This Part 107 exam tutorial covers material on various weather phenomenon that frequently appear on the test. These include atmospheric stability, air masses, cold fronts vs. warm fronts and temperature inversion.
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Helpful Resources for the FAA Part 107 Exam
A lot of the material covered in this tutorial is similar to that in the study guide I used to successfully pass the FAA Part 107 exam with a 90% score.
- Extensive Material for Part 107
- Practice questions with explanations
- Free 5 online practice tests
- Free online updates
Check out my post on my experience of preparing for the FAA Part 107 exam. There, I delve into detail which topics to study most and what to expect on the day of the exam.
The practice questions here use the Airman Knowledge Testing Supplement. You can purchase a hard copy from Amazon.
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Cold Air vs. Warm Air
The main source of weather on Earth is the sun. So, any weather process is the result of uneven heating of the Earth. This happens due to sunrays heating the Earth’s surface at varying degrees at different places. Consequently, this creates air circulation called wind as we know it.
For the weather topic on your FAA Part 107 exam, it is important to understand the properties of cold air vs. warm air.
Cold air tends to be denser, heavier and has a higher pressure than warm air. This is so because air molecules under high temperature have higher energy. As a result, warm air tends to spread out for a given unenclosed cubic meter of volume.
Since there are fewer molecules in warm air and more molecules in cold air for a given unenclosed cubic meter of volume, warm air typically rises. Conversely, cold air tends to sink. Therefore, warm air would create low pressure events, while colder air would create high pressure events.
Local Convective Currents
Different surface areas emit heat to a different degree, which creates convective current. A convective current is a small area of local circulation, which may cause turbulence at times.
For example, sand, bare land and urban places tend to emit a lot of heat, producing updraft air circulation. Conversely, lakes, rivers, trees and other vegetation typically absorb and hold on to the heat. Thus, this produces downdraft air circulation.
Temperature Inversion
Temperature inversion happens when cold air moves in and concentrates at lower altitude, while the air above it is warmer. In other words, temperature inversion occurs in a vertical layer in the atmosphere, where the ambient temperature increases with altitude.
Because cold air tends to descend, while warmer air tends to ascend, temperature inversion creates a relatively stable air mass. This is so because there is little if at all movement of air, making thermals impossible.
Wind Obstruction
Wind can greatly affect your drone performance. If there are obstructions on the ground, this may change not only the speed, but also the direction of the air current. This may cause a dangerous condition for the unmanned aircraft. Also, strong winds can impact the ability of a drone to maintain its position. If not addressed, this will consume battery power faster than expected.
For instance, consider a building. If the wind blows into the building, it may create a strong updraft on the side that faces the wind.
But, the other side of the building will have downdrafts. The same logic applies for mountains or any other tall obstruction.
Air Masses and Air Fronts
Air masses are in constant motion. The typical characteristic of an air mass is its temperature and the amount of moisture. When two air masses with different characteristics collide, the division line between them is called a front.
There are two important concepts to define, which are troughs and ridges.
Trough and Ridges
First is trough, which is an elongated area of low pressure. Troughs are associated with colder air and low pressure aloft. Because there is a low pressure in the air mass up high, the air tends to rise to the east of the trough, creating unstable weather and precipitations.
Sometimes, wind develops around a trough’s low pressure center that blows in a counterclockwise direction.
Conversely, ridges are elongated areas with high pressure. Ridges are associated with warm air and high pressure aloft. The air aloft is heavier and exerts higher pressure on the air mass below, which creates a stable atmosphere. Consequently, this produces dry, sunny weather conditions underneath the ridge. Sometimes, winds form around a ridge’s high pressure center that blow clockwise.
Front Location
As previously defined, fronts are division lines between two air masses with disparate features. As a rule, temperature is the most important feature of them all.
