Mechanisms of formation of anticyclones over continents. And what does dew on the grass in the morning have to do with it?
When do we see dew on the grass in the morning?
Everyone knows from childhood from classical literature about “fog over the river after sunset” and about “dew on the grass in the morning.”
As an example, I will provide photographs of real landscapes for these pastoral literary cliches (see Fig. 1-3)
rice.Fig. 1. A picturesque picture of an autumn dawn under a clear blue sky over frosted grass in a meadow and fog in the lowland above the river.
Fig. 2. A similar picture of an autumn dawn over frosted grass in a meadow and fog in a lowland above a river. The asphalt on the sidewalk looks dry.
Fig. 3. Frosted grass and trees, but the dirt road has no obvious traces of frost. That is, the heavy soil mass does not have time to cool down quickly during the autumn night, remaining significantly warmer than the thin grass with frost.
These were classic observations of wildlife in the countryside.
For a city dweller, other paintings are much more relevant.
So in our lifeless city courtyards after a clear night on a cool autumn morning we can see dew or even frost on the roofs of cars, while the asphalt around will be dry. (see Fig. 4-5)
Fig.4. Morning frost on the roof of the car. Frost is also visible on the grass. The asphalt around is dry.
Fig. 5. Photo of a frosted roof of a car after a clear autumn night. Interestingly, the side windows only fogged up, but did not frost over. That is, the vertical glazing radiated more into the warm surrounding landscape, and not into the cold starry sky, and therefore could not cool down as much as the horizontal roof looking into the icy blackness of space.
Dew on grass and dew on car roofs have the same physical nature.
It is interesting that if the grass or car is under the crown of a tree, then dew or frost does not form on them.
And nearby, a few meters away, there is a car parked in the open air and the entire car is literally covered in dew!
Why does dew fall on car roofs?
The appearance of morning dew on grass and car roofs has a completely understandable physical explanation, which we will now examine in this article.
From a school physics course we know that heat transfer from a heated body to the surrounding space can be accomplished in only three ways:
– infrared radiation (radiation),
– heat transfer through the surrounding air,
– convective heat transfer with the surrounding air.
The term “radiator” in heating systems comes from “radiative” heat transfer.
Heat transfer through air is mainly associated with convective heat transfer, which is tens of times more effective than direct heat transfer through still air, since the thermal conductivity of still air is extremely low.
Radiative heat transfer occurs from a hot body to a cold one in proportion to the difference in their temperatures to the fourth power (see Fig. 6-7.)
Fig. 6. Law of thermal radiation of an absolutely black body.
Fig. 7. Law of thermal radiation of bodies towards each other at different temperatures.
To calculate the cooling of a car roof under the night sky, we need to know the temperature of the sky.
But here it turns out that the radiation temperature of the sky above the roof of a car is a very variable value.
So we can easily tell what the air temperature is above the car just by looking at the thermometer.
But this will be a contact temperature, not a radiation temperature, since gases in a stable state do NOT RADIATE by themselves!
Only solid bodies emit and absorb heat in the infrared range with a broadband spectrum.
So the clouds in the sky will have a radiation temperature close to the temperature of the air in the clouds, since the fog consists of liquid water droplets or microscopic ice crystals. (see Fig. 8)
Fig. 8. Distribution of radiation flux from the sun to the Earth’s surface.
But if the sky at night is clear and the stars are visible, then the radiation temperature of such a black abyss will be far from the air temperature in the lower layer of the atmosphere.
The pyrometric temperature of the starry sky suddenly turns out to be very low, namely 20-30C lower than the air temperature according to a conventional thermometer. Let's assume that the roof temperature will be slightly below +0C (the condition of frost freezing) or 273K, and the radiation temperature of the sky is minus 20C or 253K.
Then the intensity of radiative heat transfer will be:
W=5.67*10^-8*(273^4-253^4)= 82 W/m2
With a radiation power of 82 W/m2, during 8 hours of an autumn night the roof will lose energy:
E=W*dT=82*8*3600/1000 =2361kJ/m2
With the heat of condensation of water being 2400 kJ/kg, this energy will be enough to produce dew in the amount of
2361/2400=0.98 kg/m2
Or a continuous layer of water 1 mm thick.
The roofs of the cars are sloping, so the water does not lie in such thick layers.
In reality, water falls on a dusty roof surface in small drops with a diameter of 1-2 mm.
Taking into account the area of the gaps between the drops (otherwise the drops would merge and flow down), the total amount of dew is less than 1 mm of continuous layer. (See Fig. 9.)
Fig. 9. Intensive dewfall on the roof of a car, combining into large drops and even into continuous layers of water.
