Why it rarely rains in the desert and why there is a lot of sand. Dry rain as a unique desert phenomenon Where does dry rain come from

WHY HEAT?

European Desert March

1. Problem

This July in European Russia is characterized by abnormal heat. For more than three weeks there has been practically no rain, few clouds, and the sun is scorching mercilessly all daylight hours. Meteorologists explain the reason for this phenomenon as a blocking anticyclone that has captured a significant part of Europe. It is believed that this anticyclone does not allow cold air from the areas surrounding the anticyclone to enter its area of action, which leads to abnormal heat. But Europe is not a desert. The sun continues to evaporate moisture. Where does the evaporated moisture go? Why is there no rain? Why did a blocking anticyclone arise?

It follows from the law of conservation of matter that all the moisture evaporated in the region of the blocking anticyclone must fall in the form of rain. If the evaporated moisture in the form of water vapor were to rise up, where the temperature is known to fall, then the water vapor would inevitably condense and rain would fall. Therefore, the only explanation for what is happening is that the air in the blocking anticyclone goes down and squeezes out all the evaporated water vapor near the earth's surface, preventing the water vapor from rising and condensing. Outside the blocking anticyclone, the moisture evaporated inside it falls as heavy rains. The larger the size of the anticyclone, the more heavy rains fall outside it. So, if a blocking anticyclone has formed somewhere, then a drought inside it and heavy rains are inevitable, accompanied by floods outside it.

The desert is blocked forever. In the desert, where there is no evaporation, the air always sinks down and squeezes dry air out of the desert, which does not give rain. The most important question is why a blocking anticyclone occurs over areas that are not desert. As we explained above, the answer to this question will also explain why there are heavy rains, floods, hurricanes and tornadoes outside the blocking anticyclone.

2. Evaporation, condensation and wind

The answer is as follows. Evaporation and condensation of water vapor are the main driving force behind atmospheric circulation. This is determined by the following three regularities.

1) On Earth, two-thirds of which is covered by oceans (hydrosphere), the air cannot be dry. Atmospheric air is humid and contains water vapor saturated in the area of direct contact with the surface of the oceans. (Saturated concentration is the maximum concentration of water vapor in air at a given temperature.)

2) In the gravitational field of the Earth, humid air cannot be stationary. Any arbitrarily small rise in air will lead to its cooling. (Indeed, when lifting, part of the kinetic energy of the molecules transforms into potential energy in the gravitational field. In the same way, a stone thrown up loses its speed, stops and falls down.) Cooling moist air leads to condensation of water vapor, i.e., to the elimination it from the gas phase. The air pressure during condensation decreases. The air pressure at the top becomes significantly less than at the bottom, which causes no longer an accidental upward movement of moist air.

3) The evaporation rate is determined and limited by the flow of solar energy. On average, about half of the solar energy flux is spent on evaporation, but in some cases, the entire solar energy flux reaching the earth's surface can be spent on evaporation. Consequently, the evaporation rate changes no more than twice. In contrast, the rate of condensation is determined by the rate of rise of moist air masses. It can exceed the evaporation rate by hundreds or more times, and can also vanish when the air masses sink. This difference between the possible rates of evaporation and condensation determines the diversity of air circulation in the earth's atmosphere.

In order for precipitation to almost coincide with evaporation, it is necessary that the rate of air rise be determined by the rate of evaporation. A simple calculation shows that the air should rise at a speed of about 3 mm/s. (Indeed, on average, over the entire Earth, the rates of evaporation and precipitation coincide. Over a long period of time, how much has evaporated, so much rain has fallen on the entire Earth (rain does not fall in deserts, but there is no evaporation either). Liquid water falls on average throughout On Earth, 1 m/year is the global average.In year 3× 10 7 seconds, hence the rate of liquid water falling out is 3× 10–5 mm/s. But the density of air is a thousand times (10 3 times) less than the density of water. The air contains about one percent (10 2 less) of water vapor. Therefore, in order to raise water at a rate of 1 m per year, moist air carrying water vapor must rise at a rate of 3 mm / s).This is a very small speed that we do not notice. We begin to feel the wind blowing at a speed of more than 1 m/s.

