Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “surface area”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1999

A Study of Stratospheric Aerosols and Their Effect on Inorganic Chlorine Partitioning Using Balloon, In Situ, and Satellite Observations

Heterogeneous reactions on the surface of aerosols lead to a decrease in the concentration of nitrogen radicals and an increase in the concentration of chlorine and hydrogen radical species. As a consequence, enhanced sulfate aerosol levels in the lower stratosphere resulting from volcanic eruptions lead to lower concentrations of ozone due to more rapid loss by chlorine and hydrogen radicals. This study focuses on continuing the effort to quantify the effect of sulfate aerosols on the partitioning of inorganic chlorine species at midlatitudes. The study begins with an examination of balloon-borne measurements of key chlorine species obtained by the JPL MkIV interferometer for different aerosol loading conditions. A detailed comparison of the response of HCl to variations in aerosol surface area observed by MkIV, ER-2 instruments, HALOE, and ATMOS is carried out by examining HCl vs CH4 correlation diagrams, since CH4 is the only tracer measured on each platform. Finally, the consistency between theory and observed changes in ClO and HCl due to variations in aerosol surface area is examined.

Osterman, G. B.↗
NASA NTRS1995

Condensed Water in Tropical Cyclone "Oliver", 8 February 1993

On February 8, 1993, the NASA DC-8 aircraft profiled from 10,000 to 37,000 feet (3.1-11.3 km) pressure altitude in a stratified section of tropical cyclone "Oliver" over the Coral Sea northeast of Australia. Size, shape and phase of cloud and precipitation particles were measured with a 2-D Greyscale probe. Cloud/precipitation particles changed from liquid to ice as soon as the freezing level was reached near 17,000 feet (5.2 km) pressure altitude. The cloud was completely glaciated at -5 C. There was no correlation between ice particle habit and ambient temperature. In the liquid phase, the precipitation-cloud drop concentration was 4.0 x 10(exp 3)/cu m, the geometric mean diameter D(sub g) = 0.5-0.7 mm, and the liquid water content 0.7-1.9 g m(exp-3). The largest particles anywhere in the cloud, dominated by fused dendrites at concentrations similar to that of raindrops (2.5 x 10(exp 3) m(exp -3)) but a higher condensed water content(5.4 g/cu m estimated) were found in the mixed phase; condensed water is removed very effectively from the mixed layer due to high settling velocities of the large mixed particles. The highest number concentration (4.9 x 10(exp 4)/cu m, smallest size (D(sub g) = 0.3-0.4 mm), largest surface area (up to 2.6 x 10 (exp 2) sq cm/cu m at 0.4- 1.0 g/cu m of condensate) existed in the ice phase at the coldest temperature (- 40 C) at 35,000 feet ( 10.7 km). Each cloud contained aerosol (haze particles) in addition to cloud particles. The aerosol total surface area exceeded that of the cirrus particles at the coldest temperature. Thus, aerosols must play a significant role in the upscattering of solar radiation. Light extinction (6.2/km) and backscatter (0.8/sr/km) was highest in the coldest portion of the cirrus cloud at the highest altitude.

Pueschel, R. F.↗
NASA NTRS2003

Episodic Endogenetic-driven Atmospheric and Hydrologic Cycles and Their Influence on the Geologic Records of the Northern and Southern Hemispheres, Mars

Diverse evidence shows a direct correlation between episodic endogenetic events of the Tharsis magmatic complex (TMC)/Superplume, flood inundations in the northern plains, and glacial/ lacustrine/ice sheet activity in the south polar region, which includes Hellas and Argyre impact basins, corroborating the MEGAOUTFLO hypothesis. The TMC encompasses a total surface area of approximately 2 x 10(exp 7) sq km, which is slightly larger than the estimated size of the Southern Pacific Superplume. These hydrologic events include: (1) a Noachian to possibly Early Hesperian oceanic epoch and related atmospheric and environmental change (a water body covering about 1/3 of the planet s surface area) related to the incipient development of Tharsis Superplume and the northwestern sloping valleys (NSVs) and possibly early circum-Chryse development, the northwest and northeast watersheds of Tharsis, respectively, (2) a smaller ocean inset within the former larger ocean related to extensive Late Hesperian to Early Amazonian effusive volcanism at Tharsis and Elysium and incisement of the circum-Chryse outflow system. During this time, magmatic/plume-driven tectonic activity transitioned into more centralized volcanism. This Late Hesperian water body may have simply diminished into smaller seas and/or lakes during the Amazonian Period, or renewed activity at Tharsis and Elysium resulted in brief perturbations from the prevailing cold and dry climatic conditions to later form minor seas or lakes. All of the hydrologic phases transitioned into extensive periods of quiescence.

