar cold powers global-scale movements of air that create the swirls of wind that we call storms, including the low-pressure areas at the cores of storms and the areas of high pressure away from storms. These high- and low-pressure areas, which move across the Earth and grow stronger or weaker over time, create the air pressure changes that require pilots to change altimeter settings if they fly very far or very long.
FAA exams have several questions about how water in its gaseous (water vapor), liquid (ordinary water), and solid (ice) phases acts in the atmosphere. These questions address an important topic, because the changes of water among its three forms accounts for much of the weather that can make flying dangerous, such as fog, low clouds, poor visibility, thunderstorms, or icing.
Understanding what happens when water changes from one phase to another is important for avoiding some of the weather's biggest dangers. Here's one question that asks a very basic question about water's changes of state:
Clouds, fog, or dew will always form when
- A - water vapor condenses.
- B - water vapor is present.
- C - relative humidity is 100 percent.
Let's look at the answers in order. A looks like a pretty good answer because fog droplets, cloud drops, and dew form when water vapor condenses into liquid. But, a student who's been trying to master concepts such as dew point and relative humidity could easily feel that answer is wrong because it seems too obvious. Let's put it aside for now and look at the other choices.
Answer B couldn't be correct because water vapor is always in the air. Even dry desert air has some water vapor in it. Yet, we know that clouds, fog, or dew aren't always forming.
At first glance, answer C might look good to a student who's mastered the concepts of relative humidity and dew point, because when the dew point and temperature are equal, the relative humidity is 100 percent, which means the air is saturated and condensation begins. There's one fatal error in answer C, however: What if the temperature and dew point are well below freezing? In this case frost or ice crystals will form. The ice crystals could create clouds - cirrus clouds high above the Earth are mostly ice crystals - or even ice fog. But, the question includes "dew" among the things that "always" form, and frost, not dew, forms when the air is extremely cold.
This leaves the seemingly too obvious answer A as the correct one.
While water vapor can sublimate directly into ice when it's very cold, water vapor can also condense into liquid water at below-freezing temperatures. Tiny drops of water can also cool below freezing while remaining liquid. When such "supercooled" water hits anything, it in- stantly freezes into ice. This is the cause of aircraft icing, which is what the next question is really about.
The presence of ice pellets at the surface is evidence that there
- A - are thunderstorms in the area.
- B - has been cold frontal passage.
- C - is a temperature inversion with freezing rain at a higher altitude.
Ice pellets are tiny pieces of ice, about the size of a pencil lead, that fall only in winter storms. When weather observers report "ice pellets," most people would say "sleet" is falling.
Many people, however, don't know the difference between sleet and hail and might think the two terms refer to the same thing. They probably think "ice pellets" is just a third phrase for sleet or hail. In fact, hail refers to balls of ice - sometimes very misshapen balls of ice - that fall from strong thunderstorms. Small hail might be the size of tiny green peas but will be larger than ice pellets. Large hailstones are sometimes as big as softballs, but sleet is never larger than the tip of a pencil because ice pellets are frozen raindrops, and raindrops break up before they grow very large. Another key difference between the two kinds of ice from the sky is that sleet falls in cold-weather storms when the air at the ground is below freezing or very close to freezing, while warm, humid weather is needed to energize thunderstorms that are strong enough to produce hail.
Answer A is waiting for a student pilot who doesn't know the difference between these types of precipitation, but who correctly associates hail with thunderstorms.
Knowing that ice pellets are connected with cold air might lead you to think answer B is correct. But, there is a problem. Cold fronts move in during the summer as well as the winter, and they don't always bring below-freezing air.
To reject answers A and B and select the correct answer C requires knowing how ice pellets form. An ice pellet usually begins life as a snow crystal that falls from a relatively high cloud in a winter storm. On the way down, it falls into a layer of above-freezing air - such layers are a common feature of winter storms. If the layer of warm air is thick enough, the snow crystal melts, becoming a raindrop. Often a layer of below-freezing air is sitting on the ground, below the warmer air that transformed the snowflake into a raindrop. When the raindrop falls into this below-freezing air, it becomes supercooled - that is, it becomes a drop of freezing rain. If the layer of cold air is deep enough, the supercooled drop may freeze into an ice pellet.
