DESCRIPTION

Field of the invention

The present invention relates to a method and a device for measuring characteristics of snow, in particular the density thereof.

Background art

A need is felt to obtain reliable measurements of certain characteristic parameters of snow, possibly without damaging the snow pack and with a very accurate stratigraphy.

In particular, measuring the density of snow is especially useful for several applications, among which:

- studies on the characteristics of snow for pure and applied research purposes;

- hydrogeological studies on the flow of water resulting from snow thawing (the so-called Snow Water Equivalent);

- studies on snow stratification for avalanche prevention purposes;

For such measurements, different known approaches are being used which typically have not been optimized, for example as regards the use of people and measuring means.

Commercial products are known which are widely in use, for example, in modern weather / stations. These include the following.

- Use of a load cell (snow pillow). A big load cell (e.g. 3 x 3 m) is placed on a certain geographic area whereon the falling snow settles, which load cell allows calculating the total weight of the snow incident on its area. An ultrasonic sensor then allows measuring the height of the snow pack. By combining weight and volume, a density measurement is obtained. The value thus obtained, however, is the mean density, not the stratigraphy. This instrument is not portable, is very expensive, and is difficult to install. Moreover, the instrument itself prevents percolation of the water generated by partial snow thawing. Therefore, it cannot be used for determining the dynamics of snow thawing in order to understand where water filters through the ground and according to which time law, or how much snow evaporates.

- System based on the calculation of the dielectric constant of the snow for the purpose of creating a stratigraphy thereof, commercially known as SPA (Snow Pack Analysing System). A certain number of fixed bands of a certain length (e.g. 5 m) are tensioned at different heights and at a certain distance from each other. The instrument measures the capacity between the wires immersed in the snow, from which it obtains the dielectric constant, and therefore the density, of the snow. This system is very expensive and can only analyse a few snow layers; furthermore, the measured density is not punctual, but mediated over the whole wire length: this method may sometimes prove inadequate, if the characteristics of the snow change from point to point (e.g. every one metre). The potential advantage lies in the possibility of better discriminating the water component from the ice component. Given its large dimensions, it may cause a barrier effect in the snow, resulting in possible damage due to sliding caused by the movements thereof.

- Heated pluviometers equipped with suitable snow collecting tools. The latter may however cause measurement errors, due to the fact that airborne snow may not entirely fall inside the collector, and some of it may stay out.

- According to a further method for measuring the density of snow, a snow sample of a known volume is taken after digging a vertical hole, which requires a considerable waste of energy and involves risks in the presence of potential avalanches; the sample is then weighed in order to obtain the density thereof.

Patent US-6313645-B1 describes a method for using electric conductors in order to measure the dielectric constant of snow, according to a principle similar to the one used in the commercial device called SPA.

Patent EP-0729026-A1 describes a method for using radioactive sources (e.g. thorium) for determining the density profile of a snow layer.

Patent JP-06288888-A discloses the use of an electrostatic capacitive sensor for measuring the specific permittivity of snow.

Patent RU-2004106088- A describes the use of electromagnetic waves having different frequencies for measuring, from a suitable distance, e.g. from a satellite, the thickness of the snow pack.

Patents JP-8159962-A, JP-0019771-A, JP-10142146-A, JP-102677837-A describe variants of a method based on the use of a snow sample introduced or dropped into a container. The sample is illuminated with light at infrared frequencies: it is illuminated with a first frequency (1.5 micrometres) and then with another higher frequency so as to be able to distinguish snow from ice and water from a spectroscopic viewpoint. The quantity of water in the snow is then calculated, based on the ratio between the two wavelengths, in order to obtain the snow density. The use of infrared light makes the implementation of this method complex because the snow is characterized by excessive absorption of infrared light, so that high power levels are required for frequency irradiation. Stratigraphy is not taken into account.

The above-described methods have limitations as concerns the possibility of obtaining in- depth evaluations of the density of the snow, e.g. about its stratigraphy, in addition to being difficult to use and very costly.