If the water freezes immediately as frost, then very thick layers of frost may form on the roofs of cars, and even with intricate frost patterns. (See Fig. 10-13)
Fig. 10. The roof of the car is heavily frosted, forming distinct frost patterns.
Fig. 11. The hood and windshield of the car are heavily frosted, forming clearly visible frost patterns.
Fig.12. The roof of the car is heavily covered with frost. with the formation of pronounced frost patterns, while the fence and plants in the background do not show any traces of dew or frost.
Fig. 13. A heavily frosted Jaguar figurine on the hood of a car.
Freezing of dew will not change the amount of condensate much, since the energy of condensation is 7 times greater than the energy of freezing of water, namely:
Water condensation energy – 2400 kJ/Kg,
The freezing energy of water is 333 kJ/kg. (see Fig. 14.)
Thus, freezing of the falling condensate into a solid frost will reduce the volume of condensed water by only one-seventh or 14%, provided, of course, that the heat flow from the roof of the car into the starry night sky remains constant.
Fig.14. Freezing energy of water. Also given is an assessment of the efficiency of melting snow and ice by burning diesel fuel.
Radiation temperature of black space
In open space, outside the earth's atmosphere, the concept of temperature in the usual sense no longer exists, but there is only the radiation component of heat loss from the surface of an object to the surrounding space.
Thus, the temperature of the ISS surface in orbit varies from +120C on the sun side to minus 157C (or 116K) on the shadow side.
The radiation temperature of black space is determined by the temperature of the relic radiation of 2.73 K.
This means that the temperature in the shadow of the ISS can be minus 270C.
The heat flow from the ISS wall at a temperature of 116K in the shadow towards black space with a radiation temperature of about 3K will be only about 10W/m2:
W=5.67*10^-8*(116^4-3^4)= 10.2 W/m2
That is, heat loss from the “cold” side of the ISS is one and a half times less than that of our modern houses with good facade insulation with a temperature difference from +24C in the room to minus 26C in the winter cold.
Therefore, the problem of thermal insulation and heating of the ISS is solved by relatively simple means, since the temperature range is close to the parameters on Earth, somewhere at the cold pole in Antarctica.
Thus, the wall from the inside of the ISS can be heated, for example, by direct electric heating of the inner surface or by simply blowing warm air from the volume of the ISS.
In this case, a layer of thermal insulation will also need to be built in between the internal sealed surface of the station and the external radiating body.
ISS cooling
The situation is much more interesting on the sunny side, heated to +120C (or 393K).
The heat flow from the sun heats the surface of the ISS to 393K (or +120C), and this heat is released outside into the same black space with a temperature of 3K.
In this case, the calculated heat flow from the absolutely black surface of the ISS will be a much larger value:
W=5.67*10^-8*(393^4-3^4)= 1352 W/m2.
However, this figure turns out to be higher than the energy density of the Sun in orbit, which is 1200 W/m2.
Consequently, such “calculated re-radiation” is associated with different degrees of “blackness” of the station’s shell in different ranges of light and infrared radiation from the Sun.
That is, the ISS shell can absorb more heat in the visible spectrum than it can emit in the infrared range, which leads to overheating of the ISS surface while maintaining the overall thermal balance at 1200 W/m2.
This heat power from solar radiation cannot be allowed inside the ISS, so as not to have to deal with additional cooling systems for the station later.
The ISS already has significant excess heat from operating electrical equipment, the heat from which has to be dumped on special radiating cooling panels located in open space. (See Fig. 15.)
Fig. 15. Photo of the ISS with solar photocell panels deployed perpendicular to the sun's rays (dark panels) and accordion-shaped radiating cooling panels (white accordion panels) located perpendicular to the solar batteries and along the sun's rays.
This means that the surface of the ISS on the sunny side must also be covered with thermal insulation, which reduces the heat flow into the station.
Saving weight and space on the ISS requires the use of extremely efficient thermal insulation systems, regardless of cost.
In the vacuum of space, such a thermal insulation system can only be a “multilayer re-radiating insulation”.
It would be possible to insulate with ordinary terrestrial thermal insulation materials, but they will be inferior to vacuum systems in terms of their characteristics.
So, the highest thermal insulation properties on earth are possessed by foam plastics with closed cells. But when entering the vacuum of space, the gases in the foam plastic bubbles will tear the foam plastic layer into small dust.
Therefore, insulating materials with non-hermetic cells are needed, or ordinary foam plastic must be placed inside the ISS at constant atmospheric pressure.
When moving to a deep minus in the layer of internal foam plastic, there are risks of accumulation of condensed water inside the layers of foam plastic, which can sharply worsen the thermal insulation properties of the foam plastic.
