family psychology 

What is the temperature of the snow? Factors affecting the choice of ointment What is colder snow or air

At ski waxing for professionals many factors are taken into account:

  • Temperature, humidity, snow classification.
  • The nature of snow friction.
  • Wind and more.

Lubrication of skis for sliding: paraffins, powders, accelerators.

Temperature, humidity, classification and friction of snow

Temperature indicated on the packaging of paraffin or ointment is the air temperature. It is advisable to take air temperature measurements at several points along the route. It is also necessary to know the temperature of the snow, but here it is important to remember that the snow temperature does not exceed 0 degrees. In this case, you should focus on the air temperature.

Humidity- the use of many ointments or paraffins directly depends on the level of humidity. Competitions can take place in an area with an average humidity of up to 50%, with a humidity of 50-80%, or humid climate from 80 to 100%.

Snow classification
For the choice of paraffins and ointments, the type of snow crystals is important. Falling or freshly fallen snow is the most critical situation for ski lubrication. Sharp crystals of freshly fallen snow require paraffin wax or ointment that keeps the crystals out of the lubricant layer. At positive air temperatures, when the saturation of snow with water increases all the time, water-repellent ointments are required. In addition, depending on the grain size of the snow, it is necessary to roll larger or smaller grooves on the sliding surface:

  • Fine-grained snow, sharp crystals require narrow, shallower grooves.
  • Older, stale snow at moderate winter temperatures requires medium grooving.
  • Water and large, round snow crystals require large grooves.
  • Fresh Snow - Falling and freshly fallen snow characterized by relatively sharp crystals and requiring a hard ointment.
  • Frozen granular snow, if wet snow freezes, then we get snow characterized by coarse grain with particles of frozen water, it is required to use a klister as a soil.

The friction of snow when lubricating racing skis is divided into:

  • Wet friction of snow - At a positive temperature.
  • Intermediate friction - Temperatures from about 0°C to -12°C. Friction with slip fraction dependent on temperature.
  • Dry friction - Temperatures from about -12°C and below. As the temperature decreases, the thickness of the lubricating water films decreases until their effect on snow friction becomes completely imperceptible.

Wind

Wind can easily change the surface of the snow. On wind-blown snow, skis tend to glide poorly. This is because the snow particles break up into smaller ones that rub against each other, resulting in a denser snow. Higher surface density increases the contact area between ski and snow, which leads to higher friction.

  • The atmosphere and snow conditions are constantly changing. Snow under the influence atmospheric phenomena can be heated or cooled.
  • Waterlogging of the air causes condensation on the surface of the snow, as a result of which latent heat is released, and it becomes necessary to use a warmer ointment than would be necessary, based on temperature alone.
  • In dry weather, the reverse process takes place, taking away heat from the layer of snow, requiring the use of harder ointments than dictated by air temperature.
  • Required paraffin melting point: at the level of 120 degrees, to achieve it, the iron must be heated to 150 degrees
  • Paraffin is heated by pressing several paraffin sticks stacked together against the hot surface of the iron.
  • After the position of the molten part of the paraffin on the sliding surface, it is heated and allowed to cool.
  • After that, remove the excess paraffin with a sharp plastic scraper and complete the work with appropriate brushes.

Paraffins for low temperatures should be applied in the same way, but excess wax must be removed immediately, without allowing the ski to cool. Otherwise, excess paraffin will chip off when removed. After the ski has cooled, the paraffin residue is removed with a sharp plastic scraper and the surface is treated with stiff nylon brushes.

Powder application

  • Before applying the powder, the surface of the ski must be waxed according to the snow and weather conditions.
  • Sprinkle a thin layer of powder on the sliding surface and heat with an iron (once).
  • Iron temperature approximately 150°C - heating temperature of the ointment from 110°C to 120°
  • Then let the surface cool down and then brush it with a horse hair and clean with a soft nylon polishing brush

Dry powder application method- by rubbing it into the ski surface with a clean synthetic cork. This is followed by surface treatment with a horsehair brush and a soft blue nylon polishing brush.

Snow forms at low temperatures and humidity as tiny ice crystals in the atmosphere.

When these tiny crystals collide, they join each other in the clouds and turn into snowflakes. If enough crystals are connected to each other, they become heavy and fall to the ground.

At what temperature does snow form?

Precipitation falls as snow when the air temperature is below 2°C. There is a myth that the temperature must be below zero for snow to form. In fact, the heaviest snowflakes fall already at temperatures between 0 and 2°C. Fallen snow begins to melt when the temperature rises above 0 °C, but as soon as the melting process occurs, the air temperature in the area where the snow falls begins to drop.