In general, the known methods employed so far do not allow making stratigraphic snow surveys which are economical and punctual from both the space and time viewpoints. With the known approaches, it is also difficult or even impossible to create portable instruments capable of following the ground profile.

Summary of the invention

It is therefore the object of the present invention to overcome all the above-mentioned drawbacks by providing a method and a device for measuring the characteristics of snow, in particular the density thereof, which are sufficiently versatile as regards both installation and use, as well as economical and easy to manufacture and install.

The invention is based upon the idea of using the per se known principle according to which the light within the visible range absorbed by the snow is inversely proportional to the density thereof; one or more photoemitters are inserted into the snow and emit light within the visible range towards corresponding photodetectors; attenuation is then measured between the photoemitters and the photodetectors arranged at a predetermined distance, from which attenuation the density data can finally be obtained.

It is another object of the present invention a method for measuring characteristics of snow, in particular the density thereof, comprising the steps of: placing into the snow one or more photoemitters which emit light within the visible range towards corresponding photodetectors placed into the snow at a predetermined distance from the photoemitters; measuring the light attenuation between the photoemitters and the photodetectors; determining the density of the snow between the photoemitters and the photodetectors from said attenuation readings.

It is a further object of the present invention a device for measuring characteristics of snow, in particular the density thereof, comprising: at least one pair of poles arranged side by side at a predetermined relative distance; at least one array of photoemitters, and at least one corresponding array of photodetectors installed on opposite poles of each pair, said photoemitters being so shaped as to emit light within the visible range in a direction such that said photodetectors are illuminated; means for controlling the turn-on operation of said photoemitters and the reception by said photodetectors, said poles being adapted to be driven into the snow in a manner such that the latter is interposed between said photoemitters and photodetectors.

It is a particular object of the present invention to provide a method and a device for measuring the characteristics of snow, in particular the density thereof, as will be better described in the appended claims, which are intended to be an integral part of the present description.

Brief description of the drawings

Further objects and advantages of the present invention will become apparent from the following detailed description of a preferred embodiment (and variants) thereof and from the annexed drawings, which are only supplied by way of non-limiting example, wherein:

Figures 1, 2 and 3 show some examples of embodiment of the device according to the invention.

In the drawings, the same reference numerals and letters identify the same items or components.

Detailed description of a few embodiments of the invention

As aforesaid, the idea at the basis of the invention is to use the per se known principle according to which the light within the visible range (350-750 nm) absorbed by the snow is inversely proportional to the density thereof, in particular it shows an exponential trend.

In fact, the higher the snow density, the more light goes through it: this is because the snow, as density increases, becomes more and more similar to ice, which is more transparent than normal snow, the latter consisting of flakes with air in between, which causes light scattering phenomena.

This property is known in the literature: see for example Mellor, M.(1963), "A Brief Review of the Thermal Properties and Radiation Characteristics of Snow", Polarforschung, 33, 1/2, 186-187.

The idea at the basis of the invention is therefore to exploit these optical properties in an innovative way, by inserting into the snow one or more photoemitters that emit light within the visible range towards corresponding photodetectors, and then measuring the absorption of light through the snow interposed between the photoemitter and the photodetector, and hence the attenuation between the photoemitter and the photodetector arranged at a predetermined distance, from which attenuation the density data can be obtained. The photoemitters and the corresponding photodetectors may be placed into the snow in different points, whether at the same or different depths. Furthermore, they may be driven into the ground before the snow begins to fall, or they may be inserted into snow which has already fallen.

By placing the photoemitters into the snow at different depths, one can easily obtain snow stratigraphy data.

