Baseline Flight Research Articles

Measuring aviator workload using EEG: an individualized approach to workload manipulation.

Measuring an operator's physiological state and using that data to predict future performance decrements has been an ongoing goal in many areas of transportation. Regarding Army aviation, the realization of such an endeavor could lead to the development of an adaptive automation system which adapts to the needs of the operator. However, reaching this end state requires the use of experimental scenarios similar to real-life settings in order to induce the state of interest that are able to account for individual differences in experience, exposure, and perception to workload manipulations. In the present study, we used an individualized approach to manipulating workload in order to account for individual differences in response to workload manipulations, while still providing an operationally relevant flight experience. Eight Army aviators participated in the study, where they completed two visits to the laboratory. The first visit served the purpose of identifying individual workload thresholds, with the second visit resulting in flights with individualized workload manipulations. EEG data was collected throughout both flights, along with subjective ratings of workload and flight performance. Both EEG data and workload ratings suggested a high workload. Subjective ratings were higher during the high workload flight compared to the low workload flight (p < 0.001). Regarding EEG, frontal alpha (p = 0.04) and theta (p = 0.01) values were lower and a ratio of beta/(alpha+theta) (p = 0.02) were higher in the baseline flight scenario compared to the high workload scenario. Furthermore, the data were compared to that collected in previous studies which used a group-based approach to manipulating workload. The individualized method demonstrated higher effect sizes in both EEG and subjective ratings, suggesting the use of this method may provide a more reliable way of producing high workload in aviators.

Flight capacity and behavior of Ephestia kuehniella (Lepidoptera: Pyralidae) in response to kairomonal and pheromonal stimuli.

Flight behavior is an important component to understand in the context of pest management. However, because of their small size, little is known about the flight capacity of most stored-product insects, and when a flight has been assessed, it usually consists of a propensity for initiating flight. Despite a priori expectations of the importance of flight for moths, there are no data about the flight capacity and little on the flight behavior of the Mediterranean flour moth, Ephestia kuehniella Zeller (Lepidoptera: Pyralidae). As a result, the objective of the current study was to (i) characterize the baseline flight capacity of E. kuehniella and (ii) determine how flight capacity is affected by the presence of kairomonal, pheromonal, or no stimuli. We found adult E. kuehniella flew a mean of 24-34 km in a 24-h period, and the distance flown per bout increased from 91 to 207 m in the presence of pheromones but decreased to 41 m when food was nearby compared to a negative control. The total number of flight bouts was 1.6-fold higher in the presence of pheromone compared to the negative control, but E. kuehniella flew significantly slower with pheromone and food cues present, suggesting they may be exhibiting an optimal foraging strategy. Our data on flight capacity results in qualitatively and quantitatively different conclusions about flight than those conclusions formed if only flight initiation is considered. Overall, this novel information is useful for understanding the spread within facilities and in the landscape (between facilities), as well as parameterizing ecological modeling.

Evaluating the Effects of UAS Flight Speed on Lidar Snow Depth Estimation in a Heterogeneous Landscape

Recently, sensors deployed on unpiloted aerial systems (UAS) have provided snow depth estimates with high spatial resolution over watershed scales. While light detection and ranging (LiDAR) produces precise snow depth estimates for areas without vegetation cover, there has generally been poorer precision in forested areas. At a constant flight speed, the poorest precision within forests is observed beneath tree canopies that retain foliage into or through winter. The precision of lidar-derived elevation products is improved by increasing the sample size of ground returns but doing so reduces the spatial coverage of a mission due to limitations of battery power. We address the influence of flight speed on ground return density for baseline and snow-covered conditions and the subsequent effect on precision of snow depth estimates across a mixed landscape, while evaluating trade-offs between precision and bias. Prior to and following a snow event in December 2020, UAS flights were conducted at four different flight speeds over a region consisting of three contrasting land types: (1) open field, (2) deciduous forest, (3) conifer forest. For all cover types, we observed significant improvements in precision as flight speeds were reduced to 2 m s−1, as well as increases in the area over which a 2 cm snow depth precision was achieved. On the other hand, snow depth estimate differences were minimized at baseline flight speeds of 2 m s−1 and 4 m s−1 and snow-on flight speeds of 6 m s−1 over open fields and between 2 and 4 m s−1 over forest areas. Here, with consideration to precision and estimate bias within each cover type, we make recommendations for ideal flight speeds based on survey ground conditions and vegetation cover.