If the temperature is above 2 °C, then the snowflakes begin to melt and fall, most likely in the form of wet snow, rather than in the form of ordinary snow. And if the temperature does not drop, then instead of it will snow rain.

Wet snow vs dry snow

The size and shape of snowflakes depend on the number of crystals grouped together, and this in turn is determined by the air temperature. Snowflakes that fall through dry, cold air will be small, crumbly snowfall that does not stick to each other. This dry snow is perfect for winter views sport, but in windy conditions it is more likely to slip.

When the temperature is slightly above 0 °C, the snowflakes begin to melt around the edges, thus sticking to each other and turning into large heavy snowflakes. snow flakes. This forms wet snow, which sticks easily and from which you can make a snowman.

Snowflakes

Snowflakes are several ice crystals that can have various forms and views, including prisms, hexagonal plates, and stars. Each snowflake is unique, but since they connect to each other in a hexagonal pattern, they always have six sides.

At low temperatures, small snowflakes with a simple structure are formed. At higher temperatures, each snowflake can be formed from a huge number of crystals (star-shaped snowflakes), and they can be several centimeters in diameter.

The first snow always brings joy to both kids and adults. And in the following days, the fallout of these precipitations leaves no one indifferent. Children throw snowballs at each other, build fairy-tale castles, adults get on skis. But did anyone think about the questions: “What determines the moisture content of the snow? Why on some days you can make a snowball, and on others - the snow becomes crumbly and does not want to stray into a ball on any day? But the answer lies on the surface: it all depends on the humidity and temperature of the air and soil under the snow. But what do these indicators depend on?

Soil temperature under snow.

Snow is a good thermal insulator big influence to protect the soil from freezing. And the looser the snow, the stronger the soil protection will be from the effects of low temperatures. But this value is not unambiguous and one indicator may differ from another not only from the distance of the regions, but also within the same region or district and depends on the temperature of the ground cover at the time of snowfall. If the snow falls on deeply frozen soil, and the height of the snow cover is not great, then the temperature of the soil under the snow, on its surface, and the temperature of the air above it will be almost identical. At the same time, if in these areas the snow depth reaches 15-20 cm, then the difference between the temperature of the soil and the snow surface will be 6-8 degrees; while the surface of the earth will be warmer. On the other hand, if snow falls on unfrozen ground, and the depth of the snow “cover” is large enough, then the temperature of the ground under the snow will be approximately from zero to -0.5 degrees. This suggests that snow, as a poor conductor of heat, reflecting the ultraviolet rays of the sun, reliably protects the top layer of the earth from cooling. At the same time, the soil surface cannot have a positive temperature, since in this case the snow will melt on contact with the ground.

The experiments of scientists have shown that at an air temperature of -25 ... -28 degrees and a snow cover height of 25 - 30 cm, the earth's temperature does not fall below -10 degrees, and at a depth of 35 - 40 cm - below -5 degrees. At the same time, at an air temperature of -45 gr. and a snow depth of up to 1.50 m, and provided that the snow is rather loose, the soil temperature does not fall below -8 gr. This once again proves that snow, like a reliable shield, covers the earth from freezing.

What is warmer - snow or air?

The temperature of the snow cover depends both on its thickness and on the temperature of the air above it, as well as on the temperature of the soil. The earth, accumulating heat in summer, cools down slowly with the onset of cold weather. Snow, as an excellent heat insulator, covering the ground, retains this heat even in the most severe frosts. Therefore, the temperature of the snow depends on the thickness of the snow "spread" and the temperature of the air above it. If the snow covered the ground by 10-15 cm, then its temperature and air temperature will be almost the same. In the case when snow falls to a depth of 120 - 150 cm, the temperature difference can change both directly in the snow cover itself and in relation to air temperature. The snow at the top will be colder than at the surface of the earth, since taking heat from it, it begins to warm itself. At the same time, frosty air affects the surface of the snow, cooling it. Therefore, at a depth of approximately 45-50 cm, its temperature will be higher than on the surface by approximately 1.5 - 2 grams, and near the ground - by 4-6 grams. In this case, the air temperature at a distance of up to 1 m will be the same as the temperature of the snow cover. At the same time, at a height of 1.50 m and above, this figure will be significantly lower.