Dohm, J. M.↗
NASA NTRS1962

High-Vacuum Condenser Design: Experimental Effects from Cesium and Mercury Ion Beams

Effects of variable-energy electrostatic rocket-engine ion exhaust beams that pertain to the design of high-vacuum condensers are discussed. Three ion-engine beam sources, using cesium and mercury as propellants, were operated at energy levels of 200 to 9000 electron volts and currents of 0.002 to 0.200 ampere. Five condenser geometries, having surface areas from 0.287 to 5.76 square meters, were tested. Values for a function of the accommodation and condensation coefficients, which appears in an existing theoretical analysis, were determined. Although exhibiting rather wide variations in some tests, the values were estimated at approximately 0.15 for cesium and 0.015 for mercury. A method of estimating condenser surface area requirements is given.

MERCURY /METAL/↗
NASA NTRS1989

The effects of the nonsphericity and size distribution of ice crystals on the radiative properties of cirrus clouds

Hexagonal ice crystal and equivalent ice spheres have significantly different single-scattering properties. Although the extinction cross-section for spheres with equivalent surface areas is about the same as that for nonspherical ice crystals randomly oriented in space, equivalent spheres produce more forward scattering and have smaller single-scattering albedos. On the basis of broad-band radiative transfer calculations, in this note we will illustrate that the assumption that ice particles are spheres with equivalent surface areas leads to an underestimate to the solar albedo of cirrus clouds. Furthermore, we show that, for a given optical thickness, small ice particles reflect more solar flux than large ice particles. This implies that the ice crystal size distribution could be extremely important in the determination of the solar albedo of cirrus clouds due to external radiative forcing in climatic perturbation experiments. In the thermal infrared region, absorption by ice crystals predominates and the effects of the nonsphericity and size distribution of ice crystals on infrared radiative transfer appear to be secondary.

Kinne, Stefan↗
NASA NTRS2014

Rapid Startup and Loading of an Attached Growth, Simultaneous Nitrification/Denitrification Membrane Aerated Bioreactor

The Membrane Aerated Bioreactor (MABR) is an attached‐growth biological system for simultaneous nitrification and denitrification. This design is an innovative approach to common terrestrial wastewater treatments for nitrogen and carbon removal. Implementing a biologically‐based water treatment system for long‐duration human exploration is an attractive, low energy alternative to physiochemical processes. Two obstacles to implementing such a system are (1) the "start‐up" duration from inoculation to steady‐state operations and (2) the amount of surface area needed for the biological activity to occur. The Advanced Water Recovery Systems (AWRS) team at JSC explored these two issues through two tests; a rapid inoculation study and a wastewater loading study. Results from these tests demonstrate that the duration from inoculation to steady state can be reduced to two weeks and that the surface area to volume ratio baseline used in the Alternative Water Processor (AWP) test was higher than what was needed to remove the organic carbon and ammonium from the system.

Meyer, Caitlin↗
NASA NTRS2017

The Lightweight Integrated Solar Array and anTenna (LISA-T) - Big Power for Small Spacecraft