You need to remember that all fronts lie in troughs for your FAA Part 107 exam. Each air mass has its own highest pressure center. As you move away from that center, the pressure tends to go down. As two different air masses with different temperatures collide, they do so at the periphery, away from their respective pressure centers. That boundary where they collide is a front and it will lie in low pressure areas, or troughs.
Types of Fronts
As for the fronts, there are several types of them. For the FAA part 107 exam, you need to know the associated weather conditions that occur with each one of them.
1. Cold Front
A cold front is the leading edge of a cold air mass that advances on a warm air mass.
Because cold air mass is denser than the warm air, the approaching cold air will slide under the warm air. This will cause the warm air to rise rapidly and abruptly.
As warm air rises, it cools down. This phenomenon results in condensation and the formation of thunderstorm cumulonimbus clouds, which causes severe weather conditions. These cumulus type clouds will have a tall vertical structure that is conducive to formation of thunderstorms.
Cold fronts tend to move quicker compared to warm fronts and are associated with the following:
- thunderstorms
- showery precipitations
- turbulence
- hazy visibility
As a cold front passes by quickly, the visibility improves and becomes clear.
2. Warm Front
A warm front is a leading edge of a warm air mass approaching a cold air mass.
Because a warm air mass is less dense than cold air, warm air will wedge above the cold air. Then, the warm air will continue advancing slowly for longer periods of time and for hundreds of miles.
As a result of this gradual process, more flat or slightly layered stratiform clouds will form. Stratiform clouds are associated with the following:
- calm and steady long-term precipitations
- long-term period of reduced visibility
- calm air
3. Stationary Front
A stationary front is a boundary between two air masses of different temperatures. This type of front does not move much by defintion.
Because neither the cold nor warm air mass dominates one another, some warm air mass is lifted above the cold one and there could be some light precipitation.
4. Occluded Front
An occluded front typically forms when a warm air mass gets squeezed between two cold air masses advancing from both sides.
As the two cold air masses converge on each other, the warm air mass gets pushed up and occluded or cut off from the ground. That is where the name comes from. Occluded fronts are associated with strong winds and heavy precipitation.
5. Squall Line
Finally, squall lines are narrow bands that form along or ahead of cold fronts.
Squall lines are typically associated with violent high winds and heavy precipitations. Also, hail and lightning are common too. Occasionally, tornadoes or waterspouts can form. Either way, squall lines are extremely dangerous for flight.
Frontal Passage
As the front passes, a frontal passage will have the following typical characteristics or discontinuities:
- temperature change
- steady decrease in pressure followed by an increase in pressure
- changes in wind speed and/or direction
Atmospheric Stability
Atmospheric stability is the resistance of air masses to vertical movement. So, a stable atmosphere would resist downward or any upward movement. When there is an unstable atmosphere, downward or upward movement becomes possible. This can give rise to a convective current or wind.
How to Measure Atmospheric Stability
The atmospheric stability can be determined by how temperature changes vertically with altitude compared to some standard. This measurement is referred to as temperature lapse rate. Lapse means a gradual fall since typically temperature falls with altitude. While you do not need to know any formulas for the FAA part 107 exam, the exact process of determining atmospheric stability is this.
First, you measure the environmental or actual temperature lapse rate for a given parcel of air at a given location. Then, you compare the actual temperature lapse rate with the dry adiabatic lapse rate. The dry adiabatic lapse rate is a theoretical temperature change of a dry parcel of air as you lift it fast in an adiabatic process. The adiabatic process means no heat is exchanged in the process of lifting.
The standard dry adiabatic lapse rate is 5.5°F, or 3°C per 1,000 feet. This means the temperature falls by 3°C for each 1,000 feet. If the actual temperature lapse rate is less than the dry adiabatic rate for an unsaturated air parcel, atmosphere is considered stable. This is because vertical motion of air is damped.
However, if the actual temperature lapse rate is greater than dry-adiabatic rate, there is atmospheric instability and vertical motion is possible. The reasoning behind it is this. As temperature falls faster compared to adiabatic standard, there are air masses stacked on top of each other with high contrasts in temperatures. This would lead to higher possibility of vertical movement and thus, instability.
