Weather Phenomena
Just like flying a plane, it is important to understanding some basic characteristics of the atmosphere and the effects of weather on drones. This section will focus on some of these concepts.
Density Altitude – What is it?
Density altitude is the pressure altitude that has been corrected for the actual temperature of the air. As a basic principle, aircraft performance increases with air density. Conversely, aircraft performance decreases with a decrease in air density. Things that affect air density are altitude, air pressure, temperature, and humidity.
Pressure
Assuming a constant temperature, the air pressure is directly proportional to air density. Because air is a gas, it can expand and contract. This means that when air pressure decreases, air expands and takes up a larger space. Conversely, when air pressure increases, more air occupies a given space. It is as though someone is squeezing air into a smaller space.
To visualize this concept, think of a trash compactor. When the compactor is turned on, it takes the less dense trash and puts pressure on it in order to make it occupy less space. The same concept applies when air pressure increases.
Temperature
When the temperature of a substance is increased, it expands, or its density decreases. Conversely, decreasing the temperature of a substance increases its density.
To visualize this concept, picture yourself blowing up a balloon in your house, where the temperature is a comfortable 70 degrees. You accidentally leave the balloon outside overnight and the temperature gets down to 30 degrees. The following morning, the balloon is seriously deflated because the air inside that balloon has become more dense and now occupies less space than when you blew it up at 70 degrees.
Humidity
There is always some amount of water vapor in the air. Water vapor is lighter than air and thus is lighter than dry air. So the more humid the air, the less dense it is. The less dense, the more it decreases the performance of an aircraft.
To visualize this concept, think about the idea that a cloud floats. A cloud is air with visible water (thus higher relative humidity) and is lighter than the surrounding air.
High Density Altitude vs. Low Density Altitude
High density altitude is a term that refers to thin air. Low density altitude is a term that refers to dense air. Although these terms may seem counterintuitive, think high altitude=thin air and low altitude=dense air.
Performance
Performance refers to the ability of an aircraft to accomplish its given purpose. The factors most affecting aircraft performance are weight, altitude, and changes in configuration that affect excess thrust and power.
For the remote pilot, weight is an important factor to consider. Your drone is made lightweight on purpose. The less weight on the drone, the more excess power the drone has for purposes of climbing and maneuvering. The more weight that is added to the drone (possibly in the form of a camera or other payload) decreases the excess power the drone has for climbing and maneuvering.
Atmospheric Pressure
Although understanding atmospheric pressure is important for a remote pilot, most drones have a built in barometric sensor and will measure your “altitude” from the point of take off. For this reason, a remote pilot will likely be less concerned with an actual barometric pressure reading. Despite this, understanding barometric pressure is important because changes and trends in air pressure is indicative of weather activity. Rapidly decreasing air pressure indicates approaching bad weather and possible severe storms.
Effects of Weather on Drones: Obstructions on Wind
Another unseen hazard for a remote pilot are obstructions that affect the flow of wind. This can be anything from ground topography to large buildings, but these obstructions break up the flow of the wind and cause it to gust inconsistently in both direction and speed. This is possibly even more of a concern for remote pilots in that the size and weight of an aircraft plays into the effect of the changing wind. Because drones are considerably smaller than most aircraft, and drones are typically operating in close proximity to the ground, the turbulence created by obstacles could be even more devastating than it usually is for manned aircraft.
Low-Level Wind Shear
Wind shear is a sudden, drastic change in wind speed and/or direction over a small area. Wind shear at low altitudes presents a specific danger because your drone is so close to the ground. A sudden change in altitude at a higher level may be uncomfortable, while a sudden change in altitude at a low level may destroy your drone.
Atmospheric Stability
Atmospheric stability refers to the atmosphere’s ability to resist vertical motion. In unstable atmosphere, small vertical air movements will become larger, resulting in turbulence and convective (rising air) activity. A combination of moisture and temperature determine the stability of air in the atmosphere. Cool, dry air tends to be more stable. Conversely, warm, moist air tends to be less stable.
To picture this concept, think of a place like Florida in the summer. I have friends that have lived there who tell me that thunderstorms will roll in to Tampa virtually everyday. This is because the air in Florida’s summer is hot and moist. The hot and moist air creates instability in the atmosphere and fosters the creation of thunderstorms.