NASA is developing a space power system using lightweight, flexible photovoltaic devices originally developed for use here on Earth to provide low cost power for spacecraft. The Lightweight Integrated Solar Array and anTenna (LISA-T) is a launch-stowed, orbit-deployed array on which thin-film photovoltaic and antenna elements are embedded. The LISA-T system is deployable, building upon NASA's expertise in developing thin-film deployable solar sails such the one being developed for the Near Earth Asteroid Scout project which will fly in 2018. One of the biggest challenges for the NEA Scout, and most other spacecraft, is power. There simply isn't enough of it available, thus limiting the range of operation of the spacecraft from the Sun (due to the small surface area available for using solar cells), the range of operation from the Earth (low available power with inherently small antenna sizes tightly constrain the bandwidth for communication), and the science (you can only power so many instruments with limited power). The LISA-T has the potential to mitigate each of these limitations. Inherently, small satellites are limited in surface area, volume, and mass allocation; driving competition between their need for power and robust communications with the requirements of the science or engineering payload they are developed to fly. LISA-T is addressing this issue, deploying large-area arrays from a reduced volume and mass envelope - greatly enhancing power generation and communications capabilities of small spacecraft and CubeSats. The problem is that these CubeSats can usually only generate between 7 watts and 50 watts of power. The power that can be generated by the LISA-T ranges from tens of watts to several hundred watts. A matrix of options are in development, including planar (pointed) and omnidirectional (non-pointed) arrays. The former is seeking the highest performance possible while the latter is seeking GN&C (Guidance, Navigation and Control) simplicity. In both cases, power generation ranges from tens of watts to several hundred with an expected specific power greater than 250 watts per kilogram and a stowed power density greater than 200 kilowatts per cubic meter. Options for leveraging both high performance, 'typical cost' triple junction thin-film solar cells as well as moderate performance, low cost cells are being developed. Alongside, both UHF (ultra high frequency) and S-band antennas are being integrated into the array to move their space claim away from the spacecraft and open the door for omnidirectional communications and electronically steered phase arrays.

Johnson, Les↗
NASA NTRS1995

Sensitivity of two-dimensional model predictions of ozone response to stratospheric aircraft: An update

The Goddard Space Flight Center (GSFC) two-dimensional model of stratospheric photochemistry and dynamics has been used to calculate the O3 response to stratospheric aircraft (high-speed civil transport (HSCT)) emissions. The sensitivity of the model O3 response was examined for systematic variations of five parameters and two reaction rates over a wide range, expanding on calculations by various modeling groups for the NASA High Speed Research Program and the World Meteorological Organization. In all, 448 model runs were required to test the effects of variations in the latitude, altitude, and magnetitude of the aircraft emissions perturbation, the background chlorine levels, the background sulfate aerosol surface area densities, and the rates of two key reactions. No deviation from previous conclusions concerning the response of O3 to HSCTs was found in this more exhaustive exploration of parameter space. Maximum O3 depletions occur for high-altitude, low altitude HSCT perturbations. Small increases in global total O3 can occur for low-altitude, high-altitude injections. Decreasing aerosol surface area densities and background chlorine levels increases the sensitivity of model O3 to the HSCT perturbations. The location of the aircraft emissions is the most important determinant of the model response. Response to the location of the HSCT emissions is not changed qualitatively by changes in background chlorine and aerosol loading. The response is also not very sensitive to changes in the rates of the reactions NO + HO2 yields NO2 + OH and HO2 + O3 yields OH + 2O2 over the limits of their respective uncertainties. Finally, levels of lower stratospheric HO(sub x) generally decrease when the HSCT perturbation is included, even though there are large increases in H2O due to the perturbation.

Considine, David B.↗
NASA NTRS1994

Heterogeneous Reaction of ClONO2(g) + NaCl(s) to Cl2(g) + NaNO3(s)

The heterogeneous reaction of ClON02 + NaCl yields Cl2 + NaNO3 (eq 1) was investigated over a temperature range 220-300 K in a flow-tube reactor interfaced with a differentially pumped quadrupole mass spectrometer. Partial pressures of ClON02 in the range 10(exp -8) - 10(exp -5) Torr were used. Granule sizes and surface roughness of the NaCl substrates were determined by using a scanning electron microscope, and in separate experiments, surface areas of the substrates were measured by using BET analysis of gas-adsorption isotherms. For dry NaCl substrates, both the decay rates of ClON02 and the growth rates Of C12 were used to obtain reaction probabilities, gamma(sub l) = (4.6 +/- 3.0) x 10(exp -3) at 296 K and (6.7 +/- 3.2) x 10(exp -1) at 225 K, after considering the internal surface area, The error bars represent 1 standard deviation. The Cl2 yield based on the ClONO2 reacted was measured to be 1.0 +/- 0.2. In order to mimic the conditions encountered in the lower stratosphere, the effect of water vapor pressures between 5 x 10(exp -5) and 3 x 10(exp -4) Torr on reaction 1 was also studied. With added H20, reaction probabilities, gamma = (4.1 +/- 2.1) x 10(exp -3) at 296 K and (4.7 +/- 2.9) x 10(exp -3) at 225 K, were obtained. A trace of HOCl, the reaction product from the ClON02 + H20 yield HOCl + HN03 reaction, was observed in addition to the C12 product from reaction 1. The implications of this result for the enhancement of hydrogen chloride in the stratosphere after the El Chichon volcanic eruption and for the marine troposphere are discussed.