Energy-Optimal Unmanned Aerial Vehicles Motion Planning and Control Based on Integrated System Physical Dynamics

Abstract Electric vertical-take-off-and-landing multirotor aircraft has been emerging as a revolutionary transportation mode for both manned and unmanned applications, but this technology is limited by flight time and range restrictions. In this work, an energy-efficient model-based trajectory planning and feedback control framework is developed to improve the energy performance of a multirotor unmanned aerial vehicle. Target vehicle trajectories are planned by solving a formulated energy consumption optimization problem based on a system-level model, which accommodates the integrated dynamics of key vehicle subsystems. In order to implement the generated target trajectories, the framework also includes a PID feedback control architecture for real-time trajectory following. The framework is first verified under simulation, and shows an average reduction of 10.7% in energy consumption over a range of typical hover-to-hover operations, compared to the commonly used baseline flight control architecture. Through model-based analysis, key relationships that contribute to the improvements are identified and analyzed. These results demonstrate the importance of considering and coordinating all relevant system dynamics for efficient and holistic trajectory planning and control, which is absent in existing literature. The framework also demonstrates similar performance improvement under experimental validation, with an average energy reduction of 10.2% over the baseline controller despite the presence of significant real-world disturbances including wind effect.

Diurnal and nocturnal mosquitoes escape looming threats using distinct flight strategies

Flying insects have evolved the ability to evade looming objects, such as predators and swatting hands. This is particularly relevant for blood-feeding insects, such as mosquitoes that routinely need to evade the defensive actions of their blood hosts. To minimize the chance of being swatted, a mosquito can use two distinct strategies-continuously exhibiting an unpredictable flight path or maximizing its escape maneuverability. We studied how baseline flight unpredictability and escape maneuverability affect the escape performance of day-active and night-active mosquitoes (Aedes aegypti and Anopheles coluzzii, respectively). We used a multi-camera high-speed videography system to track how freely flying mosquitoes respond to an event-triggered rapidly approaching mechanical swatter, in four different light conditions ranging from pitch darkness to overcast daylight. Results show that both species exhibit enhanced escape performance in their natural blood-feeding light condition (daylight for Aedes and dark for Anopheles). To achieve this, they show strikingly different behaviors. The enhanced escape performance of Anopheles at night is explained by their increased baseline unpredictable erratic flight behavior, whereas the increased escape performance of Aedes in overcast daylight is due to their enhanced escape maneuvers. This shows that both day and night-active mosquitoes modify their flight behavior in response to light intensity such that their escape performance is maximum in their natural blood-feeding light conditions, when these defensive actions by their blood hosts occur most. Because Aedes and Anopheles mosquitoes are major vectors of several deadly human diseases, this knowledge can be used to optimize vector control methods for these specific species.

Refining the conversion between phase and height in airborne and UAV-borne SAR interferometry

ABSTRACT Airborne or unmanned aerial vehicle (UAV)-borne interferometric synthetic aperture radar (InSAR) can offer topography and forest measurement with high accuracy and high resolution. In these measurements, the conversion between height and interferometric phase is frequently involved. The existing conversion approximations may be more suitable for the space-borne InSAR. However, for the airborne or UAV-borne InSAR in a low flight altitude, these conversion formulas cannot work well for topography and forest height measurement. To achieve high-quality InSAR measurements, we derive a refined conversion formula based on the Taylor expansion, i.e. Firsec-order conversion formula. Differing from existing studies contributed by first-order derivative, the Firsec-order conversion formula expands a second-order term. During refined phase-to-height (PHAtoH) conversion, an external low-precision digital elevation model (DEM) is introduced to calculate and then remove the second-order phase from the original interferometric phase. Thus, the high-precision DEM is reconstructed based on the refined phase rather than the differential phase. In addition, this article investigates the sensitivity of the second-order term to different baseline lengths and flight altitudes with respect to two common baseline configurations in airborne or UAV-borne InSAR. The results show that for the horizontal baseline configuration, the second-order term must be considered. In addition, the longer the baseline and the lower the flight altitude, the larger the second-order phase, especially for the UAV-borne InSAR geometries with altitudes of under 2 km. However, for the vertical baseline configuration, the adoption of the refined formula depends on the actual situation. The performance test shows that the Firsec-order conversion formula has much higher conversion accuracies than traditional ones.