The mechanism of operation of multilayer re-radiating thermal insulation in a vacuum.
Let's consider the mechanism of operation of multilayer re-radiating thermal insulation in a vacuum outside the ISS.
Thus, on the hot side of the ISS with a temperature of 393K (+120C), one re-radiation of the absolutely “black” surface of the film at a given temperature difference of dT=1K gives a heat flux of only 13 W/m2:
W=5.67*10^-8*(393^4-392^4)= 13.71 W/m2.
At a temperature of +24C (or 297K), the same heat flow of 13W/m2 will occur with a temperature difference on adjacent surfaces of dT=3K:
W=5.67*10^-8*(289.4^4-297^4)= 13.03 W/m2.
That is, a difference of dT=100K will be created on approximately 50 layers of successive re-emission.
For high gloss reflective film with low blackness factor the number of layers can be greatly reduced.
Microporous thermal insulation materials such as foam plastic are created on a similar principle of re-radiation in thin layers, where there is practically no convective component in microbubbles, and the entire heat flow occurs through the radiation of heat at the boundaries of the bubble and through direct thermal conductivity along the length of the tortuous path in the thin plastic walls of the bubbles.
The most effective foam plastics are PIR (polyisocyanurate foam), which provide a thermal conductivity coefficient of 0.022 W/m*C.
Conventional polystyrene foam has a thermal conductivity of 0.034..0.042 W/m*C (depending on the brand)
An average mineral wool slab has a thermal conductivity coefficient of 0.045 W/m*C.
The operating temperature range for PIR insulation is from minus 65C to +120C, which is quite suitable for use inside the ISS. (See Fig. 16.)
Fig.16. Characteristics of PIR foam plastic.
Thermal radiation of objects in domestic conditions
When transferring heat from a vertical wall of a house into the volume of a room, there is a well-known coefficient in construction: “alpha” = 8.9 W/m2*C.
In this case, with a difference of dT=1C from +24C to +23C, we obtain a pure radiation flow from the room to the wall at a level of only 5.9 W/m2*C:
W=5.67*10^-8*(297^4-247^4)= 5.9 W/m2.
That is, the convective fraction is only about 3 W/m2*C, or 34% of the total heat flow through the wall:
8.9-5.9=3W/m2*C
A simple calculation shows that the heat flow specified for the example with the ISS of 13 W/m2 at a temperature difference of dT=50C with the initial radiating capacity of the wall at the level of 5.9 W/m2*C provides the specified level of thermal insulation with successive re-radiation of =50/(13/5.9)=23 layers of opaque film.
Such a layered structure can be created from thin polymer films or aluminum foil.
For the outer layers of insulation of the MKS with the possibility of strong overheating and deep supercooling, aluminum foil is more preferable than rapidly degrading polymer films (see Fig. 17.)
Fig. 17. The satellite is covered on the outside with layers of heat-protective foil of a characteristic gray-matte color.
Although not only aluminum foil, but also thin polymer films with a metal coating are widely used for satellites.
It is these “golden” films that are used to wrap individual parts of the satellite from the outside to protect them from overheating in sunlight. (See Fig. 18.)
Fig. 18. The satellite is covered on the outside with reflective polymer films with a gold-colored metallized reflective layer.
ISS and dew on the grass
And what does the insulation of a space station have to do with dew on the grass on Earth?
The fact is that under the clear night sky above us, almost the same open “black” space suddenly opens up as around the ISS in Earth’s orbit.
If we go out under the starry sky on a warm summer evening, we will feel a “cold breeze” blowing from above.
This subjective sensation of “a cold breeze” means an objective sharp increase in heat loss from the surface of the skin due to increased radiation towards the cold black starry sky.
It is through infrared radiation that the human body loses the greatest amount of heat (see Fig. 19-A-B.)
Fig. 19-A. Distribution of heat loss from the human body by heat transfer methods.
Fig. 19-B. Distribution of human heat loss by types of heat transfer.
The same feeling of “a cold breath” arises in an apartment in winter if you come close to the cold glass of a window.
At the same time, the radiation heat loss from the exposed surfaces of the face and hands towards the cold window glass will sharply increase, which will cause a subjective sensation of a cold breeze from the window.
Thermograms show that the face and neck are the hottest areas of the body, which are also almost always open. (See Fig. 20-21)
It is the face and neck that radiate heat the most and react most acutely to the slightest changes in the surrounding thermal conditions.
Fig.20. Thermogram of people from behind. Zones of equal temperature according to the scale on the right are shown in one color.
Fig.21. Thermograms of the human body.