Temperature/Dew Point Relationship
Relative humidity is the relationship between dew point and temperature. The dew point is the temperature at which the air can hold no more moisture. When the temperature becomes the same as the dew point, the air is saturated with moisture and it begins to condense in the form of precipitation. If the temperature is below freezing, precipitation that is deposited ends up being frost. Frost can pose a risk to a drone because it disrupts the flow of air over the wing or rotor, which increases drag and inhibits the production of lift.
Clouds
The most dangerous type of cloud for pilots are cumulonimbus clouds (which are usually associated with thunderstorms), because they are turbulent and pose a serious hazard to flight safety. This is even more true when it comes to the flight of a drone. As you will see below in the section on thunderstorm life cycles, one of the parts of a thunderstorm is the buildup of the cumulus clouds vertically into cumulonimbus clouds. Below is a cloud that is building into a potential storm. On the other hand, a cloud type that is usually associated with stable air is a stratiform cloud. Remember this as I recall this being a test question. A stratiform cloud looks like the one pictured below.
Fronts
A front is the part of an air mass that comes in contact with another air mass. The area surrounding these two colliding air masses is the frontal zone. These areas are often associated with quickly changing temperature, humidity, and wind.
Mountain Flying
Because mountains are natural barriers to wind and air movement, it is important to be aware of air movement when flying near mountains. Do your best to gather information on wind speeds and directions before taking flight near a mountain. This could mean getting a weather report or simply paying attention to the clouds in the sky near the mountain. Take a look at the clouds near the mountain pictures below. These clouds appear to be somewhat stratified, which means that the air near this mountain is stable and is not turbulent.
When clouds appear to be rolling in over a mountain peak, however, this tends to indicate turbulence on the side of the mountain that is sheltered from the wind. Additionally, where clouds appear to be building into cumulonimbus clouds, this usually indicates turbulence on all sides of the mountain. The point here is simply to be careful and aware when you are planning to fly near a mountain. Some companies are even pioneering technology that would help provide individually tailored weather reports for what they are calling “micro weather.”
Structural Icing
This is one of the things that you need to know for the test, but I’m really not sure how it would ever be a factor in actual drone flight (at least not for the usual drone pilot). That being said, understand that ice on your drone will drastically effect the performance of your drone. Before ice can form on your drone, however, there are two things that must exist: First, your drone must be flying through visible water like rain or cloud droplets. Second, the temperature where the moisture hits your drone would need to be freezing or below.
Thunderstorm Life Cycle
A thunderstorm progresses through three stages: 1) cumulus; 2) mature; and 3) dissipation. Each of these stages is discussed below. This is one of the most obvious effects of weather on drones because who wants to fly in a storm?
Cumulus
Not all cumulus clouds turn into storms, but all storms begin as a cumulus. During this stage, the upward moving air carries the liquid in the clouds above the point where the air is freezing. As the raindrops get heavier, they begin to fall and bring cold air down with them. At this point both the updraft and downdraft exist together and the storm is now “mature.”
Mature
Precipitation in some form begins to fall, which signals that a downdraft has developed. The downward rushing air spreads out at the surface and is characterized by strong, gusty surface winds, a drop in temperature, and a rise in pressure. Updrafts within the storm continue during this stage as well, creating turbulence and wind sheer.
Dissipating
Downdrafts are characteristic of this stage and the rain will stop falling as the storm fully dissipates.
Ceiling
A ceiling of clouds is the lowest layer of clouds that is considered broken (⅝ to ⅞ cloud cover) or overcast (full cover) or vertical visibility into fog or haze. If the cloud cover at a certain point is less than ⅝ of the sky, it is not considered a ceiling.
Visibility
Visibility is the horizontal distance at which prominent objects can be viewed with the naked eye. Essentially, it is how far out you can see big things on the horizon.
When trying to fully understand the effects of weather on drones, it is important to take a big picture view of the topic. Some things, like thunderstorms, have an obvious effect on drones and for that reason, we avoid them like the plague. Other things, like wind obstruction, can be devastating for a drone, but aren’t usually very obvious. By taking every part of your drone flight into account before taking off, you are providing yourself the best opportunity to avoid all effects of weather on drones.
Now, we’ll move on to aviation weather tools.
Aviation Weather Tools for Drone Pilots
Obviously the weather is an important factor for safely flying your drone. There are a number of aviation weather tools and it is important that you are familiar with them.
Additionally, it is important to have a basic understanding of the effects of weather on the performance of a drone, as discussed in the previous section.
But as a drone pilot, aviation weather tools can sometimes seem like reading another language. The good news here is that this information only needs to be learned for the initial Part 107 knowledge test, but not the re-test you will be required to take after two years. Check out my full article on getting your Part 107 renewal.