Thus, water could fall at the rate of rain in the same place where it evaporated. But the dry component of air, containing nitrogen and oxygen, must move along a closed path containing both vertical and horizontal parts. Moreover, there should be two vertical and horizontal parts: in one vertical part, the air rises, in the other it falls. (In the upper and lower horizontal parts, the air moves in different directions.)

Therefore, precipitation cannot occur everywhere, it occurs only in the region of rising air (and not vice versa). There is no precipitation in the area of air sinking, because when the air sinks, it heats up and water vapor cannot condense. The speeds of air (wind) movement in the vertical and horizontal parts approximately coincide if the height of the vertical rise and the length of the horizontal movement are approximately equal. From personal experience of flying in airplanes, everyone knows that the height of air rise during the condensation of water vapor is less than 10 km. There are practically no clouds above this height. The air doesn't rise. Randomly emerging ten-kilometer vortices are accompanied by thunderstorm showers and heavy winds. Squall winds are the result of the pressure difference caused by the condensation of water vapor and the acceleration of air masses according to Newton's law.

3. Forest pump

Normal living conditions for people and all life on land are achieved when the rate of condensation and rainfall almost coincides with the rate of evaporation, exceeding it by the amount of river runoff, i.e. when precipitation is always equal to the sum of evaporation and river runoff. Only under this condition there are no floods, droughts, fires, hurricanes and tornadoes. This equality can be achieved by extremely complex and subtle management of the water regime on land. Such management is carried out by the biota that exists on land in the form of ecosystems of undisturbed forest cover. This control has been called the forest biotic pump. Before the evolutionary formation of forests on land and the activation of the action of the biotic moisture pump, the entire land was a lifeless desert.

Vladimir Mayakovsky, revealing the theme of good and evil, wrote:

– If the wind
roofs tear,
if
the city rumbled -
Everyone knows -
this is
for walking
poorly.
Rain dripped
and passed.
The sun
in the whole world.
This -
very good
and big
and children.

This is really good, but to achieve such an idyll, it is necessary to solve two physical problems by taming chaotic, uncontrollable vortices and turning them into ordered ones:

1) On land, part of the precipitation flows into the ocean in the form of river runoff, and the evaporation of this river runoff occurs in the ocean, and not on land. It is necessary to return the moisture of this evaporation in the ocean back to the land so that it rains where the river flow came from.

2) It is necessary to slow down the increasing wind speed, since the air during the entire movement from the ocean to the continent is under the influence of a pressure difference, i.e. constant force accelerating the air masses according to Newton's law. It is easy to see that if there were no braking, then the wind speed at the end of the lift at a height of about 10 km and, consequently, the speed of the horizontal wind compensating for the lift, would be hurricane-like, about 60 m/s. And in order not to tear the roof, it is necessary, as we found out, that the vertical speed does not exceed 3 mm / c!

(Indeed, if there were no braking, then the wind speeduat the end of the ascent at a height of about 10 km would be equal to the value calculated from the equality of the kinetic energy of the windr u 2 /2, where r - density of air, and potential energy of condensation. The latter is equal to the partial pressure of water vapor - all water vapor disappeared (condensed) up to a height of 10 km. Partial pressure of water vaporp vat the surface is 2% of the total air pressure. The air pressure at the earth's surface is equal to the weight of the atmospheric column,p = r gh, g\u003d 9.8 m / s 2, h~ 10 km. The wind speed is obtained from the equalityr u 2 /2 = 2 × 10 –2 r gh, that after reducing the air densityr gives u= 0.2 ~ 60 m/s.)