Asymptotic tracking position control with active oscillation damping of a multibody Mars vehicle using two artificial augmentation approaches

The Valles Marineris Explorer Cooperative Swarm navigation, Mission and Control research project aims to explore the Valles Marineris canyon system on Mars with, among others, multibody rotary-wing unmanned aerial vehicles (UAVs) comprising of a hexrotor system and a helium-filled balloon being attached to it by means of a rope. In this paper, we develop a high-fidelity closed-loop control system in MATLAB® and Simulink™ to present the application of an adequate flight controller guaranteeing (1) asymptotic tracking position control of the multibody flight system, (2) suppression of the balloon’s swinging motion in forward flight case, and (3) stabilization of the rope angle around its equilibrium for steady-state conditions. Applying feedback linearization for the outer loop and analytical backstepping for the inner loop of a nonlinear cascaded control design model of the hexrotor system, we propose an extension of the baseline flight controller by two artificial augmentation approaches to cope with the balloon dynamics. Basically, by utilizing oscillation damping feedbacks of the uncertain plant which are applied as additional commands to either the inner or the outer loop’s reference model. Simulation results are presented for an eight-shaped flight maneuver at the bottom of Valles Marineris proving that the augmentation units increase the flight controller capabilities to suppress modeling errors artificially—without changing the baseline control laws. The augmentation units actively damp the balloon motion in the forward flight case for non-steady-state conditions to counteract the rope oscillations and finally stabilize the rope angle around its equilibrium, so that the Mars vehicle is able to reach a steady-state in position when its extraterrestrial mission profile is successfully completed.

Baseline Flight Potential of Euschistus servus (Hemiptera: Pentatomidae) and Its Implications on Local Dispersal.

The brown stink bug, Euschistus servus (Say), is a damaging pest of multiple crops in the southeastern United States. In addition to crops, both the weedy field borders and wooded areas of a typical farmscape in this region harbor E. servus host plants, many of which are temporally and spatially limiting in availability or nutritional suitability. Therefore, local dispersal is required so that individuals efficiently track and utilize host resources. This research sought to establish the baseline flight capacity of adult E. servus across the season in relation to body weight, sex, and plant host use with a flight mill system. Across this 2-yr study, among the individuals with a flight response in the flight mill, 90.1% of individuals flew in a range of >0-1 km, with an individual maximum flight distance of 15.9 km. In 2017, mean total distance flown varied across the season. Except for the individuals collected from corn in 2019, during both 2017 and 2019, the highest numerical mean flight potential occurred soon after overwintering emergence and a relatively low flight potential occurred during the cropping season. Individuals collected from wheat, corn, and early season weeds lost a higher proportion of body weight after flight than did individuals from soybean and late season weeds. The baseline dispersal potential information generated from this study can be extrapolated to the farmscape level aiming to develop, plan, and implement E. servus management programs.

Roll performance assessment of a light aircraft: Flight simulations and flight tests

A methodology is presented for the roll performance assessment of a light aircraft. The study is based both on flight simulations and flight tests, focusing on the accurate determination of the lateral control power. The chosen test airplane was a Tecnam P92, a two-seat ultralight aircraft which was specifically equipped with a lightweight and accurate instrumentation for the planned set of flight data measurements. A matrix of flight experiments for the test campaign was established with the support of 6-degree-of-freedom simulations, implementing a carefully constructed baseline dynamic model of the aircraft. The article discusses first the general problem of a reliable evaluation of aircraft roll performance indicators, i.e. the estimation of the aerodynamic derivatives that mainly influence the airplane ability to roll. Next, the results of extensive flight test activities are presented. The analysis of several roll maneuvers performed at different flight speeds and with different aileron maximum deflections showed interesting rolling characteristics for this non-aerobatic aircraft. One notable finding was a clear nonlinear dependency of the aileron efficiency index on aileron deflection amplitude. A control power derivative extracted from flight data in the form of lookup table was used to correct the baseline flight dynamics model. Flight simulations outputs based on the updated model showed a satisfactory agreement with experimental time histories. According to this, the present effort proposes a new method to estimate the aileron control derivative in whole the flight envelope for light aircraft.