According to the experiments of scientists, the temperature of the air, as well as snow, also depends on the time of day. By observing the studies, they concluded that the most heat snow (-0.5 gr.) is observed during the day from 13:00 to 15:00, and the lowest (-10) from 02:00 to 03:00. During the same period, the air temperature during the day rose to +6 degrees, and at night it dropped to -15 degrees. Thus, we can conclude that the snow temperature is controlled by three indicators - air temperature, snow depth and soil temperature. Having studied these indicators, it is possible to make forecasts in many sectors of the national economy.

The impact of snow on the environment.

Snow, covering the ground, keeps it warm, protects the soil from freezing. And this is a very important factor in the first place for Agriculture and primarily for the preservation of winter crops. Grains sown in autumn and germinated under a snow cover calmly endure even severe frosts, while in places where there is no snow, and frost binds the earth, they freeze out. The same thing happens with garden plants. In snowless winters, the soil freezes, which contributes to cracking and freezing of the roots, “burns” on the bark of trees.


At the same time, sudden temperature changes can also have a negative impact on both nature and human activities. So, with an hourly change in air temperature from + to -, the snow begins to melt at positive temperatures, and then, when it decreases, it freezes, which contributes to the appearance of a frozen crust. Nast complicates the use of winter pastures. Melt waters wash away the fertile layer of the earth, which often leads to soil erosion. Accumulating in the lowland, they contribute to the soaking of winter crops. But now people have learned to control the level of snow. So, in areas where there is little snow, special shields are placed on the fields that trap snow. And in places where a lot of melt water accumulates, drainage channels break through.

And yet, despite all the negative factors, we are always happy with these white, fluffy stars. Again and again, with a smile, we follow the children descending on sleds from snow slide, do beautiful photos snow covered trees, together with the kids we sculpt a snowman. And laugh, laugh, laugh...

Effect of snow-covered surface on air temperature

A number of familiar gardeners contacted me by phone with a request to talk about the effect of snow cover on the air temperature above it. They motivated their request with the current enough harsh winter. My colleagues in my main job addressed me with the same request, after I had to explain to them for a long time what the mechanism of air temperature change at different heights from the snow surface is. Actually, my article on this topic already published in US (No. 7/2004), and I referred all those interested to this article. But requests to republish such an article were very insistent. And I decided that indeed six years have already passed since the first publication, many new gardeners have appeared, and winters bring constantly unexpected surprises every year and the reprint of this article will be very useful for most gardeners. Therefore, below, with minor modifications, this article is reprinted.

Research by specialists noted a special temperature variation on the snow surface and near it in the air compared to the air temperature at a height of 1-1.5 m. fruit trees in many regions of Russia and former Union, including our Sverdlovsk region.

At night, the surface of the snow and the adjacent layers of air cool much more strongly (on average by 5-9 ° C) than the overlying ones. During the day, the temperature rises to positive. In the air at a height of 50-100 cm, this phenomenon is practically not observed. Sharp fluctuations in the temperature of snow layers of air and plant tissues located here are caused by a number of circumstances: the special thermal properties of snow, exposure to the sun, the state of the atmosphere, and the plants themselves. Snow loses heat to radiation, especially at night in calm, clear weather (the long-wave radiation coefficient of freshly fallen snow is 0.82, and of old snow is 0.89). Severe and prolonged frosts in Siberia, the Urals and even Ukraine are observed precisely under such conditions. The very rough surface of the snow also contributes to large heat losses. Increased air dryness in winter in Siberia and the Urals leads to big losses snow for evaporation, causing an additional still significant heat consumption. In addition, the cooling of snow layers of air is also associated with the cessation of heat from the depth of the soil. Snow, as a poor conductor of heat, breaks the heat exchange between soil and air. As a result, its surface is very much cooled, although small negative temperatures (-5 ... -12 ° C) are observed in it.

An increase in the temperature of the upper snow horizons and near-snow layers of air during the day is associated with solar radiation (the short-wave absorption coefficient of freshly fallen snow is 0.13, and of stale snow is 0.33). Part of the solar radiation penetrates the thickness of the snow and heats it up. This is facilitated by branches of fruit and berry plants, penetrating it in all directions. They are heated to positive temperatures at negative temperatures ah air. Snow during the day in January-February thaws around the branches at night temperatures on the snow surface up to -40 ° C, which is largely facilitated by the so-called greenhouses around the branches. The ice crust at the beginning forms around the branches, then it grows, freely transmits light rays and prevents thermal radiation from branches and snow into the atmosphere. As a result, under the surface of the ice in the snow, plant tissues are heated to high positive temperatures, and their vital activity begins, and at night they cool to very low temperatures. Such sharp fluctuations are most often manifested in the second half of winter, causing the death of the bark - "burns".