Moreover, by turning on the emitters at subsequent time intervals, one can also easily measure dynamic characteristics of the snow, e.g. not only its dynamic stratigraphy, but also the solar or lunar light that goes through the snow pack, and hence insolation and cloudiness, the height of the snow deposited on the ground (which is usually measured by means of supplementary sensors), the weight of the snow on the ground (and hence on roofs), in addition to the hydrogeological parameters for which the instrument has been designed. The dynamic stratigraphy may relate to slow density variations, e.g. for current or subsequent snowfalls, or to fast or sudden variations, e.g. avalanches.

Some examples of devices which may be used for implementing the method will now be described.

With reference to Fig. 1, a device according to the invention comprises two poles PI, P2, arranged side by side at a predetermined relative distance and having a certain length, on which arrays of photoemitters El, ... En are installed, with respective photodetectors Rl, ...Rn installed on opposite poles. A suitable electronic circuitry controls the photoemitters' emission and the photodetectors' reception over time.

When the poles are driven into the snow, the various photoemitters and photodetectors arrive at different depths in the snow pack. The poles may be positioned in a fixed manner on the ground, e.g. through a base B, even before any snowfall takes place, or they may be subsequently inserted into existing snow.

The poles may be mutually connected through suitable connection fittings, not shown in the drawing, so that they can be kept at a fixed mutual distance.

In a first embodiment, both poles are made of solid plexiglass, with a square cross-section (e.g. 40 x 40 mm) and a length of, for example, 2.7 m; they show a recess, e.g. a milling Fl, F2, on the side that faces the other pole, whereto respective arrays of photoemitters and photodetectors are applied, e.g. at a distance of 3 cm from each other, equipped with a respective power supply circuit. A plexiglass cover is glued onto the front face of the poles, over the photoemitters and photodetectors, for sealing and protection purposes. The use of plexiglass solves a number of problems, including:

- condensed air in front of the photodiodes and photodetectors: the air at low temperature within the interspace of the poles' milled part causes the water vapour contained therein to condense, which then tends to freeze and darken the windows whereto photodiodes and photodetectors have been applied. The plexiglass cover allows to solve this problem by creating a vacuum in the cavity that houses the photodiodes and photodetectors and/or by filling it with nitrogen.

- thermal bridge caused by the pole's material: the portion of the pole that protrudes from the snow is heated by the sun, and then the heat goes down along the pole and tends to thaw the snow around it; on the other hand, in cold weather (e.g. at night) the snow around the pole tends to freeze and form a neck on the pole, thus heavily affecting the reading. The ideal situation is one wherein the thermal transmission coefficient is the same as that of snow, so that the heat is transferred in the same manner in both the snow and the pole. In truth, the thermal transmission coefficient of snow varies over time as a function of its composition. Plexiglass has a thermal transmission coefficient which is similar to that of snow after some time has passed since it has fallen. This is an acceptable condition in that, as time passes after a snowfall, the thermal transmission coefficient of snow increases, and therefore heat is transferred more easily through the snow than through the pole, thereby minimizing the problem. For the purpose of reducing heat transmission by convection along the milled parts of the pole (unless a vacuum has been created), it is preferable to use transversal septa placed at a distance of, for example, 3 cm, so as to make the formation of convective cells very difficult.

On each pole there is an array of monochromatic light emitters El, ...En, e.g. LEDs, e.g. arranged in triples with red, green and blue light, which are turned on sequentially in order to get a response in the various visible bands. For example, they may be positioned side by side in a triangular fashion. The three colours allow making a spectral analysis. LEDs with specific bands for specific requirements, still within the visible range, may also be used.

For example, if the snow on a certain area is known to contain different substances, such as pollutants, which have a certain light absorption spectrum, then it is possible to use LEDs emitting light at those frequencies, perhaps in addition to those used for measuring the snow at frequencies other than the pollutant's absorption frequency.