Timonen, Raimo S.↗
NASA NTRS2003

Stereology techniques in radiation biology

Clinicians involved in conventional radiation therapy are very concerned about the dose-response relationships of normal tissues. Before proceeding to new clinical protocols, radiation biologists involved with conformal proton therapy believe it is necessary to quantify the dose response and tolerance of the organs and tissues that will be irradiated. An important focus is on the vasculature. This presentation reviews the methodology and format of using confocal microscopy and stereological methods to quantify tissue parameters, cell number, tissue volume and surface area, and vessel length using the microvasculature as a model tissue. Stereological methods and their concepts are illustrated using an ongoing study of the dose response of the microvessels in proton-irradiated hemibrain. Methods for estimating the volume of the brain and the brain cortex, the total number of endothelial cells in cortical microvessels, the length of cortical microvessels, and the total surface area of cortical microvessel walls are presented step by step in a way understandable for readers with little mathematical background. It is shown that stereological techniques, based on a sound theoretical basis, are powerful and reliable and have been used successfully.

Non-NASA Center↗
NASA NTRS2017

The Lightweight Integrated Solar Array and anTenna (LISA-T) Big Power for Small Spacecraft

NASA is developing a space power system using lightweight, flexible photovoltaic devices originally developed for use here on Earth to provide low cost power for spacecraft. The Lightweight Integrated Solar Array and anTenna (LISA-T) is a launch stowed, orbit deployed array on which thin-film photovoltaic and antenna elements are embedded. The LISA-T system is deployable, building upon NASA's expertise in developing thin-film deployable solar sails such the one being developed for the Near Earth Asteroid Scout project which will fly in 2018. One of the biggest challenges for the NEA Scout, and most other spacecraft, is power. There simply isn't enough of it available, thus limiting the range of operation of the spacecraft from the Sun (due to the small surface area available for using solar cells), the range of operation from the Earth (low available power with inherently small antenna sizes tightly constrain the bandwidth for communication), and the science (you can only power so many instruments with limited power). The LISA-T has the potential to mitigate each of these limitations, especially for small spacecraft. Inherently, small satellites are limited in surface area, volume, and mass allocation; driving competition between their need for power and robust communications with the requirements of the science or engineering payload they are developed to fly. LISA-T is addressing this issue, deploying large-area arrays from a reduced volume and mass envelope - greatly enhancing power generation and communications capabilities of small spacecraft and CubeSats. The problem is that these CubeSats can usually only generate between 7W and 50W of power. The power that can be generated by the LISA-T ranges from tens of watts to several hundred watts, at a much higher mass and stowage efficiency. A matrix of options are in development, including planar (pointed) and omnidirectional (non-pointed) arrays. The former is seeking the highest performance possible while the latter is seeking GN&C simplicity. Options for leveraging both high performance, 'typical cost' triple junction thin-film solar cells as well as moderate performance, low cost cells are being developed. Alongside, UHF (ultrahigh frequency), S-band, and X-band antennas are being integrated into the array to move their space claim away from the spacecraft and open the door for more capable multi-element antenna designs such as those needed for spherical coverage and electronically steered phase arrays.

Space Exploration↗
NASA NTRS2004

Impregnation of Catalytic Metals in Single-Walled Carbon Nanotubes for Toxic Gas Conversion in Life Support System

