Twilight is characterised by changes in both quantity ("irradiance") and quality ("colour") of light. Animals use the variation in irradiance to adjust their internal circadian clocks, aligning their ...behaviour and physiology with the solar cycle. However, it is currently unknown whether changes in colour also contribute to this entrainment process. Using environmental measurements, we show here that mammalian blue-yellow colour discrimination provides a more reliable method of tracking twilight progression than simply measuring irradiance. We next use electrophysiological recordings to demonstrate that neurons in the mouse suprachiasmatic circadian clock display the cone-dependent spectral opponency required to make use of this information. Thus, our data show that some clock neurons are highly sensitive to changes in spectral composition occurring over twilight and that this input dictates their response to changes in irradiance. Finally, using mice housed under photoperiods with simulated dawn/dusk transitions, we confirm that spectral changes occurring during twilight are required for appropriate circadian alignment under natural conditions. Together, these data reveal a new sensory mechanism for telling time of day that would be available to any mammalian species capable of chromatic vision.
A number of inorganic (nitryl chloride, ClNO2; chlorine,
Cl2; and hypochlorous acid, HOCl) and chlorinated, oxygenated volatile
organic compounds (ClOVOCs) have been measured in Manchester, UK during
...October and November 2014 using time-of-flight chemical ionisation mass
spectrometry (ToF-CIMS) with the I− reagent ion. ClOVOCs appear to be
mostly photochemical in origin, although direct emission from vehicles is
also suggested. Peak concentrations of ClNO2, Cl2 and HOCl reach
506, 16 and 9 ppt respectively. The concentrations of ClNO2 are
comparable to measurements made in London, but measurements of ClOVOCs,
Cl2 and HOCl by this method are the first reported in the UK. Maximum
HOCl and Cl2 concentrations are found during the day and ClNO2
concentrations remain elevated into the afternoon if photolysis rates are
low. Cl2 exhibits a strong dependency on shortwave radiation, further
adding to the growing body of evidence that it is a product of secondary
chemistry. However, night-time emission is also observed. The contribution of
ClNO2, Cl2 and ClOVOCs to the chlorine radical budget suggests
that Cl2 can be a greater source of Cl than ClNO2, contributing
74 % of the Cl radicals produced on a high radiant-flux day. In contrast,
on a low radiant-flux day, this drops to 14 %, as both Cl2 production
and loss pathways are inhibited by reduced photolysis rates. This results in
ClNO2 making up the dominant fraction (83 %) on low radiant-flux
days, as its concentrations are still high. As most ClOVOCs appear to be formed
photochemically, they exhibit a similar dependence on photolysis,
contributing 3 % of the Cl radical budget observed here.
We describe and validate a Monte Carlo model to track
photons over the full range of solar wavelengths as they travel into
optically thick Antarctic blue ice. The model considers both reflection and
...transmission of radiation at the surface of blue ice, scattering by air
bubbles within it, and spectral absorption due to the ice. The ice surface is treated as planar whilst bubbles are considered to be spherical scattering
centres using the Henyey–Greenstein approximation. Using bubble radii and
number concentrations that are representative of Antarctic blue ice, we
calculate spectral albedos and spectrally integrated downwelling and
upwelling radiative fluxes as functions of depth and find that, relative to
the incident irradiance, there is a marked subsurface enhancement in the
downwelling flux and accordingly also in the mean irradiance. This is due to the interaction between the refractive air–ice interface and the scattering interior and is particularly notable at blue and UV wavelengths which correspond to the minimum of the absorption spectrum of ice. In contrast the absorption path length at IR wavelengths is short and consequently the attenuation is more complex than can be described by a simple Lambert–Beer style exponential decay law – instead we present a triple-exponential fit to the net irradiance against depth. We find that there is a moderate dependence on the solar zenith angle and surface conditions such as altitude and cloud optical depth. Representative broadband albedos for blue ice are calculated in the range from 0.585 to 0.621. For macroscopic absorbing inclusions we observe both geometry- and size-dependent self-shadowing that reduces the
fractional irradiance incident on an inclusion's surface. Despite this, the
inclusions act as local photon sinks and are subject to fluxes that are
several times the magnitude of the single-scattering contribution. Such
enhancement may have consequences for the energy budget in regions of the
cryosphere where particulates are present near the surface. These results
also have particular relevance to measurements of the internal radiation
field: account must be taken of both self-shadowing and the optical effect
of introducing the detector. Turning to the particular example of englacial
meteorites, our modelling predicts iron meteorites to reside at much reduced
depths than previously suggested in the literature (< 10 cm vs.
∼ 40 cm) and further shows a size dependency that may explain
the observed bias in their Antarctic size distribution.