Temperature lapse rate < dry adiabatic lapse rate ⇒ stable atmosphere
Temperature lapse rate > dry adiabatic lapse rate ⇒ unstable atmosphere
Of course, in reality you will have moisture present in the air. For this reason, there are formulas that make corrections for humidity to determine atmospheric stability. If there is anything you need to remember from this material is that atmospheric stability is determined by measuring the temperature lapse rate.
The ideal condition for atmospheric stability is temperature inversion, where a warmer parcel of air is situated above cold air, making the ambient temperature rise with altitude. Conversely, warming from below would decrease the stability of air.
Characteristics of Stable vs. Unstable Atmosphere
The typical conditions for stable vs. unstable air are shown in this table.
| Unstable Air (e.g. cold fronts) | Stable Air (e.g. warm fronts) |
|---|---|
| Cumuliform clouds | Stratiform clouds and fog |
| Showery precipitations, thunderstorms | Continuous precipitations |
| Rough air (turbulence) | Smooth air |
| Good visibility except in blowing obstructions | Fair to poor visibility in haze, smoke or fog |
Note how these two columns correlate with conditions for cold fronts and warm fronts that we just covered.
Practice Question for the FAA Part 107 Exam
Let’s test your understanding of the covered material with practice questions.
Practice Question #1
Each physical process of weather is the result of
A. movement of air
B. pressure differential
C. heat exchange
The first question is about how weather events occur. Because we know that uneven heating of the Earth by the sun produces various weather processes, the answer is C, which is heat exchange. Answers A and B are incorrect because they are the consequences of unequal heating and not the causes of weather.
Practice Question #2
Thermals develop as a result of
A. a counterclockwise circulation of air
B. temperature inversions
C. solar heating
The next question tests your knowledge on the source of thermals. Thermals are small-scale currents of updraft air produced as the surface of the Earth is heated by the sun. So, we immediately see that the answer is C, which is solar heating.
Choice A is incorrect because thermals are updrafts and have nothing to do with counterclockwise direction.
Choice B is interesting. Temperature inversion happens when cold air moves in and concentrates at lower altitude, while the air above it is warmer.
In other words, temperature inversion occurs in a vertical layer in the atmosphere, where the ambient temperature increases with altitude. Because cold air tends to descend, while warmer air tends to ascend, temperature inversion creates a relatively stable air mass since there is little if at all movement of air, making thermals impossible. Therefore, choice B is incorrect.
Practice Question #3
Refer to Figure 20. Above what area there should be the highest amount of thermal currents under normal conditions?
A. Area 2
B. Area 7
C. Area 5
The next question wants to know which areas on the map will produce the most amount of thermals. Remember, a thermal is an updraft air current typically produced by barren terrain or sand. We also know that areas with a lot of water are poor sources of thermals and typically cause downdrafts.
Looking at the map, we can see that areas 5 and 7 are both located above the water, which is unlikely to cause a thermal. Area 2 on the other hand is located on land. Even though there is a lake and a swamp at area 2, it is still much likelier to be a source of a thermal, making choice A the answer.
Practice Question #4
There is a strong wind from North. You need to fly your drone south from where you are. Your location is in an open field with no obstructions. Which of the following is not a problem during your flight?
A. Strong winds may consume more battery power than under calm conditions
B. Turbulence will be a significant factor
C. Strong wind may exceed aircraft’s performance, making it impossible to recover
This question tells you that there is a strong wind from the north and you are in an open field with no obstructions. You will fly your drone to the south of your location. The question wants to know what should not be a concern for your drone operation.
Because there are no obstructions in the field, this means that turbulence is very unlikely. However, we know that strong winds can consume a drone’s battery and make it hard to recover. This excludes choices A and C and makes B the answer to this question.
Practice Question #5
Where would wind shear exist?