Both tasks are solved by the forest due to its large length, which is several thousand kilometers, and the high height of the closed cover of trees, which is 20–30 m. The forest pulls an air “train” of enormous length from the ocean above it (the length of the “train” is several thousand kilometers). The movement of the train is "slowed down" by the closed crowns of trees of great height, which extinguishes all the acceleration of the air, which appeared from a constant pressure gradient. At the same time, complex and largely unexplored processes of evaporation control (biological control of evaporation by leaves and interception of rain by leaves and branches) and condensation (by emitting biological condensation nuclei) operate in a natural forest.

Over a distance of several thousand kilometers from the ocean, the excess of evaporation from the surface of the forest over the evaporation of the ocean by almost a factor of two creates an increased rate of condensation over the forest and a constant air pressure gradient, which decreases with increasing distance from the ocean. Thus, the ocean becomes an area of sinking air, low condensation and high pressure, and the forest - a zone of rising air, high condensation and low pressure. This creates a horizontal flow of air from the ocean to land, carrying water vapor evaporated in the ocean and compensating for the amount of river runoff with precipitation on land. The rotation of the Earth modifies the movement of air provided by the action of the forest pump; at the same time, air currents twist in a horizontal plane, forming cyclones over the forest and anticyclones over the ocean. This is the idyll.

Evaporation of moisture by the forest itself maintains the concentration of water vapor close to the saturating value, despite the decrease in the total air pressure with distance from the ocean. Local evaporation by the forest is compensated by local condensation with rainfall. This process forms an ordered local air vortex with a scale of condensation and rainfall heights of the order of 10 km. At the bottom, the air flow in a locally ordered vortex moves in the same direction as the air flow from the ocean. Deceleration of air acceleration in this vortex along the vertical occurs due to the deceleration of falling raindrops. Squall winds associated with a local eddy are extinguished by a continuous flow of air from the ocean. River flow compensation must be accurate, i.e. the amount of moisture brought from the ocean should not be more or less than the river runoff. This is achieved by the correlated actions of the species of the entire undisturbed ecosystem.the woods. In an undisturbed forest, there are no droughts, floods, hurricanes and tornadoes.

Why the heat, what's going on? Destruction of the forest pump.

Now we can answer the question of what is happening now in Europe. The Siberian forest, including the forests of the Far East, is unique; it draws moisture from three oceans - the Atlantic, the Arctic and the Pacific. Therefore, even after the destruction of the undisturbed forest over the whole of Western Europe, the Siberian forest did not dry up (unlike the continental forests of Australia, Arabia and the Sahara, which could not withstand the destruction of the coastal forest belt). Continuously supported by moisture from the Arctic and Pacific Oceans, it continued to draw moisture from the Atlantic Ocean across Western Europe. The course of the westerly winds over Europe was regular and orderly. Only thanks to the Siberian forest and the forests of Eastern Europe, Western Europe did not turn into the Sahara, despite the almost complete destruction of its forests.

The clearing of forests in most of Europe led to the chaotization of westerly wet winds. The ongoing destruction of the intact forests of Eastern Europe has led to what we are seeing this July. A significant part of Europe has become a zone of air sinking, giving up its moisture and flooding with rain the surrounding zones of air rise, including the adjacent oceans. With the correct operation of the forest pump, the dry zone of air sinking should have been over the ocean, and not over land. What is happening today is not safe and is the threshold of turning Europe into a desert. It should be noted that June was relatively cool, because secondary deciduous forests with strong evaporation pulled moisture from the Arctic Ocean, heating it with reverse air currents. In July, after the cessation of active vegetation in the secondary forests, the heated ocean became a zone of air rise, pulling the rains needed by the land from a large part of Europe.

A.M. Makaryeva, V.G. Gorshkov

Desert Gobi. We camped in the sands of Khongoryn-Els for two days, in tents right under the dunes…Photos and text by Anton Petrus

1. The sun burned mercilessly, well, that's why it is a desert. But closer to sunset, the weather began to change, and obviously not for the better.

Black clouds swirled over the dunes, and a sharp wind blew. Not even the wind, but the windmill! Yes, such that I had to stand at the tents so that they would not be carried away into the desert distances.