Strong cooling of the snow layers of air depends on the climatic features of the region, winter and weather. Cooling of snow layers of air is observed, in fact, in all areas where a permanent snow cover is established. However, its frequency and intensity are far from the same in different areas. In the European part of Russia, cooling is less common and the difference in the temperatures of the upper and lower layers of air is smaller (no more than 3-5°C). Only in the Volga region, temperature differences on the snow surface reach large values, causing significant tissue damage on the snow line, especially in young trees. The sharpness of fluctuations increases significantly in the Urals, in Western Siberia and reaches his the greatest value in Eastern Siberia and on Far East due to the predominance of calm cloudless dry anticyclonic weather without thaws.

The lowest temperatures on the snow surface are most often observed in snowy winters. After heavy snowfalls on long time clear calm weather sets in, contributing to increased cooling of the snow layers of air. For example, in the Sverdlovsk region, such were the winters of 1966-67, 1968-69, 1978-79, 1984-85. In winters with little snow, fluctuations on the snow surface are also large, but they are observed at lower absolute minimum temperatures, and the plants are almost not damaged. In the second half of winter, the temperature on the snow surface fluctuates most strongly. At this time, calm, clear, dry frosty weather usually prevails in the Urals, and in rarer years January-March are characterized by heavy snowstorms, snowfalls and high humidity. In November-December, as a rule, winds, increased cloudiness and heavy precipitation are most frequent, which does not contribute to the cooling of the snow surface. Less cooling of snow layers of air in the first winter months Other reasons also contribute, in particular, the low snow depth and the still weak cooling of the soil. Heat from it comes to the upper horizons of the snow, since its small height does not yet prevent the penetration of heat. But, despite the above, there are some rare winters (for example, the winter of 1998-99 with a temperature of about -30 ° C in the air, observed on November 10-12), when there are early, not particularly low, short-term drops in temperature on the snow surface, causing significant damage to plants and in their consequences are not much inferior to winter ones.

The most detrimental effect on plants is exerted not so much by lowering temperatures as by the rate of their manifestation during the day. Observations show that in the morning the temperature on the snow is the lowest, but by 10 o'clock, when Sun rays touch its surface, it rises and is held at this level until sunset, after which it sharply decreases and drops to the lowest limits by 22:00, after which the cooling of the snow surface slows down and the overlying air layers begin to cool. Usually, the temperature increase on the snow surface is observed from 08:00 to 14:00, and a decrease - from 14:00 to 20:00, while the heating of plant tissues is more intense than the subsequent cooling in the evening. The speed of thawing is of decisive importance for the survival of tissues of fruit plants. Strong freezing of plant tissues in snow layers of air is also associated with the duration of exposure to low temperatures. For example, in one of the observations, low critical temperatures on the snow surface were maintained for 5-6 hours during the day, while at a height of 50 cm - only no more than 1 hour. Thus, sharp fluctuations in temperature on the snow surface, depending on the time and duration of their manifestation, as well as the condition of plants, cause various damage to tissues (cracking of bark and wood, sunburn of bark and wood, damage to wood), often leading to the death of individual branches and trunk. , and sometimes the entire above-ground part of the crown above the snow cover.

For a better understanding of the features of the establishment of snowy air temperatures and in some form of influence on them, I want to continue in more detail in popular form consider the mechanism of this phenomenon. As you know, the earth receives energy through solar radiation (wavelength 0.3-2.2 microns), and the loss of energy into space occurs due to long-wave radiation (wavelength 6-100 microns). The high reflectivity characteristic of the snow cover changes so rapidly with wavelength that at longer wavelengths the snow turns out to be a poor reflector, but a good emitter. Although a significant part of the long-wave radiation emitted by the snow-covered earth's surface returns to it due to absorption and emission by the atmosphere, a significant part of it (about 20%) is lost in space. If these losses are not compensated by the supply of energy from other sources, the resulting effect is expressed in a decrease in air temperature, especially in the lower layers of the atmosphere. The temperature profile of air subjected to radiative cooling for a long time is characterized by a very low surface temperature.

A region where intense radiative cooling is observed in Russia, as a result of which air masses characterized by very low surface temperatures, light winds and clear skies is Siberia. When the Siberian anticyclone captures the Ural zone, such temperatures are often set in our region.