Carbon nanotubes (CNTs) possess extraordinary properties such as high surface area, ordered chemical structure that allows functionalization, larger pore volume, and very narrow pore size distribution that have attracted considerable research attention from around the world since their discovery in 1991. The development and characterization of an original and innovative approach for the control and elimination of gaseous toxins using single walled carbon nanotubes (SWNTs) promise superior performance over conventional approaches due to the ability to direct the selective uptake of gaseous species based on their controlled pore size, increased adsorptive capacity due to their increased surface area and the effectiveness of carbon nanotubes as catalyst supports for gaseous conversion. We present our recent investigation of using SWNTs as catalytic supporting materials to impregnate metals, such as rhodium (Rh), palladium (Pd) and other catalysts. A protocol has been developed to oxidize the SWNTs first and then impregnate the Rh in aqueous rhodium chloride solution, according to unique surface properties of SWNTs. The Rh has been successfully impregnated in SWNTs. The Rh-SWNTs have been characterized by various techniques, such as TGA, XPS, TEM, and FTIR. The project is funded by a NASA Research Announcement Grant to find applications of single walled nanocarbons in eliminating toxic gas Contaminant in life support system. This knowledge will be utilized in the development of a prototype SWNT KO, gas purification system that would represent a significant step in the development of high efficiency systems capable of selectively removing specific gaseous for use in regenerative life support system for human exploration missions.

Li, Jing↗
NASA NTRS1975

Water Hyacinths for Upgrading Sewage Lagoons to Meet Advanced Wastewater Treatment Standards, Part 1

Water hyacinths, Eichhornia crassipes Mart. Solms, have demonstrated the ability to function as an efficient and inexpensive final filtration system in a secondary domestic sewage lagoon during a three month test period. These plants reduced the suspended solids, biochemical oxygen demanding substances, and other chemical parameters to levels below the standards set by the state pollution control agency. The water hyacinth-covered secondary lagoon utilized in this experiment had a surface area of 0.28 hectare (0.70 acre) with a total capacity of 6.8 million liters (1.5 million gallons), receiving an inflow of 522,100 liters (115,000 gallons) per day from a 1.1 hectare (3.8 acre) aerated primary sewage lagoon. These conditions allowed a retention time of 14 to 21 days depending on the water hyacinth evapotranspiration rates. The desired purity of final sewage effluent can be controlled by the water hyacinth surface area, harvest rate, and the retention time.

Wolverton, B. C.↗
NASA NTRS1992

Localized sources of propagating acoustic waves in the solar photosphere

A time series of Doppler measurements of the solar photosphere with moderate spatial resolution is described which covers a portion of the solar disk surrounding a small sunspot group. At temporal frequencies above 5.5 mHz, the Doppler field probes the spatial and temporal distribution of regions that emit acoustic energy. In the frequency range between 5.5 and 7.5 mHz, inclusive, a small fraction of the surface area emits a disproportionate amount of acoustic energy. The regions with excess emission are characterized by a patchy structure at spatial scales of a few arcseconds and by association (but not exact co-location) with regions having substantial magnetic field strength. These observations bear on the conjecture that most of the acoustic energy driving solar p-modes is created in localized regions occupying a small fraction of the solar surface area.

Brown, Timothy M.↗
NASA NTRS2002

Evaluation of SAGE II and Balloon-Borne Stratospheric Aerosol Measurements: Evaluation of Aerosol Measurements from SAGE II, HALOE, and Balloonborne Optical Particle Counters

Stratospheric aerosol measurements from the University of Wyoming balloonborne optical particle counters (OPCs), the Stratospheric Aerosol and Gas Experiment (SAGE) II, and the Halogen Occultation Experiment (HALOE) were compared in the period 1982-2000, when measurements were available. The OPCs measure aerosol size distributions, and HALOE multiwavelength (2.45-5.26 micrometers) extinction measurements can be used to retrieve aerosol size distributions. Aerosol extinctions at the SAGE II wavelengths (0.386-1.02 micrometers) were computed from these size distributions and compared to SAGE II measurements. In addition, surface areas derived from all three experiments were compared. While the overall impression from these results is encouraging, the agreement can change with latitude, altitude, time, and parameter. In the broadest sense, these comparisons fall into two categories: high aerosol loading (volcanic periods) and low aerosol loading (background periods and altitudes above 25 km). When the aerosol amount was low, SAGE II and HALOE extinctions were higher than the OPC estimates, while the SAGE II surface areas were lower than HALOE and the OPCS. Under high loading conditions all three instruments mutually agree to within 50%.