Volumetric Change Detection with using Structure from Motion – The Impact of Repeat Station Imaging

Repeat Station Imaging (RSI) for image acquisition is compared with non-RSI to assess the methods’ effects on vertical and volumetric estimation using structure from motion (SFM). Aerial triangulation (i.e., SFM) is used to create three-dimensional reconstructions of the study area using unmanned aerial vehicle-acquired imagery. Targets of known volume were deployed throughout the scene and manipulated to create changes between the first and subsequent flights. An RSI flight and two non-RSI flights were compared to a baseline flight in order to estimate a series of introduced volumetric changes, which were then compared to known volume changes. Using images with a nominal ground sampling distance of 1.96 cm, results show a total root-mean-squared-error (RMSE) of 0.035 m3 and mean percent error (MPE) of 25.9% for the RSI flight, and average RMSE of 0.057 m3 and MPE 33.3% for the two non-RSI flights. For the measurement of volumetric changes to extant features, the RSI flight had an RMSE of 0.026 m3 and an MPE of 17.6%; the average RMSE and MPE of the two non-RSI flights were 0.071 m3 and 39.4%. These results show that RSI has the potential to improve the accuracy of volumetric and height change estimation.

Flight behavior of foraging and overwintering brown marmorated stink bug, Halyomorpha halys (Hemiptera: Pentatomidae).

Brown marmorated stink bug, Halyomorpha halys (Stål), is a highly polyphagous invasive species attacking both cultivated and wild plants increasing its threat to ecosystems as a global pest. However, dispersal biology of this invasive species is not well understood. This study evaluated the flight capacity and behavior of H. halys under laboratory, semi-field, and field conditions. Flight mills were used to measure the baseline flight capacity of adults collected year round from the field and included both foraging and overwintering populations. The effects of abiotic conditions such as wind speed and temperatures on the free flight parameters of H. halys were evaluated under semi-field and field conditions. The mean flight distances over a 22-h period were 2442 and 2083 m for male and female, respectively. Most individuals (89%) flew <5 km, though some flew much further with a maximum flight distance observed of 117 km. Flight distances by H. halys increased after emergence from overwintering sites in spring and reached their highest point in June. The incidence of take off by H. halys was significantly affected by the wind speed; when provided with still air conditions, 83% of individuals took off, but the rates decreased to <10% when wind speed was increased to or above 0.75 m s-1. The incidence of take off by H. halys was significantly affected by ambient temperature and light intensity in the field, whereas relative humidity and insect sex did not. When the temperature was at 10-15°C, 3% of individuals took off, but the proportion of H. halys taking flight increased to 61, 84, and 87% at 15-20, 20-25, and 25-30°C, respectively. In the field, prevailing flight direction was biased toward the opposite direction of the sun's position, especially in the morning. The implications of H. halys flight biology are discussed in the context of developing monitoring and management programs for this invasive species.

Real-time wind speed estimation and compensation for improved flight

This paper presents the development and experimental validation of a prototype system for online estimation and compensation of wind disturbances onboard small Rotorcraft unmanned aerial systems (RUAS). The proposed approach consists of integrating a small pitot-static system onboard the vehicle and using simple but effective algorithms for estimating the wind speed in real time. The baseline flight controller has been augmented with a feed-forward term to compensate for these wind disturbances, thereby improving the flight performance of small RUAS in windy conditions. The paper also investigates the use of online airspeed measurements in a closed-loop for controlling the RUAS forward motion without the aid of a global positioning system (GPS). The results of more than 80 flights with a RUAS confirm the validity of our approach.

Factors Influencing Flight Capacity of the Mountain Pine Beetle (Coleoptera: Curculionidae: Scolytinae)

The mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae: Scolytinae) is the most damaging pest of mature pine (Pinaceae) in western North America. Although mountain pine beetles have an obligate dispersal phase during which adults must locate a new host for brood production, dispersal is a poorly understood aspect of its ecology. This flight mill study was designed to test the effects of beetle size, sex, and age on flight capacity. Energy use during flight was assessed through measurements of weight before and after flight and fat content of flown versus control beetles. The mean flight distance achieved by mountain pine beetles varied between 2.12 and 5.95 km over the 23-h bioassay, but the longest total flight of an individual beetle was >24 km. Beetle preflight weight influenced flight initiation, flight distance, and duration. Bigger beetles are more likely to fly and once in flight fly longer and farther than smaller beetles. There was no direct effect of beetle sex on flight capacity. Flight capacity of beetles declined with age postemergence. Although individual flight capacity was variable, flight velocity was relatively constant between 1.55 and 1.93 km/h. Lipids are used to power flight in mountain pine beetles, as lipid content was lower in beetles flown on the flight mills compared with beetles that did not fly. Flight distance was negatively correlated with beetle postflight lipid content. The baseline flight capacity data revealed in this study have implications for understanding the population dynamics of this eruptive forest pest.