A layer of warmer air above cold air is called an inversion. Answer C is a good, brief description of what's going on aloft when ice pellets are hitting the ground. The important point for a pilot is this: If ice pellets are hitting the ground, don't take off unless you're flying a jet or another aircraft with a very good deicing system. Otherwise, the freezing rain aloft could coat your airplane with a fatal load of ice.
The concept of air stability is a topic that's important to pilots, and FAA exams reflect this with several questions. In brief, when the atmosphere is unstable, air that is given an upward push will continue rising. Such rising air creates cumulus clouds, turbulence, and - if all of the conditions are right - thunderstorms. Stable air, on the other hand, tends to keep air from rising, which means it is associated with smooth air and flat clouds. Precipitation in stable air tends to be slow and steady, unlike the on-and-off downpours of thunderstorms.
Cold air, the colder the better, atop warm air will make the air unstable. Think of the warm air as wanting to rise and the cold air wanting to sink, causing an up-and-down motion. An inversion of warmer air above cold air is stable. The air is also stable when it cools relatively slowly as you go up. You need to know these basic facts to answer some FAA questions, such as:
What would decrease the stability of an air mass?
- A - Warming from below.
- B - Cooling from below.
- C - Decrease in water vapor.
First, this is a question you need to read carefully. If you rush through it, you could well come up with the wrong answer. It asks what would "decrease the stability," which means, of course, what would make the air more unstable?
Looking at the answers, A seems to be the winner because if you warm the air from below, the air near the ground will grow warmer while the air aloft does not warm up. In other words, warming the air from below increases the warm-cold contrast, with the warmer air wanting to rise even more.
As always, before deciding on an answer, it's a good idea to eliminate the other two choices. We can throw out B because cooling the air from below will make the air more stable, not less. Remember, cold air "wants" to stay near the ground.
Throwing out C is a little more complicated, but there are two reasons why decreasing the water vapor in the air will make the air more stable, not less. High humidity, which means there is a lot of water vapor in the air, makes the air unstable in two ways. First, humid air is lighter than dry air, and lighter air tends to rise. If you could reduce the humidity in the air, it would make the air heavier and less inclined to rise. Second, when humid air rises and cools to the point at which the humidity begins condensing to form cloud droplets, condensation releases heat. This makes the air rise even more - in other words, it makes the air more unstable.
Another important topic is wind shear, which refers to a large change in wind speed or direction over a relatively short distance. Such shear can cause turbulence and other hazards.
During the 1980s, a particular kind of wind shear received a lot of attention because it was blamed for some major airline crashes and because weather researchers and the aviation community launched a major effort to understand and cope with the problem. I am referring to the wind shear caused by microbursts, which are winds that blast down from showers or thunderstorms. If a microburst happens to hit an aircraft that's taking off or landing, the sudden change in wind direction can cause a severe loss of lift and a crash. Many news stories surrounding these accidents used the term wind shear to refer to microbursts, giving the impression that microbursts and wind shear are the same thing.
Pilots need to know that they should be concerned with many kinds of wind shear, not just microbursts. Knowing that wind shear occurs in a variety of circumstances supplies the correct answer to at least one test question:
Where does wind shear occur?
- A - Only at higher altitudes
- B - Only at lower altitudes
- C - At all altitudes, in all directions.
The correct answer is C because wind shear can occur at any altitude, not just close to the ground as some people believe and because winds can differ in speed or direction either horizontally or vertically.
The FAA's test questions about weather are not a good guide to what pilots need to know. They don't cover some important areas and, in other areas, they focus on minor details. Still, anyone who earns a pilot's certificate is going to have to cope with exam questions about the weather. Making the extra effort to learn the concepts behind the questions can pay off in the air.
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