By the way, pay attention to the tracks on the left on the dune - this is the track of the "climbers", who were brought in batches by cars. UAZ arrives, the Mongolian hand points to the dune, and everyone meekly rushes up. And gaining almost 200 meters in the sand is really difficult ...

2. For almost two hours we stood with tents in an embrace. During this time, we all managed to go through the peeling procedure with a gentle sand scrub, we also had a bite to eat with it. Well, dandruff in the hair has increased. Special desert.

3. But when the wind died down, you could take the camera and go shoot the impending storm. A beautiful, magical spectacle that can scare and charm at the same time.

4. There was a lot of greenery at the foot of the dunes, such a threshold of a sandy hell)

5. There were also small reservoirs where goats, sheep, camels and other hairy people came to drink in the morning.

6. The contrast of wet and dry sand and lead clouds on the horizon. The combination is wild.

7. In the distance, beautiful vymyaobrazny clouds appeared in the sky. A rare and beautiful sight, it's a pity they were far away ...

8. Meanwhile, the storm was approaching. Traditionally, it is assumed that there is no rain in the desert. But this is not about the Gobi, they go there. And in winter there is not only no heat, but wild cold reigns there up to 40 degrees!

9. But the spectacle is amazing. Black, dramatic clouds over golden sands! It's exciting. And if you add heavy thunder peals to this ...

10. Panorama of the coming storm from 7 vertical frames to create the effect of presence)

11. The thunderstorm came already at night, when it was blazing, thundering and pouring. But the worst was in the middle of the night. I am lying in a tent, listening to a raging thunderstorm and I hear a terrible groan-cry, as if something ghostly rose under the flashes of lightning. And this groan echoed through the dunes ... We decided that it was a camel that had strayed from its own in the darkness of the night. But anything is possible, and the answer is not always so obvious...

The question is put "upside down". It is not in the desert that it rarely rains and there is a lot of sand, but on the contrary, deserts form where it rarely rains and there is a lot of sand. Rains come from clouds. Clouds bring cyclones. Cyclones are formed mainly on the coast of the seas and oceans. Until the cyclones reach the central regions of the continent, all the water from the clouds in the form of rain spills along the road, so there is little rain in the central regions of the continents. If there are not sandy soils, then the water remains on the surface (it is not deeply absorbed into the soil), therefore, the existence of vegetation is possible. If there are sandy soils, then water from rare rains easily seeps deep into the sand and there is little water on the surface. Plants do not have enough water and they do not grow. Such a place is called a desert.

8 years back from Natalia Lisovskaya

WHY IS THERE NO WATER IN THE DESERT?

What is a desert? The desert is a region where only special forms of life can exist. All deserts experience a lack of moisture, which means that the existing forms of life had to adapt to do without water.
The amount of precipitation determines the volume and types of plant life in the region. Forests grow where there is sufficient rainfall. Grass cover is common where there is less rainfall. Where there is very little rainfall, only certain plant species characteristic of deserts can grow.
Hot deserts near the equator, such as the Sahara in Africa, are located in the subtropical zone, where the descending air becomes warmer and drier. The land in these areas is very dry, despite the proximity of the ocean. The same can be said about the deserts in northwestern Africa and western Australia.
Deserts located far from the equator formed because of their remoteness from the oceans and their damp winds and because of the presence of mountains between the desert and the sea. Such mountain ranges trap the rain on their seaward slopes, while their back slopes remain arid.
This phenomenon is called the "rain barrier" effect. The deserts of Central Asia are located behind the barrier of the Himalayan mountains and Tibet. The deserts of the Great Basin, in the western part of the United States, are protected from rain by mountain ranges such as the Siera Nevada.
Deserts are very different in appearance. Where there is enough sand, the winds create sand hills, or dunes. There are sandy deserts. Rocky deserts consist mainly of rocky soil, rocks that form fantastic cliffs and hills, as well as uneven plains. Other deserts, such as those in the southwestern United States, are characterized by barren rocks and arid plains. The winds blow away the smallest particles of soil, and the gravel that remains on the surface is called "pavement desert".
In most deserts, there are various types of plants and animals. Plants growing in deserts have practically no leaves to reduce the evaporation of moisture from the plant. They may be equipped with spines or spikes to scare away animals.
Animals living in deserts can go without water for a long time and get water from plants or in the form of dew.