•Google Street View images can be used to obtain urban canyon geometries worldwide.•Hugin software was used to reconstruct fisheye polar projections of urban canyons.•Rayman software was used to ...model total shortwave irradiances at reconstructed canyon.•Models combined with full-sky measurements returned good approximations to measured canyon irradiances.•Proposed procedures approximate urban sky irradiances for all sky conditions.
This paper presents an implementation of digital image processing tools, in order to reconstruct building geometries and urban sky view factors for individual street canyons around the world. By using the Google Street View image database, it is possible to reconstruct canyon fisheye images at any point where a Google panorama is available. The Hugin software allows the geometrically correct merging of independent images in order to create a fisheye projection, covering the upper hemispheric view of the sky. Sky view factor calculations may then be performed by specialised software on the reconstructed fisheye image. In order to analyse the fitness of reconstructed panoramas, control images taken with a fisheye lens camera were used for comparison. The Rayman model was then used to calculate total shortwave global irradiances from the restricted canyon view and from a full hemispheric view. Finally, total shortwave global irradiances were measured for the same position in the street canyon and compared against observatory measurements under a full sky view, this in order to evaluate the performance and usefulness of idealised calculations. The reconstructed urban geometries returned acceptable fine-scale descriptions, and their potential utility for scientific purposes in the atmospheric sciences was demonstrated with the examples provided.
Long-term trends of total column ozone (TCO), assessments of stratospheric ozone recovery, and satellite validation are underpinned by a reliance on daily best representative values from Brewer ...spectrophotometers and other ground-based ozone instruments. In turn reporting of these daily total column ozone values to the World Ozone and Ultraviolet Radiation Data Centre (WOUDC) has traditionally been predicated upon a simple choice between direct sun (DS) and zenith sky (ZS) observations. For mid- and high-latitude monitoring sites impacted by cloud cover we discuss the potential deficiencies of this approach in terms of its rejection of otherwise valid observations and capability to evenly sample throughout the day. A new methodology is proposed that makes full use of all valid direct sun and zenith sky observations, accounting for unevenly spaced observations and their relative uncertainty, to calculate an improved estimate of the daily mean total column ozone. It is demonstrated that this method can increase the number of contributing observations by a factor of 2.5, increases the sampled time span, and reduces the spread of the representative time by half. The largest improvements in the daily mean estimate are seen on days with the smallest number of contributing direct sun observations. No effect on longer-term trends is detected, though for the sample data analysed we observe a mean increase of 2.8 DU (0.82 %) with respect to the traditional direct sun vs. zenith sky average choice. To complement the new calculation of a best representative value of total column ozone and separate its uncertainty from the spread of observations, we also propose reporting its standard error rather than the standard deviation, together with measures of the full range of values observed.
The Winchcombe fireball—That lucky survivor McMullan, Sarah; Vida, Denis; Devillepoix, Hadrien A. R. ...
Meteoritics & planetary science,
05/2024, Letnik:
59, Številka:
5
Journal Article
Recenzirano
Odprti dostop
Abstract On February 28, 2021, a fireball dropped ∼0.6 kg of recovered CM2 carbonaceous chondrite meteorites in South‐West England near the town of Winchcombe. We reconstruct the fireball's ...atmospheric trajectory, light curve, fragmentation behavior, and pre‐atmospheric orbit from optical records contributed by five networks. The progenitor meteoroid was three orders of magnitude less massive (∼13 kg) than any previously observed carbonaceous fall. The Winchcombe meteorite survived entry because it was exposed to a very low peak atmospheric dynamic pressure (∼0.6 MPa) due to a fortuitous combination of entry parameters, notably low velocity (13.9 km s −1 ). A near‐catastrophic fragmentation at ∼0.07 MPa points to the body's fragility. Low entry speeds which cause low peak dynamic pressures are likely necessary conditions for a small carbonaceous meteoroid to survive atmospheric entry, strongly constraining the radiant direction to the general antapex direction. Orbital integrations show that the meteoroid was injected into the near‐Earth region ∼0.08 Myr ago and it never had a perihelion distance smaller than ∼0.7 AU, while other CM2 meteorites with known orbits approached the Sun closer (∼0.5 AU) and were heated to at least 100 K higher temperatures.