METARs
The Part 107 knowledge exam focuses on one of a few aviation weather tools, a METAR, more than the others. A METAR, which stands for a METeorological Aerodrome Report, provides current and accurate weather depictions that are in a standard format. Below is an example of such a format and then I will be breaking the METAR down into its individual parts. The applicable part of this METAR will be bolded.
METAR KGGG 161753Z AUTO 14021G26KT 3/4SM +TSRA BR BKN008 OVC012CB 18/17 A2970 RMK PRESFR
- Type of report (METAR) – There are two types of METARs, which are indicated by the words METAR or SPECI for a special report. A special report is typically issued for quickly changing weather.
- Station identifier (KGGG) – Each airport has a four letter code assigned to it. In the lower 48 of the US, these codes begin with “K.” KGGG identifies the Gregg County Airport in Longview, Texas. “K” is the country designation and GGG is the airport designation.
- Date and Time of Report (161753Z) – The first two digits indicate the date, while the next four digits indicate the time of the METAR in coordinated universal time (UTC). The letter “Z” at the end indicates that the time is given in Zulu. Zulu time is the standard use of time in the aviation industry and is the time at the Zero Meridian (Greenwich Mean Time).
- Modifier (AUTO) – When a modifier is shown, it will be shown here. Here, AUTO indicates that the report is from an automated source.
- Wind (14021G26KT) – Wind is reported in five digits (14021) unless windspeed is greater than 99 knots, when it will be reported in six digits. The first three digits indicate the direction that true wind is blowing from in tens of degrees (140). Alternatively, VRB indicates variable winds. The next two (or three) tell the windspeed (21). The above reading also indicates that the wind is also gusting at 26 knots (G26KT).
- Visibility (3/4SM) – this shows the prevailing visibility measured in statute miles (SM).
- Weather (+TSRA BR) – First, the existence of a + or – would indicate the intensity of the weather (heavy or light). The remaining letters indicate the existence of some type of weather. Here, TS is Thunderstorm, RA is rain and BR is mist. A full list of these codes is linked below.
- Sky condition (BKN008 OVC012CB) – The sky condition is always reported in the sequence of amount of sky cover, height, and type of cloud. The height of the cloud bases are reported in three digit numbers in hundreds of feet above ground level. Here, the METAR states that there are broken clouds at 800 feet, and it is overcast at 1200 feet with cumulonimbus clouds. A complete breakdown of sky cover and the proper code used is available in the downloadable FAA Study Guide. (See page 17).
- Temperature and dew point (18/17) – air temperature and dew point are always in degrees Celcius (C). If the temperatures indicated are negative or minus they will be preceded by an “M.”
- Altimeter setting (A2970) – an altimeter setting is measured in inches of mercury and is represented by the letter “A” and a four digit number. While this is not necessary for remote pilots of drones, this is necessary for manned pilots to ensure that their altimeter is reading correctly.
- Remarks (RMK PRESFR) – The remarks section will always start with the code “RMK.” It can include anything from additional wind data, pressure information or any other weather phenomenon that does not fit elsewhere. Here, the remark section indicates that the altimeter pressure is falling rapidly.
In the METAR we just looked at, the indication in the remarks section that the pressure was falling rapidly would be important to know and understand before taking your drone out. Also, if you are looking to get some more practice on METARs, the National Weather Service provides a list of the all the codes used in a METAR. There are also awesome services that let you plug in the METAR codes and it reads them out for you. This is another great way to practice.
If you’re still having trouble, I have a short video that I put together dissecting a METAR/TAF.
Aviation Forecasts
Terminal Aerodrome Forecasts (TAF)
This report is for a five statute mile radius around an airport and uses the same descriptors and abbreviations as used in the METAR report. Each TAF is valid for a 24 or 30 hour period of time, and they are updated four times a day. The beauty of a TAF is that although it provides a forecast instead of current conditions, it will use the same codes as a METAR. So if you learn to read a METAR, you will also be learning to read a TAF.
Convective Significant Meteorological Information
SIGMETs are inflight weather advisories associated with large meteorological events that are not thunderstorm related. Convective SIGMETs are issued for severe thunderstorms with winds greater than 50 knots, large hail, or tornadoes. It is called a SIGMET because it is issued to advise of SIGnificant METeorological events. The Aviation Weather Center provides a map of all active SIGMETs and convective SIGMETs.