According to the rules of radiant heat transfer, the amount of heat released from the snow surface during radiation is directly proportional to the emissivity of the snow surface, its area, as well as the temperature difference between this surface and the air layers in contact with it. The snow-covered surface, formed by the accumulation of numerous individual snowflakes and individual various blocks consisting of them, is an extremely rough surface. In addition, the snowflakes themselves (atmospheric and snow crystals) are also extremely rough formations. The total area of ​​such a surface turns out to be much larger than the area limited only by the length and width of the surface. The roughness and the total area of ​​the snow-covered surface increase especially strongly when it is formed by freshly fallen snow.

On fig. Figure 2 shows the change in the emissivity of bodies with a rough (1) and smooth surface (2) depending on the angle of radiation (A. Machkashi, L. Bankhidi "Radiant Heating", Moscow, Stroyizdat, 1985). From fig. 2 it can be seen that the emissivity of rough surfaces is much greater than that of smooth ones. In addition, the emissivity of rough surfaces decreases more slowly as the radiation angle approaches 75–90° than for smooth surfaces. That is, the more rough the radiation surface, the greater its emissivity and the greater the angle of radiation. And taking into account the increase in this case to the maximum possible and most radiating surface, we can also talk about the maximum possible heat loss by this radiating surface.

Where does the heat consumed in the process of radiation come from? This heat is taken from the layers of snow adjacent to the surface. But the snow cover, due to the content of a significant amount of air in it, has good thermal insulation properties. Therefore, the negative temperatures of the near-snow layers of air extend to a shallow depth. It is from these layers of snow that the heat expended on radiation is released. On fig. Figure 3 shows the dependence of the attenuation of diurnal temperature fluctuations with depth in the snow layer, taken from the "Snow Handbook", Leningrad, Gidrometeoizdat, 1986. From fig. 3 shows that already at a depth of 40 cm the amplitude of daily fluctuations in snow temperature is completely absent, and at a depth of 20 cm it is insignificant. Therefore, approximately a layer of snow 20 cm thick can be considered responsible for the release of heat spent on radiation. True, when standing for a long time severe frosts the amplitude of daily temperature fluctuations will be absent at a depth somewhat greater than 40 cm, but in this case, for a rough estimate, a layer of snow of 20 cm can be considered responsible for the release of heat spent on radiation.

The specific heat capacity of snow is 2.115 kJ/kg°C. That is, when 2.115 kJ of heat is taken away from 1 kg of snow for radiation by the snow surface, its temperature should decrease by 1°C. But the density of snow is very low (freshly fallen snow has 50-300, snow compacted by the wind - 150-400, firn - 450-700 kg / m3). Therefore, this 20-cm layer of snow adjacent to the snow surface, having a low mass in its volume, is forced to cool by a large amount of degrees to compensate for the heat spent on radiation. The heat inside the 20 cm layer of snow is transferred to its surface due to heat transfer due to thermal conductivity. The greatest heat losses due to radiation and the greatest decrease in the temperature of snow and near-snow layers of air, as already mentioned above, occur on clear, quiet, calm nights with a snow surface formed by freshly fallen snow, at least 40 cm thick, excluding heat from the ground.

When considering the features of the formation of near-snow air temperatures and the temperature of the snow surface, its flat surface was taken into account. However, in the forest, and in the field, and in the garden there are various irregularities, and snow is deposited unevenly during the winter due to them. Let's try to consider how such snowy elevations affect the temperature of the snow surface and the temperature of the snow layers of air at their tops.

On fig. 4, for example, two snow structures are shown: one with a round flat surface of radius r and a heat-releasing layer thickness of 20 cm, the other with a spherical surface of radius r with a spherical heat-releasing layer thickness of 20 cm (for clarity, both structures do not one quarter is shown). A comparison of these structures shows that the surface area of ​​the sphere of the second structure is 2 times larger than the flat surface of the first structure. Let's try to estimate the ratio of the volume of a 20-cm layer of snow involved in the delivery of heat to the snow surface for radiation. In the first structure, this volume is constant and the ratio of this volume to the radiating surface is constant. In the second structure, this volume depends on the radius of the sphere and is the smallest at small radii of the sphere. The ratio of this volume to the corresponding surface of the sphere also turns out to be dependent on the radius of the sphere. Comparison of the ratios of the 20-cm layer of snow to the radiation surface for the first and second structures showed that for the second spherical structure at r=0.5 m it was 35% less than for the first flat structure with the same radius r, at r= 1.0 m - 18.5% less, at r=1.5 m - 14.5% less, at r=2.0 m - 10% less.

