J. A. Curcio

The Near Infrared Absorption Spectrum of Liquid Water

The near infrared absorption spectrum of liquid water at 20°C has been reinvestigated using a PbS cell detector system. The total spectral range investigated was from 0.70 to 2.50μ. A curve is included which shows five prominent absorption bands at 0.76, 0.97, 1.19, 1.45, and 1.94μ; and a table gives experimental results of water absorption at 20°C.

Evaluation of Atmospheric Aerosol Particle Size Distribution from Scattering Measurements in the Visible and Infrared

The atmospheric aerosol particle size distribution was examined using measured spectral scattering coefficients in the wavelength region 0.40–2.27 μ. It was found that the aerosol on a particular day can be represented by a two-component composite, the main component having a Junge distribution of the form dN/d logr = Cr−n and the other having either a distribution of larger particles similar to that of aerosols found in maritime air or a relatively monodisperse distribution contained in a narrow radius interval. This study shows that estimates of aerosol size distribution may be in error if based only on attenuation measurements in the visible region.

An Atlas of the Absorption Spectrum of the Lower Atmosphere from 5400Å to 8520Å

This paper presents microphotometer traces which show, as film density vs wavelength, the absorption spectrum of an uncontaminated 16-km path in the lower atmosphere. The spectra were recorded with a small grating spectrograph over the spectral range from 5400A to 8520A with a resolution of about 0.2A. Wavelengths are presented for the absorption lines, and most lines are identified by absorbing species which are considered to be either water vapor or molecular oxygen.

Correlation of Atmospheric Transmission with Backscattering

This is a report of experimental work performed during the period of September 1956 to June 1957 on the correlation of backscattering with atmospheric transmission. Measurements were made for a variety of conditions where the meteorological range varied from less than 0.10 mile to more than 40 miles in atmospheres which were free of industrial pollution. Analysis of the data shows the following relationship: V=C/S1.5,where V=meteorological range, C=constant, and S=backscattered signal. The point spread about the curve indicates that meteorological range can be determined from the backscattered signal with an accuracy of 20% for all visibilities in the ranges studied.

Atmospheric Scattering Coefficients in the Visible and Infrared Regions

Atmospheric spectral attenuation coefficients have been measured in ten narrow wavelength bands between 0.4 and 2.3 μ for a variety of weather conditions for two overwater, sea-level paths of 5.5 and 16.3 km. The wavelength bands were chosen so as to avoid molecular absorption and were isolated by interference filters. A 60-in.-diameter high-intensity source and a 24-in.-diameter narrow-field receiver were combined to yield relative scattering attenuation coefficients (σ) as a function of wavelength λ. These were then scaled using values obtained at one wavelength with a visual telephotometer. Logσ vs logλ curves show a wide range of slopes and shapes, with a tendency toward less slope in the infrared (indicating that σ is becoming independent of λ in the infrared). Some correlation with relative humidity was found for relative humidities greater than 70%. An anomalous slope reversal between 1.68 and 2.27 μ is discussed, and a possible explanation for the reversal is given as selective scattering by the aerosol at these wavelengths.

The Influence of Field of View on Measurements of Atmospheric Transmission

When transmittance of the atmosphere is determined by comparing the irradiancy of a distant light source to the calculated value for the irradiancy in vacuum, the result obtained is dependent on the field of view of the receiver. Measurements of this dependence over water have been made at sea level. For an uncollimated source, the results for any wavelength in the visible and near ultraviolet regions of the spectrum are represented approximately by the relation Tθ=T+0.5(1−T)(1−e−θ),where Tθ is the transmittance at a particular wavelength measured with an instrument having a field of view θ radians in diameter, and T is the transmittance which would be obtained if unscattered light alone were accepted by the measuring equipment.Observations have been made in several wavelength regions between 3600A and 6235A for atmospheres in which the transmission for visible light was between 0.35 and 0.85 per sea mile. Two-mile and nine-mile optical paths were used. Observations of Tθ through the nine-mile path were made with a fixed field of view 13.5 degrees in diameter, and those through the two-mile path were made with several values of θ between 5° and 25°.

Measurements of spectral radiance of the horizon sky.

Typical spectral radiance data are presented for a rectangular portion of the horizon sky under various weather conditions and at different solar positions. Representative curves were selected from a total of seventy-nine spectral radiance measurements which covered meteorological ranges (at 665 mmu) from 10 km to 106 km and solar altitudes from -6 degrees to 74 degrees . The north horizon at noon with a 30-km or greater meteorological range typically produced a peak spectral radiance of 5 microW cm(-2)sr(-1)mmu(-1). The maximum spectral radiance value obtained in the study was 38 microW cm(-2)sr(-1)mmu (-1) for the south horizon in winter. The reciprocal dispersion varied from 1.8 mmu/mm at 400 mmu to approximately 8 mmu/mm at 1000 mmu. A mixture of fog and haze produced a spectral radiance curve with two maxima.

An Experimental Study of Atmospheric Transmission

An experimental investigation was made to determine the general characteristics of the spectral transmission of the atmosphere in the vicinity of Washington, D. C, on the Chesapeake Bay, in the Gulf of Mexico, and in the Central Pacific. Transmission measurements were made at the wavelengths of approximately 15 of the Hg discharge lines in the interval 2500A to 6000A. Values of the spectral atmospheric attenuation coefficients (km−1) have been computed. For all observations made, these values increase toward shorter wavelengths and this is most pronounced for wavelengths below 3200A, where true absorption as well as scattering contributes to the attenuation.

Transmission of Infrared Radiation from Flames by CO 2

The transmission of infrared radiation from flames through various pathlengths of CO(2) was determined in the laboratory. The results show that a considerable amount of flame energy is transmitted in the regions 2.5 micro to 3.5 micro and 4.0 micro to 5.5 micro. For example, a cell length of 1500 cm of CO(2) at 760-mm pressure, equivalent to the CO(2) content in 50,000 m of atmosphere at sea level, transmitted 20% of the energy in the band region from 4.0 micro to 5.5 micro. Integrated transmittances for the bands at various pressures and cell lengths are presented in tabular form.

Measurements of Ultraviolet Spectral Radiance of the Horizon Sky

The spectral radiance of the horizon sky between 3000 A and 4200 A was measured in a maritime environment under clear and cloudy sky conditions. The spectral radiance was found to be mainly a function of the position of the sun and of the amount of cloud cover. A typical value for the spectral radiance at 3600 A at noon with clear sky is 5 microW cm(-2) sr(-2) nm(-1).