8 years back by kulisvet

why it rarely rains in the desert and why there is a lot of sand and got the best answer

Answer from aircraft aircraft[guru]
Deserts arise where dry air ALWAYS comes, from which all the rains have already poured out before. Sand, these are small pebbles, of a certain size, why are there no pebbles of a different size in the desert? Because the smaller ones are carried away by the wind (from the Sahara, to the very middle of the Atlantic Ocean, for example), and the larger ones cannot be moved by the wind, so they roll under the wind, forming dunes and dunes of only one size of pebbles.

Answer from ~+ Katty +~[active]
An area is considered a desert if it receives no more than 25 cm of precipitation per year. As a rule, deserts form in hot climates, but there are exceptions. Most deserts have a lot of rocks and stones, and there is very little sand. In many deserts, there is no rain for several years in a row, then there is a short downpour, and everything starts anew. The driest is the Atacama Desert in South America. Until 1971, not a drop had been spilled there for 400 years. Artesian waters are known to exist in several places in the desert, but the high boron content makes them unsuitable for irrigation.

Answer from Rafael Ahmetov[guru]
The question is put "upside down". It is not in the desert that it rarely rains and there is a lot of sand, but on the contrary, deserts form where it rarely rains and there is a lot of sand. Rains come from clouds. Clouds bring cyclones. Cyclones are formed mainly on the coast of the seas and oceans. Until the cyclones reach the central regions of the continent, all the water from the clouds in the form of rain spills along the road, so there is little rain in the central regions of the continents. If there are not sandy soils, then the water remains on the surface (it is not deeply absorbed into the soil), therefore, the existence of vegetation is possible. If there are sandy soils, then water from rare rains easily seeps deep into the sand and there is little water on the surface. Plants do not have enough water and they do not grow. Such a place is called a desert.

Answer from Anna Osadchaya[guru]
The rain comes from the evaporation of water, which is very abundant in the desert =)))

Answer from Yoman Kavun[expert]
WHY IS THERE NO WATER IN THE DESERT?
What is a desert? The desert is a region where only special forms of life can exist. All deserts experience a lack of moisture, which means that the existing forms of life had to adapt to do without water.
The amount of precipitation determines the volume and types of plant life in the region. Forests grow where there is sufficient rainfall. Grass cover is common where there is less rainfall. Where there is very little rainfall, only certain plant species characteristic of deserts can grow.
Hot deserts near the equator, such as the Sahara in Africa, are located in the subtropical zone, where the descending air becomes warmer and drier. The land in these areas is very dry, despite the proximity of the ocean. The same can be said about the deserts in northwestern Africa and western Australia.
Deserts located far from the equator formed because of their remoteness from the oceans and their damp winds and because of the presence of mountains between the desert and the sea. Such mountain ranges trap the rain on their seaward slopes, while their back slopes remain arid.
This phenomenon is called the "rain barrier" effect. The deserts of Central Asia are located behind the barrier of the Himalayan mountains and Tibet. The deserts of the Great Basin, in the western part of the United States, are protected from rain by mountain ranges such as the Sierra Nevada.
Deserts are very different in appearance. Where there is enough sand, the winds create sand hills, or dunes. There are sandy deserts. Rocky deserts consist mainly of rocky soil, rocks that form fantastic cliffs and hills, as well as uneven plains. Other deserts, such as those in the southwestern United States, are characterized by barren rocks and arid plains. Winds erode the smallest particles of soil, and the gravel that remains on the surface is called "pavement desert".
In most deserts, there are various types of plants and animals. Plants growing in deserts have practically no leaves to reduce the evaporation of moisture from the plant. They may be equipped with spines or spikes to scare away animals.
Animals living in deserts can go without water for a long time and get water from plants or in the form of dew.