Thus, with a spherical snow structure, a 20-cm layer of snow contains a smaller volume of snow, which is used to transfer heat from a certain snow surface to radiation than the same layer of snow with a flat structure with the same surface. In addition, the roughness and surface area of ​​the sphere of such a snow structure turns out to be much larger than that of a flat snow surface equivalent in geometrical dimensions. From this follows the manifestation of greater cooling of the snow surface and near-snow layers of air at the top of such a spherical snow structure than on a flat snow surface. Such a decrease in air temperature at the tops of snow structures is observed only on calm nights. Freshly fallen loose snow also contributes to this, delaying the flow of colder air from the peaks.

Observations of air temperature on snowy hills in Siberia, in the European part of Russia and in a number of other places have shown that, indeed, on clear, calm nights, these temperatures are several degrees lower than on a flat snow surface. In Siberia, according to the observations of GV Vasilchenko, the difference between these temperatures reaches 2-4°C. The same can be considered for our region. Such an establishment of negative temperatures, greater on elevations than on a flat snow surface, requires a very careful attitude to the hilling of trees and shrubs with snow. We must always remember and evaluate: will hilling plants with snow benefit them? Hilling plants with snow contributes to favorable climatic conditions their parts and at the same time worsens temperature conditions on the border of the snow of their unhilled parts. Under these conditions, it is advisable to hill the plants completely. But such hilling of large plants is not feasible in practice. In addition, with a large hilling, it is possible for plants to warm up and not complete their dormant period, which affects their growth in spring and fruiting.

Given all of the above, amateur gardeners must be aware of and take into account the possibility of reducing the air temperature on a flat snow surface by 5-9 ° C, and on the tops of hills and snowdrifts by 8-12 ° C compared to the air temperature at a height of 1-1, 5 m from these snowy surfaces in any winter. To exclude the influence of these extreme temperatures, all low-hardy garden plants should be bent to the ground and completely covered with snow. Garden plants wintering in an open form - standard apple trees, plums, cherries, apricots, sweet-fruited mountain ash, large-fruited hawthorns - should be grown on highly winter-hardy standard formers, grafting cultivars to a height of about 1.5 m. No hilling of such plants with snow is carried out. When hilling garden plants with average winter hardiness, grown in an open form, they strive to completely spud the base of the crown with forks of branches in order to preserve it in winter and restore from it, in case of freezing, parts of the crown located above the snow cover. To this end, when forming the crown of a tree, a low location of its base should be provided. Young fruit trees grafted into the root collar, whose winter hardiness is always less than adult fruit trees, must be hilled to the highest possible height. But in order to avoid the possibility of warming up and not passing the dormant period, the diameter of the snow hill should be small. Mature fruit trees with a high base of skeletal branches are also better not to hill up, since the dead part of the bark below is thicker and has great heat-insulating properties. When living tissues are protected when such trees are hilled with snow, the zone of extreme snowy temperatures approaches the forks of the bases of the skeletal branches of the crown, which are most vulnerable to such temperatures. The crowns of all low-growing fruit trees, even without hilling them with snow, only with its natural snow transfer, fall into the zones of extreme snow temperatures and in more at the same time, they are subject to freezing than the crowns of tall fruit trees. For this reason, in our conditions, it should not be promising to grow dwarf, columnar and bushy fruit trees in an open form. These trees should be grown in slate form.

V. N. Shalamov

(Ural gardener)

It is no coincidence that most natural avalanches descend during or immediately after snowfalls, since the snow mass is not able to withstand a significant amount of fresh snow falling in a short period of time on the slope. The weather, even more than other factors, affects the stability of the snow cover by changing the balance between the forces of adhesion and load. Let's see how rainfall, wind and air temperature affect this equilibrium.

Precipitation (type, amount, duration, intensity)

The effect of precipitation is to increase the weight of the snow mass, and hence the load on it. New snowfall or rain, especially heavy rain, can make the snow extremely unstable. An important difference between these two types of precipitation is that fresh snow can increase the strength of the snow mass by binding it to some extent. The downpour adds weight without adding strength to the layers. In addition, it weakens the holding forces, destroying the bonds between snow grains and between snow layers. While wet snow can be extremely unstable, once it freezes, it can also be strong and stable. Rain-soaked layers turn into ice crusts, helping to solder the structure of the snow mass. However, these crusts form randomly within the strata and on the surface. Especially smooth ones form an excellent bed for a future avalanche.

