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Sports and Outdoors Meet the Reader. Want to support independent local journalism? Scotland sees the most snow in the UK, with snow or sleet falling on The weather station which recorded the most snowfall in the UK was the Cairngorm Chairlift, with snow falling on Part of the enduring appeal of snowflakes is their intricate appearance and near-infinite variation, meaning that all snowflakes are unique. The number of possibilities of nuances in temperature and humidity as the snowflake falls to the ground is limitless.
If you look closely at a snowflake you will see countless individual features, all having formed ever so slightly differently in direction or shape, owing to the slightest change in the environment in which it formed.
While snowflakes appear white as they fall through the sky or as they accumulate on the ground as snowfall, they are actually totally clear. The ice though is not transparent like a sheet of glass is but rather is translucent, meaning light only passes through indirectly. The many sides of the ice crystals cause diffuse reflection of the whole light spectrum which results in snowflakes appearing to be white in colour.
Most snow falls at a speed of between 1 - 4 mph dependent upon the individual snowflake's mass and surface area, as well as the environmental conditions surrounding its descent. Snowflakes which collect supercooled water as they fall can fall at up to 9 mph, but snowflakes, as most people recognise them, will tend to float down at around 1. The first person to capture a photograph of a snowflake was a farmer from the small town of Jericho in Vermont, US.
After years of experimenting with connecting microscopes to a bellows camera, in Wilson Bentley succeeding in capturing the first ever snowflake photograph. Bauer , , N. Viltard , , D. Johnson , , W-K. Meneghini , , and L. Part I: Model formulation and comparison with observations. Ralph , F. Nieman , , D. Smith , P. Steiner , M. Bousquet , , R. Smull , , and M.
Mancini , : Airflow within major Alpine river valleys under heavy rainfall. Stewart , R. Marwitz , , J. Pace , , and R. Carbone , : Characteristics through the melting layer of stratiform clouds. Stoelinga , M. Tao , W-K. Taylor , G. Oregon State University Press, pp. Army Corps of Engineers , : Summary report of the snow investigations—Snow hydrology.
North Pacific Division Rep. Waldvogel , A. Weber , B. Wuertz , , D. Welsh , , and R. White , A. Jordan , , B. Martner , , F. Ralph , , and B. Bartram , : Extending the dynamic range of an S-band radar for cloud and precipitation studies. Willis , P. Heymsfield , : Structure of the melting layer in mesoscale convective system stratiform precipitation. Zikmunda , J.
Accumulated joint size and fall speed distributions of observed particles after quality control is applied section 2 : a rain-only event at McKenzie Bridge between and UTC 17 Dec and b dry-snow event at Storm Peak from to UTC 27 Feb During the mixed-precipitation event, the time periods A, B, and C correspond to precipitation samples when air temperature decreased from 1. As in Fig. Times represented in a — c correspond to intervals A, B, and C defined by temperature in Fig.
The time periods A, B, and C are as shown in Fig. The intensity of the radar bright band varies during this example. Horizontal bands of blue and green correspond to increasing minimum detectable reflectivity with increasing height. The signal was attenuated by snow accumulation on the radar antenna during portions of the time period shown.
Wind barbs point into the wind. Application of masks is to data in Fig. Note that x -axis scale differs between plots. Note that x -axis scale differs between left and right columns. The y axis is at same relative scale 3. Datasets as in Fig. Accumulated fall velocity distributions n V for datasets as in Fig.
Coexisting rain and snow particles are distinguished using a classification method based on their size and fall speed properties. As air temperatures increase to 1. In this setting, very large raindrops appear to be the result of aggregrates melting with minimal breakup rather than formation by coalescence. In contrast to dry snow and rain, the fall speed for wet snow has a much weaker correlation between increasing size and increasing fall speed.
The large standard deviation is likely related to the coexistence of particles of similar physical size with different percentages of melting. These results suggest that different particle sizes are not required for aggregation since wet-snow particles of the same size can have different fall speeds.
Given the large standard deviation of fall speeds in wet snow, the collision efficiency for wet snow is likely larger than that of dry snow. As air temperatures increased above 0.
The value of 0. Corresponding author address: Prof. Sandra Yuter, Dept. An accurate description of the physical characteristics of coexisting rain and snow near the freezing level is important not only to cloud and regional forecast models Tao et al. The assumptions about the particle size distributions PSD and fall speed relations used in bulk microphysics parameterizations are similar to those required to determine the absorption, scattering, and extinction coefficients within radiative transfer calculations.
For the purposes of this paper, we will use the term wet snow to refer to partially melted aggregates of snowflakes and the term mixed precipitation to refer to coexisting rain and wet-snow mixtures excluding graupel or hail. Melting natural and artificial aggregates of snow crystals have been investigated in the laboratory Mitra et al. However, examination of naturally occurring populations of melting snowflake aggregates has been lacking because of the practical difficulties of distinguishing among the coexisting particle distributions of rain and snow.
