What are the cloud clearance requirements for UAS operations?

No person may operate an ultralight vehicle when the flight visibility or distance from clouds is less than that in the table found below. All operations in Class A, Class B, Class C, and Class D airspace or Class E airspace designated for an airport must receive prior ATC authorization as required in § 103.17 of this part.

[Amdt. 103-17, 56 FR 65662, Dec. 17, 1991]

The following state regulations pages link to this page.

Recently, it came to our attention a software company was coding a product that uses a temperature and dewpoint spread technique to estimate cloud bases. Weather is a complex science, and while we all seek to simplify our understanding to cope with the weather, there is a reason aviation weather services are the most regulated of any industry.  

This story caused us to write this blog. First, to demonstrate to software developers why using peer-reviewed science or consulting with a trained aviation meteorologist is essential to producing an aviation weather product, not using rules of thumb. Second, to discuss the merits of the temperature and dewpoint spread method to estimate cloud height for pilots operating in the field

There are many risks a remote pilot must consider when it comes to weather effects on operations. A key FAA concern surrounds the danger of drone pilots violating the Part 107 rule that states a UAV must maintain a 500′ clearance below a cloud deck.  This is important so other aircraft descending from clouds can detect and avoid the drone or allow the drone pilot time to avoid the path of the descending aircraft.  The FAA is especially worried about mishaps in LAANC areas, where micro-climate clouds and visibility do not necessarily match the airport METARs. 

As a weather observer for over twelve years, I can say that estimating cloud heights is not always straightforward. Low stratus clouds are notoriously difficult to estimate height due to the milkiness of the clouds.  Unfortunately, UAV operators are at most risk of violating the 500 ft clearance when the clouds are most challenging to discern height.

               The “rule of thumb” or technique we have been hinting at is the “dew point depression method.” This technique computes the difference between the temperature and dew point in Celsius at the surface and multiplies that number by 400 to get a cloud height in feet.  For example, the dew point depression between a temperature of 10C and a dew point of 5C is 5 degrees. Multiply the 5C difference by 400 to get an estimated cloud base of 2,000 feet.  As is traditional, it is a little more challenging with Fahrenheit. In this case, take the dew point depression, multiply by 4.4, then multiply by 1,000 to get the height in feet. This technique will give a reasonable estimate of the cloud base in feet, but only under certain atmospheric conditions.

               The FAAs Pilot’s Handbook of Aeronautical Knowledge (2014) explains this method as such:

               “As moist, unstable air rises, clouds often form at the altitude where temperature and dew point reach the same value. When lifted, unsaturated air cools at a rate of 5.4 °F per 1,000 feet, and the dew point temperature decreases at a rate of 1 °F per 1,000 feet. This results in a convergence of temperature and dew point at a rate of 4.4 °F. Apply the convergence rate to the reported temperature and dew point to determine the height of the cloud base.”

               I couldn’t have said it better myself if the technique is used in specific atmospheric conditions.  As with all “rules of thumb,” understanding when to use it is important.  This rule is most accurate during the daytime when the boundary layer becomes well mixed. It works for the bases of cumuliform clouds the best. In some cases, it can work in some low-level stratus situations, but not consistently. In fact, these layers often form through different processes that render this quick and easy method unreliable. So, the types of clouds that matter most to low-level operations are often underserved by this process. According to the Air Force observing manual, of which I am fondly familiar with:

 “This method is most accurate in determining cloud bases below 5,000ft. This method cannot be used to determine heights of non-cumuliform clouds or for locations in mountainous/hilly terrain.”

To prove my case, take the following METAR from Reading, Pennsylvania:

KRDG 020554Z 35013KT 10SM OVC030 19/16 A2969 RMK AO2 SLP057 60017 T01890156 10189 20183 51015 $

               The dew point depression in this observation is 3C, which would mean a cloud base of 1200 feet. However, the observed height is 3,000 feet. There are many reasons the method can fail, such as a moist layer above the ground level, with a low-level dry air intrusion near the ground, or the opposite, where there is low-level moisture but a dry layer aloft.   

A cloud product like TWS offers through the TruFlite V360 API or web portal is often the best data type for cloud height estimation. The TruFlite V360 cloud product uses real-time observations and cloud physics to provide the best estimate of cloud heights. The dew point depression method has its place but is generally not relevant for drones limited to flight at 400 FT AGL.  Using it in the wrong situation without a cloud height service to validate can increase the risk and liability of unknowingly breaking the 500-foot rule.

References

Federal Aviation Administration. (2014). Pilot’s handbook of aeronautical knowledge. Simon & Schuster.

HQ USAF/A3W. (2019). Surface weather observations (AFMAN 15-111). U.S. Air Force.