How fresh snow is related to old snow is as important as the type and amount of precipitation. As a general rule, rough, irregular, and uneven pitted surfaces promote stronger traction by acting as natural anchors than smooth surfaces. For example, a thin layer of unconsolidated (unbound) snow overlying a very smooth ice lens can form a very large avalanche zone after new snow falls.

There is no unequivocal answer to the question of how much snow is sufficient for the occurrence of instability and subsequent avalanches. During some snowfalls, more than 60 cm of fresh snow can fall and avalanches practically do not occur, during others - 10 cm falls and there is a high avalanche danger. This depends in part on the binding properties of freshly fallen snow and on the strength of the layers within the snowpack. However, as a rule, avalanches come down under the influence of an additional load from a large amount of snow that has fallen or carried by the wind.

The reaction of the snow mass to the load depends to a large extent on the weight of the fallen snow and the rate of its accumulation. With heavy snowfall (from 2 cm / h), the snow mass instantly reacts to the critical mass of freshly fallen snow, as it is unable to withstand this load. Often, with such intensity of snow accumulation, 90% of avalanches descend during a snowfall or within a day after it. But the avalanche period persists for another 2-3 days, depending on the processes occurring inside the snow mass. It's like stretching a rubber band until it breaks. The slowly growing snowpack gradually responds to changes by plastically flowing, bending and deforming, although collapse can still occur, especially if there are weak layers in the underlying horizons. The faster the accumulation of snow, the faster the snow mass will respond to the additional weight. Under the same conditions, 50 cm of new snow falling in 10 hours is more likely to create a critical situation than 50 cm of snow falling within 3 days. Add the factor of wind, temperature changes and - the task becomes much more complicated.

Temperature (snow and air temperature, direct and reflected solar radiation, gradients)

Changes in snow temperature can significantly affect its stability. These changes, in turn, are associated mainly with changes in air temperature, direct solar radiation (directly received from the sun) and reflected radiation (from earth's surface in atmosphere). The air temperature is transferred to the snow mass by turbulent heat transfer - conduction (from grain to grain) and by convection (from free air flow). As a result of this process, the surface of the snow can be significantly warmed or cooled.

The intensity of solar radiation reaching the earth's surface depends on latitude, time of day and season, slope exposure and cloud cover. Although only a small amount of thermal energy is absorbed by the snow surface, significant heating is possible. Snow also radiates heat very efficiently and, in clear frosty weather, can cool down to temperatures much lower than the air temperature. This radiation from the surface can be counteracted by counter radiation from a warm layer of clouds in cloudy weather.

The significance of such processes lies in the fact that the temperature of the snow affects the rate of changes within the snow mass, which characterize the stability of the snow cover on the slope.

The warmer the snow thickness, the faster the changes occur inside it. Warm snow thickness (warmer - 4 ° C) usually quickly settles, becoming denser and stronger. As it compacts, it becomes more resistant to further subsidence. In cold snowpacks, unstable snow conditions last longer because shrinkage and compaction processes are slowed down. Ceteris paribus, the colder the snow layer, the slower the shrinkage process.

Another temperature effect is that the snowpack can weaken over time if there is a significant difference in the temperature of the individual layers. For example, between isolated warm snow at depth and colder layers near the surface. The temperature difference under certain conditions contributes to the formation of weak layers caused by the temperature gradient, especially in loose snow. Well-defined snow crystals formed as a result of gradient metamorphism (under the influence of temperature differences) are called deep hoarfrost (deep frost) or sugar snow. Such a layer at any stage of formation poses a serious threat to the stability of the snow mass on the slope.

The change in air temperature during snowfall also has great importance, as it affects the connectivity of layers. Snowfalls that start off "cold" and then gradually "warm up" are more likely to trigger an avalanche than those that warm snow lays down on a warm surface. The fluffy cold snow that falls at the start of a snowfall often does not bond well to the old snow surface and is not strong enough to support the denser wet snow that falls on top of it.

The impact of solar radiation can be twofold. Moderate warming of the snow thickness contributes to strength and stability due to shrinkage. However, intense sudden warming, which occurs mainly in spring, makes the upper layers of snow wet and heavy and weakens the bond between snow grains. An avalanche can come down the slope that was stable in the morning.

Direct sunlight is not the only danger. Weak layers last longer on shaded slopes, where the snow thickness is not as compacted as on the illuminated slope, and where the formation of deep frost is often enhanced by cooling (cooling) of the snow surface.