Dual-polarization radar measurements can distinguish between rain-only and mixed-phase precipitation, but these methods have primarily been applied to the discrimination of hail and graupel from coexisting rain e. In addition, aircraft-icing safety concerns limit the time aircraft can spend in the melting layer. Ground-based instruments that simultaneously measure particle size and fall speed provide an opportunity to distinguish the characteristics of coexisting rain and snow particles within mixed precipitation.
We apply a classification method based on particle size and fall speed properties to separate rain particles from snow particles. The implications of these results for bulk microphysics parameterizations and hydrological modeling are also examined. A laser diode produces a horizontal sheet of light 30 mm wide, mm long, and 1 mm high.
The horizontal sampling area is mm 2 , which is similar to the mm 2 sampling area of the Joss—Waldvogel disdrometer Waldvogel The laser light is received at a photodiode that samples at 50 Hz. When particles pass through the light sheet, a portion of the transmitted laser light is blocked and the voltage produced by the photodiode is reduced relative to when no particles are present in the beam. The amplitude of the voltage drop is related to the size of the particle.
The duration of the voltage drop is related to the fall speed of the particle. The instrument measures the maximum diameter of the one-dimensional projection of the particle, which is smaller than or equal to the actual maximum diameter.
The particle size D and fall speed V for every particle detected over the measuring period are tabulated in an array whose dimensions are the number of size bins by the number of fall speed bins.
For this study, the raw output arrays, which represent 1-min samples, are accumulated into longer time periods following the method of Joss and Gori Because the height of the sample volume is a function of fall speed, n D , V ij , the number concentration of particles per unit size and per unit velocity interval is first computed for each size interval i and fall speed interval j. These values are summed over all of the velocity bins to determine n D i and over all of the size bins to determine n V j.
A spheroid model 1 derived from Andsager et al. For particles with D between 1 and 5 mm, the assumed axis ratio varies linearly from 1 to 1. Rain particles are assumed to be symmetric in the horizontal plane. The effect of the porosity of snowflake aggregates on the measured size and fall speed has not been investigated. Such a juxtaposition of particles would yield data indicating a large particle falling at either the same speed or slower if the particles were slightly offset in the vertical direction relative to the constituent particles.
When only a portion of a particle intersects the beam, the sensor registers a small particle falling faster than other particles observed at that size. Blahak , personal communication requires assumptions on the natural distribution of fall speeds for small particles.
For rain, the natural fall speed distribution is relatively narrow and margin fallers can be distinguished and removed. For snow, the situation is more ambiguous because the size—fall speed distributions for small snow particles and margin fallers overlap.
Empirical studies have addressed average fall speeds of ice particles e. The manufacturer recommends against deployment of the instrument in windy conditions. The instrument was sheltered from the wind at both locations at which data were collected for this study section 3.
A possible complication in the measurement of particle size and fall speeds of snow aggregrates is that they may exhibit complex fall trajectories including spinning, spiraling, and shaking Lew et al.
Because the depth of the light beam is only 1 mm, it is assumed that the influence of complex fall trajectories on the results is negligible. The instrument site was in the McKenzie River valley within a flat grassy area adjacent to a rarely used airstrip.
The MKB site was well sheltered by surrounding trees. The objective was to obtain measurements in dry snow. Because the SPL is a mountaintop facility that often experiences high winds, the PARSIVEL disdrometer was placed in an open-topped enclosure near the center of the roof where snowflakes were observed to fall close to vertically.
The horizontal light sheet of the disdrometer was about 24 cm from the top of the enclosure. The perturbed airflow over the enclosure would have largest impact on the trajectories of the smallest, lightest particles and least impact on the largest, heaviest particles Folland During the 5-h observation period, the air temperature varied from 2. The plot shows the quality-controlled matrices of raw particle counts by size and fall speed.
Superimposed on the color-coded matrix Fig. As expected, the distribution of observed fall speeds as a function of diameter clusters closely around the empirical fall speed relation for rain. A snow event at SPL from to UTC 27 February is used to illustrate the size and fall speed distribution of dry-snow particles. The quality-controlled data matrix from SPL is shown in Fig.
The mode of the joint size—fall speed distribution is centered on the empirical fall speed relation for dendrites. The average wind speed was 4. The rain-only and dry-snow observations contrast with those in mixed precipitation from MKB on 18—19 December Fig. The air temperature dropped from 1. Quality-controlled matrices of raw particle counts by size and fall speed are shown in Fig.
The precipitation was associated with several prefrontal and postfrontal rainbands. The vertically pointing S-band radar data Fig. The hourly averaged profiles of horizontal wind velocity shown in Fig.
Near-surface winds at the observation site not shown , measured at 10 m AGL, were light and averaged 0. Further details on the environmental setting and observations at McKenzie Bridge and Storm Peak are presented in Table 1. The distribution of fall speeds for particles of a given size at temperatures from 1. The PSD distribution from 1. At temperatures between 0. The particles in the raw data matrix are classified into rain, not-rain, and ambiguous subsets using the masks shown in Fig.
The rain classification is based on the identification of rain particles by their size and fall speed characteristics. The bottom edge of the rain mask is defined as the velocity bin that is two bins lower than the velocity bin nearest the empirical fall speed relation for rain.
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