A. At all altitudes.
B. At low altitude.
C. At high altitude.
The next question asks you where wind shear would exist. Wind shear is a change in wind speed and/or direction that occurs in a sudden manner. This can cause major issues for drone operations. Wind shear can occur at any altitude. This makes choice A the answer to this question.
Practice Question #6
Which one is the most recognizable discontinuity across a front?
A. A change in temperature.
B. An increase in cloud coverage.
C. An increase in relative humidity.
This question asks you to identify the most recognizable discontinuity across a front. Choice B is incorrect because cloud coverage is not a necessary condition to exist across a front. Likewise, choice C is incorrect too. Changes in relative humidity are not easy to recognize across a front. This makes choice A the answer. Temperature changes are indeed one of the most reliable indicators for a frontal passage.
Practice Question #7
Ridges are elongated areas of what?
A. Steady precipitations.
B. Turbulent air.
C. High pressure.
The next question tests your knowledge of the definition of ridges. If you recall, ridges are elongated areas of high pressure. This makes choice C the answer to this question.
Practice Question #8
Fronts are located?
A. in ridges
B. behind an advancing cold air mass
C. in troughs
The next question is about fronts and their location. As explained earlier, fronts are located at the lowest pressure zone where two air masses collide with each other. Recall, troughs are low-pressure areas. Thus, fronts are always located in troughs, making choice C the answer.
Practice Question #9
A weather phenomenon that always happen when flying across a front is a change in the
A. wind direction
B. type of precipitation
C. stability of the air mass
The next question asks you about one weather phenomenon that is always present when flying across the front. You can immediately discard choice B. This is so because precipitations may not be necessarily present in a front. Also, there could be stability across two colliding air masses that form a front, making choice C incorrect. However, change in wind direction is always present across a front. This makes choice A the answer.
Practice Question #10
What type of weather event is common in cold fronts?
A. Long-term reduced visibility.
B. Long-term steady precipitations.
C. Thunderstorms and heavy rain.
The next question tests your knowledge of cold fronts and weather phenomenon associated with them. We know that cold fronts cause the most severe changes in weather. These include thunderstorms, heavy rain, turbulence and relatively good visibility.
Warm fronts are characterized by long-term periods of reduced visibility and steady precipitations. So, we can immediately see that choices A and B are relevant for warm fronts. Choice C is relevant for cold fronts, making it the answer.
Practice Question #11
What type of weather produces the best flying conditions?
A. Warm, moist air.
B. Cool, dry air.
C. Turbulence.
The next question wants to identify the best flying conditions. We can immediately discard choice C, as turbulence is terrible for flights. Choice A is incorrect. Warm moist air has few air molecules in it and contains a lot of water vapor. These conditions are not good for flying.
Conversely, cold dry air is denser and contains more air molecules compared to warm moist air for a given volume. Also, cool dry air is very stable and resists vertical movement, making it perfect for flying. This makes choice B the answer.
Practice Question #12
What is the name of the zone between air masses with different temperature, humidity and wind?
A. A front.
B. An air mass.
C. Wind shear.
The next question wants to know the name of the zone between two different air masses of temperature, humidity and wind. From before, we know that such a zone is a front. Temperature, humidity and wind can rapidly change across any front. This makes A the answer.
Choices B and C are incorrect because wind shear is a separate weather phenomenon that has nothing to do with the name of the zone the question is asking about. Also, air mass has relatively uniform properties of temperature and moisture, which makes choice B incorrect.
Practice Question #13
Thunderstorms which generally produce the most intense hazard to flying aircraft are
A. squall line thunderstorms.
B. steady-state thunderstorms.
C. warm front thunderstorms.
The next question wants to know which thunderstorms produce the most hazard for an aircraft. From before, we know that warm front thunderstorms are relatively mild. This excludes choice C. Steady-state thunderstorms by their definition are stable precipitations and are not very dangerous. However, squall lines are narrow bands that form along or ahead of a cold front and are very dangerous for flying. This makes choice A the answer.