Periods of clear frosty weather contribute to the formation of frost on the snow surface. These light pinnate crystals can form thin, very weak layers within the snow mass, which are covered by subsequent snowfalls and blizzards.


Such conditions also favor the emergence of a temperature gradient and the formation of deep frost in the lower layers.

In warm and cloudy weather, the snow can warm up, which contributes to its settling and hardening. Although such periods may contribute to greater snow stability on the slope, avalanches still occur quite often during warming, especially when this warming is fast and pronounced. Any rapid, sustained rise in temperature after a long period cold weather leads to instability and should be noted as "tip of nature".

Wind (direction, speed, duration)

When snow falls without wind on slopes with a steepness of less than 50 °, regardless of orientation, a snow cover is formed of approximately the same height, however, the thickness of the cover will be less on steeper slopes than on gentle slopes.

The direction and speed of the wind during a snowfall is of great importance, because these indicators determine which slopes the snow accumulates or is transported to. As a rule, at wind speeds of 7−10 m/s, most of the snow remains on the windward slope. If the wind blows more than 10 m/s, then the snow is transferred to the leeward slope, settling immediately behind the ridge. The stronger the wind, the further down the slope the snow accumulates. In the ridge parts, on the sharp ledges of the relief, snow cornices are formed. Being a good indicator of the dominant wind directions in the area. Eaves collapse is often the cause of larger avalanches on the leeward, snow-laden slope.


An increase in wind causes a general blizzard, which dramatically changes the conditions for the formation of snow cover, depending on the local orographic features of the mountain surface. Significant redistribution of snow in the snow cover occurs during low snowstorms, which often occur some time after the snowfall has stopped. The wind lifts previously fallen loose snow into the air and transports it to another location, forming compact, often well-knit layers that serve as suitable material for the formation of snow slabs.

When snow drifts, a very large heterogeneity of the snow cover can be created due to the redistribution of previously deposited snow, its blowing on positive landforms, and the creation of large blows in depressions and formations of snow cornices. On an uneven surface of the earth with small landforms, the blizzard transfer levels the irregularities and makes them hardly noticeable on the snow cover. Close to obstacles, snow transport causes the formation of snowdrifts complex shape. The density of snow cover after a blowing blizzard increases significantly and can reach 400 kg/m 3 .

Snow accumulation on side slopes occurs when the wind blows across the slope, transporting snow from left to right (or vice versa) on the leeward side of the ridges or ridges that divide the slope.

Note that while the lee slopes become more unstable due to snow overload, the pressure on the windward slopes decreases as the snow blows away. For this reason, windward slopes are often suitable for routes. But remember that a change in the wind in the mountains is a common occurrence. The slopes windward today may have been loaded with snow yesterday when they were leeward.

The wind speed required to transport snow depends in part on the type of snow surface. For example, 20 cm of loose, unbound fresh snow under the influence of wind speeds of 10–15 m/s can form an unstable snow cover in a couple of hours. An old slab of wind-compacted snow is relatively stable and rarely comes off except when it is impacted. external factors. A good indicator of wind-compressed snow is sastrugi on the snow surface.

Height above sea level. Temperature, wind and precipitation change significantly with altitude. Typical differences are rain at the bottom and snow at the top (there is a snow line between the two), or differences in rainfall and wind speed. Never assume that the conditions at one control site will reflect the situation at another height!

Findings:

Examples of typical weather conditions contributing to the instability of the snow cover on the slope

— A large number of snow falling in a short period of time;

— Heavy rain;

— Significant wind transport of snow

— Prolonged cold and clear period, followed by intense snowfalls or blizzards. It contributes to the emergence of a temperature gradient inside the snow mass and the formation of deep frost, and subsequent snowfalls contribute to the formation of a critical mass;

- Snowfalls at first "cold", then "warm";

- Temperature changes:

- Rapid warming (above 0 ° C) during the day - Leads to a critical increase in avalanche danger!

- Gradual (moderate) warming - compaction, increase in communication between layers - reduction of danger!

- Frosty weather - slowdown (preservation) existing danger and processes inside the snow mass!

– Long periods (more than 24 hours) with temperatures close to or above 0 ° C

- Intense solar radiation - the slopes that are in the sun the longest, in the afternoon can be dangerous!

In summary, the weather is the architect of avalanches and as such it draws the blueprint for changing the stability of the snowpack. By anticipating the effects of weather conditions, and matching different variations with the structure of the snowpack, you can greatly increase your safety when traveling through avalanche areas.