The breath of cold space from the starry sky is felt not only by us, as living people, but also by the completely inert surface of objects on Earth or the thin leaves of grass and trees.
Under the starry sky, thin leaves and blades of grass quickly cool through radiation into space, while cooling the air around them through convective heat exchange with the air.
When the air in the grass layer cools, water vapor condenses on the thin, cold grass in the form of dew or frost (at sub-zero temperatures).
Since cold air is heavier than warm air, the air, having cooled down among the grass, does not go anywhere and spreads over the very ground.
This is how the “ground frosts” announced in weather forecasts occur.
That is, when they talk about “ground frosts” in the forecast, they mean a clear night without wind, during which the grass in the thin ground layer will be sharply overcooled and freeze, while the higher layers of air will remain at a positive temperature.
With low, continuous cloud cover, there is no frost on the ground.
It is precisely according to these principles that they try to protect fruit orchards from frost, creating smoke from fires among the trees, thereby reducing the possibility of radiation into the starry sky.
Why is there no dew under the trees?
If you look closely at the dew or frost on the grass and cars in the morning, you will see that there is no dew on the grass or on the roofs of cars under the tree crowns.
There are two aspects to this interesting phenomenon:
1. The tree crown with foliage effectively screens the ground surface beneath it from direct radiation into the cold starry sky. (See Fig. 22.)
2. The crown of a tree without leaves is itself a powerful emitter into space, cooling and drying the air between its thin branches. Thus, the air cooled and dried in the crown of the tree descends under the tree to the warmer ground and is no longer capable of causing dew to fall on the warmer grass or on the roof of a car under the tree. (See Fig. 23.)
Fig.22. Photo of frost on the grass in a meadow during spring frosts on the soil. In the background, the spring forest with young foliage looks green and without traces of frost underneath.
Fig. 23. Photo of an autumn landscape after frost. There is no frost on the grass under the tall birches on the left, although frost is clearly visible on the tall grass along the road in the open area. The road is also dry, having dried out in the frost to a dusty state by sublimation of water from the surface layers of the soil.
In some situations, entire systems of intensive frost generation on trees arise. This happens if frost occurs under the starry autumn sky near unfrozen bodies of water. (See Fig. 24.)
Fig. 24. Photo of an autumn landscape after frosts. Trees without leaves are heavily frosted next to an unfrozen river. That is, the freezing tree crowns under the clear night sky become surfaces for intensive condensation of evaporation, and the evaporation itself comes from the surface of warm unfrozen water with a positive temperature.
And in special cases, when there is frost near large unfrozen bodies of water, a light layer of frost can suddenly turn into an ice shell on the trees and cars surrounding the body of water (see Fig. 25-26.)
Fig.25. Icy trees and cars on the embankment after a winter storm. At slightly below zero, small splashes from the waves are supercooled in the air, turning into “freezing rain”.
Fig.26. The unfrozen sea in a frosty wind creates spray from the crests of the waves, which are supercooled in the air and turn into “freezing rain”. The spray of “freezing rain” did not reach the house standing in the distance, although the trees and benches standing nearby on the embankment were covered with thick layers of ice.
Conclusion: To prevent your car from being flooded with dew or covered with frost near your house, you don't need a warm and heated garage, but a simple canopy, perhaps even without walls on the sides.
For a long time I had a shell-type shelter that had a lot of cracks on all sides.
So, inside the “shell” it was always dry, and the snow carried in on the body fell off onto the ground and dried out over time without even becoming liquid.
That is, the snow evaporated at subzero temperatures with a continuous flow of heat from the relatively warm ground to the cooled roof. This effect of drying in a frozen state is called “sublimation”.
Ground frost and anticyclone
What else interesting can be gleaned from the topic of frost on the grass and frost on the ground?
It turns out that the cooling of the thin ground layer of air with the formation of frost under a clear cloudless sky is a global climatic phenomenon, and not just a subject of rural morning beauty.
Due to the cooling of the ground layers of air over a large area of cooling earth, a phenomenon called an “Anticyclone” occurs.
Earlier in the previous article, the mechanism of the formation of “tropical cyclones” was analyzed in detail.
https://habr.com/ru/articles/832582/
https://habr.com/ru/articles/834254/
Now let's look at the mechanism of the emergence of the “ANTIcyclone”.
As the name suggests, in an “anticyclone” the mechanism works in the opposite way to the functioning of a “cyclone”, namely:
In the center of the anticyclone there is an area of high pressure, and surface dry winds blow from the center of the cyclone to the periphery (see Fig. 27.)
Fig.27. A simplified explanation from the Internet of the difference between a Cyclone and an Anticyclone.