Octopaminergic modulation of the visual flight speed regulator ofDrosophila

Recent evidence suggests that flies' sensitivity to large-field optic flow is increased by the release of octopamine during flight. This increase in gain presumably enhances visually mediated behaviors such as the active regulation of forward speed, a process that involves the comparison of a vision-based estimate of velocity with an internal set point. To determine where in the neural circuit this comparison is made, we selectively silenced the octopamine neurons in the fruit fly Drosophila, and examined the effect on vision-based velocity regulation in free-flying flies. We found that flies with inactivated octopamine neurons accelerated more slowly in response to visual motion than control flies, but maintained nearly the same baseline flight speed. Our results are parsimonious with a circuit architecture in which the internal control signal is injected into the visual motion pathway upstream of the interneuron network that estimates groundspeed.

Helicopter Pilot Use of a See-Through, Head-Mounted Display With Pathway Guidance for Visually Guided Flight: Observations of Navigation Behavior and Obstacle Avoidance

An exploratory study was conducted to evaluate synthetic imagery, presented in a binocular, stereoptic, head-mounted see-through display (HMD) for providing destination guidance and navigation data to helicopter pilots. The intended application was to Helicopter Emergency Medical Services (HEMS). Participants flew a simulator from an airfield to a remote, unimproved site to pick up an accident victim. Synthetic cues included approach and landing-zone signposts, highway-in-the-sky guidance for a 6-degree approach, approach direction lights, landing direction lights, and a synthetic helipad. Performance measures included destination visual acquisition time, transit time, obstacle avoidance, landing accuracy, and workload. Signposts allowed significantly shorter destination visual acquisition times. There were no significant differences in perceived workload. The baseline flights using the Global Positioning System (GPS) terminated further from the desired landing zone than flights using the HMD imagery. Successful visual avoidance of obstacles in the out-the-window view varied for different types of obstacles.

Helicopter Pilot Use of a See-Through, Head-Mounted Display with Pathway Guidance for Visually Guided Flight: Observations of Navigation Behavior and Obstacle Avoidance

An exploratory study was conducted to evaluate synthetic imagery, presented in a binocular, stereoptic, head-mounted see-through display (HMD) for providing destination guidance and navigation data to helicopter pilots. The intended application was to Helicopter Emergency Medical Services (HEMS). Participants flew a simulator from an airfield to a remote, unimproved site to pick up an accident victim. Synthetic cues included approach and landing-zone signposts, highway-in-the-sky guidance for a 6-degree approach, approach direction lights, landing direction lights, and a synthetic helipad. Performance measures included destination visual acquisition time, transit time, obstacle avoidance, landing accuracy, and workload. Signposts allowed significantly shorter destination visual acquisition times. There were no significant differences in perceived workload. The baseline flights using the Global Positioning System (GPS) terminated further from the desired landing zone than flights using the HMD imagery. Successful visual avoidance of obstacles in the out-the-window view varied for different types of obstacles.

Design of a Variable Stability Flight Control System

A design objective for variable stability flight control system is to develop a controller of in-flight simulation capability that forces the aircraft being flown to follow the dynamics of other aircraft. This paper presents a model-following variable stability control system (VSS) for in-flight simulation which consists of feedforward and feedback control laws, the aircraft dynamic model to be simulated, and switching and fader logics to reduce the transient effect between two aircraft dynamics. The separate design techniques for feedforward and feedback control law proposals are based on model matching and augmented linear quadratic (LQ) techniques. The system allows pilots to select and engage VSS mode, and when deselected, the aircraft reverts to the baseline flight control system. Both the baseline flight control laws and VSS control laws are computed continuously during flight. Initialization of the state values are necessary to prevent instability, since VSS control laws have integrators and filters in longitudinal, and lateral/directional axes. This paper demonstrates and validates the effectiveness and quality of VSS with F-16 models embedded in T-50 in-flight simulation aircraft.