Practice Question #14
A stable air mass would have the following characteristic:
A. Showery precipitations.
B. Turbulence.
C. Poor surface visibility.
This question asks you to identify the characteristics of a stable air mass. We know that stable air is associated with steady precipitations and smooth calm air. This rules out choice A and B. Conversely, poor visibility is one of the characteristics of a stable air, making choice C the answer.
Practice Question #15
A moist unstable air mass would have the following characteristic:
A. Turbulent air and showery precipitations.
B. Poor visibility and smooth air.
C. Haze, smoke, fog.
The next question tests your knowledge of unstable air mass. We know that unstable air mass is associated with turbulence and showery precipitations with thunderstorms. This makes choice A the answer. Choices B and C are wrong because these are characteristics of a stable air mass.
Practice Question #16
Which measurements can determine the atmospheric stability?
A. Atmospheric pressure.
B. Actual temperature lapse rate.
C. Surface temperature.
The next question wants you to know which measurements would determine atmospheric stability. Recall, the difference between the actual temperature lapse rate and adiabatic lapse rate determines the stability of an air mass. This makes choice B the answer. Other choices cannot determine the vertical stability of an atmosphere.
Practice Question #17
Which factor would decrease the stability of a given air mass?
A. Warming from below.
B. Cooling from below.
C. Decrease in water vapor.
The next question wants to know which factors would reduce the stability of an air mass. Let’s examine incorrect choices first.
Choice B is wrong because cooling from below means that the air at the surface would become denser and therefore resists upward movement. This makes the air mass actually more stable.
Next is choice C. Recall that if the water vapor or humidity goes up in the air, it becomes less dense and prone to vertical movement. So, an increase in water vapor would decrease atmospheric stability. Conversely, decrease in water vapor would actually increase atmospheric stability. This makes choice C incorrect. Choice C would have been correct if it was phrased “increase in water vapor”.
On the other hand, warming from below would make an air mass at the surface less dense and prone to vertical movement. Hence, atmospheric stability goes down with it, making choice A the answer.
Practice Question #18
Weather forecasts states that there is an unstable air mass approaching your flight location. Which would not be a concern for your planned flight?
A. Thunderstorms.
B. Stratiform clouds.
C. Turbulence.
The next question tests your knowledge of the characteristics of an unstable air mass. We know that unstable air masses are associated with thunderstorms, turbulence and cumulus clouds. This rules out choices A and C. But, stratiform clouds are present under stable atmospheric conditions. This makes choice B the answer.
Practice Question #19
You received weather forecast, which indicates a low-level temperature inversion with high relative humidity. Which weather conditions should you expect?
A. Smooth air, poor visibility, fog, haze or low clouds.
B. Light wind shear, poor visibility, haze, light rain.
C. Turbulent air, poor visibility, fog, low stratus type clouds, and showery precipitation.
The question states that there is a low-level temperature inversion with high relative humidity present. You need to identify weather conditions that can be expected in this case.
Temperature inversions are associated with stable air conditions. These include such characteristics as smooth air, poor visibility, fog, haze and low stratiform clouds. This is so because with temperature inversion, the temperature of the air rises with altitude. As a result, you have layers of smooth, stable air close to the ground.
Under the inversion conditions, temperature will rise up to a certain point though. Thus, air at the top of the layer where temperature inversion will stop acts as a lid. This top layer keeps various weather and pollutants trapped underneath. With relatively high humidity, cloud formation is possible closer to the ground and you will have fog, haze, smooth air and poor visibility. This makes choice A the answer.
As for the other choices, choice C is incorrect because it mentions turbulent air, which is not a characteristic of a stable air. Also, choice B is incorrect because you are unlikely to have wind shear under stable air conditions.
Concluding Remarks
That is is it for this tutorial. We made many more FAA Part 107 exam tutorials that will help you prepare for the test. If you have any questions about this material, let me know in the comments section.