Externally, anticyclones look so inexpressive that it is difficult to find independent photographs of anticyclones.
Usually paired photos are shown as a contrast (see Fig. 28.)
Although I am not at all sure that the image in the photo on the right is actually an anticyclone, and not an ordinary cyclone in the initial phase of formation.
Fig. 28. Picture with paired photos of a cyclone and anticyclone for comparison (from the Internet).
Below is a photo that most likely shows a high pressure zone with a clear sky, surrounded by clouds at the edges. That is exactly how a real “Anticyclone” should look from space (see Fig. 29.)
Fig. 29. It is possible that this is a real photo of an anticyclone over the cold ocean between Australia and Antarctica: a vast area of completely cloudless clear sky hundreds of kilometers in size, surrounded by a fine ripple of thin clouds.
Anticyclones look much more expressive on climate maps, where closed isobar lines with a pronounced center of high or low pressure are clearly drawn (see Fig. 30-32)
Fig.30. Climate map with isobars. Low pressure zones with a cyclone in the center are highlighted in blue. (letter L in the center). High pressure zones (yellow with the letter H) surround the cyclones on the periphery. Small closed high pressure isobars with the letter H are visible only over the Caucasus and Central Asia, as well as a large high pressure zone on the left over the ocean above the cold Canary Current off the coast of northwest Africa.
Fig.31. A large high-pressure zone over Siberia. Low-pressure cyclones are visible over warmer seas (gray zones over the Arctic Ocean).
Fig.32. A vast zone of high pressure (H) over Eastern Siberia and Primorsky Krai. A clear powerful cyclone with low pressure (L) is present over Sakhalin Island.
Can these vast high pressure zones on climate maps be called “anticyclones” at all?
Or is an “anticyclone” simply large gaps between cyclones where dried air flows from the periphery of the cyclones?
How can you even call a weather phenomenon characterized by clear sunny weather with very light wind an “atmospheric vortex”?
On the global map, the geographic reference of high pressure zones is especially noticeable (see Fig. 33), if we consider them in relation to the map of warm and cold sea currents (see Fig. 34).
Fig. 33. Wind pattern and distribution of high (H) and low (L) pressure zones on planet Earth in July. There is a high pressure spot hanging over southern Australia, and the photo with the “anticyclone” in this very place was considered earlier as a hypothetical view of the anticyclone from space (see Fig. 29.)
Fig. 34. Map of large warm and cold sea currents in the oceans.
A comparison of the pressure zone map and the ocean current map clearly shows that the location of the “anticyclones” is simply a zone of dry air flowing from neighboring cyclones.
Knowing this conclusion, one can immediately easily find confirmation in space images of cloud formations (see Fig. 35.)
Fig.35. Photo of the Atlantic anticyclone with the “center” southeast of Greenland. In fact, an open arc-shaped cloud formation is observed over the warm Gulf Stream as it turns from the Canadian coast to Spain. On either side of the cloud band hanging over the warm current are areas of the ocean with lower water temperatures, over which the dehydrated air masses descend, which is accompanied by a slight increase in atmospheric pressure in cold areas with clear or almost cloudless skies.
Conclusion:
Cyclones are generated over powerful heat sources, which are thick layers of sea or ocean water warmed by the sun.
Water has a very high heat capacity and a high rate of heat transfer from the lower layers to the upper ones due to the mixing of the water column by storm winds and waves.
It is this ability to organize a salvo release of a huge amount of heat from sea water that makes tropical cyclones so powerful and destructive.
Land has a heat capacity 5 times lower (than water) and at the same time it transfers heat from the depths to the surface extremely slowly through transmission heat transfer, and therefore any vortex formations have a much smaller span over land than over the ocean.
So a land vortex – “tornado” although it breaks houses, but does it in a very narrow corridor of its movement. Whereas tropical cyclones are capable of razing entire cities on the ocean coast over a large area.
Anticyclones are very large atmospheric formations over land, where the density of heat flows is tens of times weaker than that of cyclones over tropical seas. As a result, the anticyclone is large, but very uneven in shape and with very weak pressure gradients over the area.
As a result, it turns out that the anticyclone is surrounded by cyclones along the perimeter, and it becomes impossible to separate the cyclone boundary and the beginning of the anticyclone.
Thus, it can be said that an “anticyclone” is not an independent atmospheric vortex with a reverse direction of twisting, but merely the internal space between cyclones, or a “donut hole”.
The space under the anticyclone is characterized by a relatively cold surface, where the general flow of dried air from the cyclones goes.
That is, a group of cyclones can form one common “anticyclone” among themselves, in which they combine their peripheral drains of dried air from the stratosphere in a common area within their circle.