On the other pole there are respective photodetectors Rl,...Rn, e.g. photodiodes, which are more accurate but slower than photoresistors (which are less accurate but faster), depending on cost and detection passband, which can take readings over a wide spectrum, including the infrared and ultraviolet ranges near the visible range.

the detectors' broadband (or the possible presence of multiple detectors, each sensitive within a certain frequency band) is useful, for example, for taking preliminary readings in order to eliminate any contributions to snow lighting coming from external sources, such as the sun or other sources, within the visible range and possibly also within the near infrared and ultraviolet ranges. In fact, since the emitters are not active outside the visible range, any contributions, e.g. within the near infrared range, may be used for validating the data even in the presence of strong external solar illumination.

The LEDs' on times are preferably short, so as to avoid heating the snow and altering the readings. It is preferable to turn on multiple LEDs simultaneously in order to optimize the currents' circulation and absorption time.

One can take real-time measurements in dynamic conditions, with rapidly moving snow, e.g. during an avalanche, or measurements in static conditions, with time constants variable over a wide spectrum. The conditions during a snowfall are considered to be similar to static conditions, because they change slowly over time.

The LEDs are preferably turned on sequentially, and anyway in a manner such that the emission of each LED is only picked up by the respective photodetector, without interfering with the others. For example, one may turn on simultaneously LEDs being at a fixed relative distance, e.g. one every three, also to avoid any effects due to anomalous reflections in the snow. On the other hand, an excessively directional emission might give rise to problems of misalignment with the respective detector; therefore, LED emission directionality should be neither too narrow nor too broad.

In static conditions it is preferable to turn on the LEDs sequentially, as aforesaid, because there is sufficient time to complete the measurement. In this case, photodiodes may be used as photodetectors.

A two-wire serial bus may be used, e.g. the I2C industrial bus, over which an already numerical signal can be transmitted: a read command is sent which comprises the address of the LED that must be read, which then returns a numerical measurement signal.

For dynamic conditions, however, it is preferable that the LEDs are turned on simultaneously. In this case, it is better to use faster photoresistors, even though they require a more expensive electronic part, e.g. for the bus that must ensure simultaneous, i.e. parallel, transport of a large number of signals and channels; in fact, many photoemitters are normally fitted to the pole (e.g. 90).

In dynamic conditions it is also preferable to use metal, e.g. aluminium, e.g. a "C" section, as a material for the poles, which is stronger than plexiglass. Furthermore, in this case the thermal bridge problem is negligible, given the speed at which the snow slides around the poles.

The device of the invention makes it possible, therefore, to follow the density and temperature variations over time of the snow, which normally gets compacted and forms layers which may also include ice; the surface snow may thaw at daytime, and the resulting percolating water will alter the structure and density of the snow of the underlying layers; at nighttime the snow may then freeze, thus forming a crusty layer over which an upper layer of fresh snow may slide, thereby causing an avalanche.

The array of diodes must not necessarily be straight and vertical. It is sufficient that the diodes are placed in different points of the snow pack, at equal or different depths. Therefore, the poles may be applied not only vertically, but also at different angles. The poles may even be curved instead of straight. The condition of vertical and straight poles generally corresponds to less interference with the snow, especially if the poles are driven into the snow pack after one or more snowfalls.

The realization of the electronic circuits for controlling the photodiodes and the photodetectors and of the power circuits, e.g. by using special batteries for low outside temperatures, poses no problems to the man skilled in the art, nor do the storage and transmission of the detected signals, which can be implemented, for example, by means of suitable antennas arranged on the poles.

The above-described embodiment example may be subject to variations without departing from the protection scope of the present invention, including all equivalent designs known to a man skilled in the art.

The advantages deriving from the application of the present invention are apparent.

- flexibility of use in many situations (the sensors can be arranged horizontally, vertically or following the ground profile, with variable geometries as necessary, or they may be made portable);

- low production cost, also due to the physical principle adopted, which does not require particularly sophisticated electronics;

- ability of supplying data which at present cannot be easily obtained or cannot be obtained at all (such as, for example, a very accurate stratigraphy, even during a snowfall, or the possibility of quickly taking manual measurements through portable devices).

From the above description, those skilled in the art can produce the object of the invention without introducing any further construction details.