두루미-II 무인기 기반의 조종력 할당 기법 성능 평가

논문에서는 두루미-II 무인기 모델을 이용하여 다양한 조종력 할당 기법의 성능을 평가하였다. 조종력 할당 기법은 먼저 조종면 고장을 고려하지 않은 상태에서 대상 항공기의 기준 비행제어 시스템을 설계한 후, 조종력 할당 알고리즘을 결합하여 구현할 수 있다. 본 논문에서 대상 항공기 모델인 두루미-II 무인기의 기준 비행제어 시스템을 고전제어 기법에 근거하여 설계하였다. 그리고 Psuedo-inverse CA 기법과 Direct CA 기법 및 Optimization CA 기법을 구현하여 두루미-II 무인기의 기준 비행제어 시스템과 결합한 후, 조종면 고장 시나리오에 따른 비선형 시뮬레이션을 통해 각 조종력 할당 기법의 성능을 평가하고 비교하였다. This paper focuses on the performance evaluation of various control allocation methods applied on DURUMI-II UAV system. In order to implement control allocation scheme to aircraft control system, control system can be designed through two step design procedure. The first step is to design a baseline control system for an aircraft without consideration of control surface failure. The second step is to design a control allocator that maps the total control command on the individual control surfaces. In this paper, several control allocation methods such as Psuedo-Inverse CA method, Direct CA method, and Optimization CA method are implemented and integrated to the baseline flight control system of DURUMI-II UAV. The performance of these control allocation methods is evaluated by nonlinear simulation under the flight scenario of control surface failure.

Noise impact study of a new 2004 noise abatement procedure at the Louisville airport

A flight demonstration test in September 2004 at Louisville was a continuation of research conducted in 2002 by a team sponsored by the Federal Aviation Administration’s Center of Excellence for Air Traffic Systems. A continuous descent procedure was designed primarily to minimize environmental impacts such as community noise and aircraft emissions, and to maximize savings in fuel and flight time. The test was designed to show the operational suitability of the new area navigation arrival procedure that begins at cruise altitude and which may be used in daily operation on two opposite facing runways. Flyover noise measurements were taken during a two‐week testing period, and a three‐week baseline period. The latest research focused on detailed analysis of aircraft performance, collecting noise data and noise prediction. The noise measurements confirm increased repeatability and predictability of noise levels that resulted from the well‐designed procedure. The Integrated Noise Model was used to compare noise levels of test and baseline flights. And noise predictions using precise flight data confirm that a continuous descent approach reduces noise levels by 4 to 6 decibels which in turn reduces contour area by as much as 30 percent.

Wintertime/summertime contrasts of cloud condensation nuclei and cloud microphysics over the Southern Ocean

Wintertime/summertime contrasts of cloud condensation nuclei (CCN) and cloud microphysics observed in clean maritime environments off the west coast of Tasmania, Australia in the Southern Ocean Cloud Experiment, are presented. The average wintertime CCN concentration (NCCN) was 32 cm−3 at 1% supersaturation (S), but it was 19 cm−3 when only baseline (maritime airflow with minimal anthropogenic influences) flights were considered. In contrast, the average summertime NCCN were more than a factor of 5 higher than wintertime for all S ranges. The seasonal contrast was larger when only baseline flights were considered, especially at lower S: summertime more than an order of magnitude higher than wintertime at S below 0.1%. Corresponding cloud droplet concentrations (Nc) showed similar contrasts but to a smaller extent. Summertime average cloud droplet concentrations [Nc(ave)] were only 2.5 times higher than wintertime concentrations (70 cm−3 versus 28 cm−3). This difference was nearly a factor of 3 when only baseline flights were considered (57 cm−3 versus 20 cm−3). Flight‐average NCCN and various representations of adiabatic cloud droplet concentrations (Na) generally showed good correlations, indicating that the original effects of CCN are somehow retained in the Nc(ave). The average mean diameter (MD) of the cloud droplets was 13.9 and 17.1 μm for the summer and winter clouds, respectively. For baseline only, average MDs were 15.4 and 18.2 μm, respectively. Average MD was thus above or close to the 15‐μm threshold for drizzle production, except for the summertime nonbaseline clouds, which had an average MD smaller than 10 μm. Because of the larger droplet sizes, conversion to drizzle was more efficient in the winter clouds, where the average drizzle liquid water content (Ld) of 0.12 g m−3 was twice that of the summer Ld. The Ld for the summer nonbaseline clouds was negligible. Average Ld was also highly dependent on cloud depth. Winter baseline clouds were sometimes too thin to produce significant drizzle even though they contained very low Nc and very large MDs. The thickest clouds contained the highest Ld although their MD was not always the largest.