Hervig, Mark↗
NASA NTRS2024

Post-Synthetic Modification of Yttria-Stabilized Zirconia Aerogels with Silica Coatings for Enhanced Thermal Stability

Maintaining high surface area and porosity at high temperatures is important when considering aerogels for use in thermal management systems. The mesoporous structure of aerogels results in extremely low thermal conductivity, making them lightweight, high performance insulating materials. Maintaining these properties requires innovative routes to suppress sintering and pore collapse as use temperatures rise. The current work aims to improve the pore structure stability of yttria-stabilized zirconia aerogels by the addition of a SiO 2 coating. The functionalization of surface hydroxyl groups by the addition of SiO 2 is hypothesized to mitigate condensation reactions, which are a driving force for shrinkage and pore structure collapse. Zirconia aerogels with 0, 10 and 30 mol% yttria (YO 1.5 ) additions were coated in a tetraethyl orthosilicate (TEOS) solution and exposed to temperatures up to 1200°C. Crystal structure, pore structure, and aerogel morphology were investigated to understand changes in aerogel thermal stability. The SiO 2 coating exhibited a greater influence on pore stability over yttria concentration, with specific surface area of the coated aerogels being twice that of the uncoated aerogels up to 1000°C. However, the presence of the SiO 2 coating promoted rapid sintering and densification at 1200°C, establishing an upper use temperature for SiO 2 and a need to develop other coating chemistries.

aerogel↗
NASA NTRS2005

Carbon-Nanotube-Based Electrochemical Double-Layer Capacitor Technologies for Spaceflight Applications

Electrochemical double-layer capacitors, or supercapacitors, have tremendous potential as high-power energy sources for use in low-weight hybrid systems for space exploration. Electrodes based on single-wall carbon nanotubes (SWCNTs) offer exceptional power and energy performance due to the high surface area, high conductivity, and the ability to functionalize the SWCNTs to optimize capacitor properties. This paper will report on the preparation of electrochemical capacitors incorporating SWCNT electrodes and their performance compared with existing commercial technology. Preliminary results indicate that substantial increases in power and energy density are possible. The effects of nanotube growth and processing methods on electrochemical capacitor performance is also presented. The compatibility of different SWCNTs and electrolytes was studied by varying the type of electrolyte ions that accumulate on the high-surface-area electrodes.

Arepalli, S.↗
NASA NTRS2005

Formation of a Tropopause Cirrus Layer Observed over Florida during CRYSTAL-FACE

On July 13, 2002 a widespread, subvisible tropopause cirrus layer occurred over the Florida region. This cloud was observed in great detail with the NASA Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment (CRYSTAL-FACE) instrumentation, including in situ measurements with the WB-57 aircraft. In this paper, we use the 13 July cloud as a case study to evaluate the physical processes controlling the formation and evolution of tropopause cirrus layers. Microphysics measurements indicate that ice crystal diameters in the cloud layer ranged from about 7 to 50 microns, and the peak number mode was about 10-25 microns. In situ water vapor and temperature measurements in the cloud indicated supersaturation with respect to ice throughout, with ice saturation ratios as large as 1.8. Even when the ice surface area density was as high as about 500 sq microns/cu cm, ice supersaturations of 20-30% were observed. Trajectory analysis shows that the air sampled near the tropopause on this day generally came from the north and cooled considerably during the previous few days. Examination of infrared satellite imagery along air parcel back trajectories from the WB-57 flight track indicates that the tropopause cloud layer formation was, in general, not simply left over ice from recently generated anvil cirrus. Simulations of cloud formation using time-height curtains of temperature along the trajectory paths show that the cloud could have formed in situ near the tropopause as the air was advected into the south Florida region and cooled to unusually low temperatures. If we assume a high threshold for ice nucleation via homogeneous freezing of aqueous sulfate aerosols, the model reproduces the observed cloud structure, ice crystal size distributions, and ice supersaturation statistics. Inclusion of observed gravity wave temperature perturbations in the simulations is essential to reproduce the observed cloud properties. Without waves, crystal number densities are too low, crystal sizes are too large, and the crystals fall out too fast, leaving very little cloud persisting at the end of the simulations. In the cloud simulations, coincidence of high supersaturations and high surface areas can be produced by either recent nucleation or sedimentation of crystals into supersaturated layers. The agreement between model results and observed supersaturations is improved somewhat if we assume that the steady state relative humidity within cirrus at T<200 K is enhanced by about 30%. The WB-57 measurements and the model results suggest that the cloud layer irreversibly dehydrated air near the tropopause.

Jensen, Eric↗