Two radiative transfer models are presented that simplify calculations of street canyon spectral irradiances with minimum data input requirements, allowing better assessment of urban exposures than ...can be provided by standard unobstructed radiation measurements alone. Fast calculations improve the computational performance of radiation models, when numerous repetitions are required in time and location. The core of the models is the calculation of the spectral diffuse-to-global ratios (DGR) from an unobstructed global spectral measurement. The models are based on, and have been tested against, outcomes of the SMARTS2 algorithm (i.e. Simple Model of the Atmospheric Radiative Transfer of Sunshine). The modelled DGRs can then be used to partition global spectral irradiance values into their direct and diffuse components for different solar zenith angles. Finally, the effects of canyon obstructions can be evaluated independently on the direct and diffuse components, which are then recombined to give the total canyon irradiance. The first model allows ozone and aerosol inputs, while the second provides a further simplification, restricted to average ozone and aerosol contents but specifically designed for faster calculations. To assess the effect of obstructions and validate the calculations, a set of experiments with simulated obstructions (simulated canyons) were performed. The greatest source of uncertainty in the simplified calculations is in the treatment of diffuse radiation. The measurement-model agreement is therefore dependent on the region of the sky obscured and ranges from <5 % at all wavelengths to 20–40 % (wavelength dependent) when diffuse sky only is visible from the canyon.
Summary
Purpose: To determine the potential risk of epileptic seizures from wind turbine shadow flicker under various meteorologic conditions.
Methods: We extend a previous model to include ...attenuation of sunlight by the atmosphere using the libradtran radiative transfer code.
Results: Under conditions in which observers look toward the horizon with their eyes open we find that there is risk when the observer is closer than 1.2 times the total turbine height when on land, and 2.8 times the total turbine height in marine environments, the risk limited by the size of the image of the sun’s disc on the retina. When looking at the ground, where the shadow of the blade is cast, observers are at risk only when at a distance <36 times the blade width, the risk limited by image contrast. If the observer views the horizon and closes their eyes, however, the stimulus size and contrast ratio are epileptogenic for solar elevation angles down to approximately 5°.
Discussion: Large turbines rotate at a rate below that at which the flicker is likely to present a risk, although there is a risk from smaller turbines that interrupt sunlight more than three times per second. For the scenarios considered, we find the risk is negligible at a distance more than about nine times the maximum height reached by the turbine blade, a distance similar to that in guidance from the United Kingdom planning authorities.
This study describes a dual-channel array spectrometer system designed to make high-frequency simultaneous spectral global irradiance and direct solar irradiance measurements covering the visible and ...ultraviolet wavelength ranges. The dual-channel nature of the instrument allows spectrally integrated quantities (e.g., erythema or vitamin D) to be calculated at a rate similar to broadband instruments while retrieving total column ozone (TCO) from the direct solar channel. The characterization and calibration of the instrument is discussed, with emphasis on temperature stabilization (< plus or minus 0.01 degree C) and stray light removal. Focusing on the TCO retrieval from direct spectra, results are compared to a collocated Brewer spectrophotometer during the study period of May 2013-January 2014. Agreement for individual measurements made within 20 min of a reference Brewer direct sun observation on relatively clear example days is <1.5%. For all valid individual measurements, the study found an overall bias of 1.1 Dobson units (DU; 0.4%) and scatter of plus or minus 6.7 DU (2.2%) for retrievals obtained at airmass values < 4. A dependence on air mass of 6.3 DU (2.0%) per airmass unit is observed and a correlation of R super(2) = 0.954 is found for all individual measurements, although this is reduced to 0.908 for daily means. TCO retrievals are limited to airmass values < 4 primarily because of residual structure in the transmission spectrum that cannot be attributed to other trace gases. These results are encouraging and suggest that similar instrument designs could make a significant and relatively low-cost contribution to surface measurements of atmospheric radiation.
Aerosols play an important role in key atmospheric processes and feature high
spatial and temporal variabilities. This has motivated scientific interest in
the development of networks capable of ...measuring aerosol properties over
large geographical areas in near-real time. In this work we present and
discuss results of an aerosol optical depth (AOD) algorithm applied to
instruments of the European Brewer Network. This network is comprised of
close to 50 Brewer spectrophotometers, mostly located in Europe and adjacent
areas, although instruments operating at, for example, South America and Australia are
also members. Although we only show results for instruments calibrated by the
Regional Brewer Calibration Center for Europe, the implementation of the AOD
algorithm described is intended to be used by the whole network in the
future. Using data from the Brewer intercomparison campaigns in the years
2013 and 2015, and the period in between, plus comparisons with Cimel
sun photometers and UVPFR instruments, we check the precision, stability, and
uncertainty of the Brewer AOD in the ultraviolet range from 300 to
320 nm. Our results show a precision better than 0.01, an uncertainty
of less than 0.05, and, for well-maintained instruments, a stability similar to that of the ozone measurements. We also discuss future improvements to our
algorithm with respect to the input data, their processing, and the
characterization of the Brewer instruments for the measurement of AOD.
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