Question No. 27

Why the HEAT?
Answered 25 July 2010.
Question author: question in the air.
Asked 25 July 2010.

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1. The problem

This July is anomalously hot in most part of Europe. In St. Petersburg, for example, there has been practically no rain for over three weeks. The earth is exposed to relentless baking by the Sun that travels over cloudless sky all day long (and the day is long). Meteorologists explain that it is the so-called blocking anticyclone to blame. This weather beast does not allow the cool, wet air masses to penetrate to the continent from the Atlantic. But European Russia is not a desert yet. Moisture continues to evaporate intensely from the earth surface. Where does this moisture disappear? Why are rains so rare or absent altogether? What is the cause of the blocking anticyclone itself?

From the matter conservation law we know that all moisture evaporated in the region occupied by the anticyclone must ultimately precipitate. If the evaporated moisture moved upwards where, as it is well-known, the temperature is lower then the water vapor would inevitably condense to form rain. But this apparently does not happen. From this observation it is clear that within the anticyclone the air goes downwards. This descending motion of air pushes the evaporated moisture away from the anticyclone region near the earth surface and prevents the condensation process. Outside the blocking anticyclone the air ascends and the moisture it contains rains out. The larger the anticyclone, the more intense it rains outside. Thus, we summarize, if a blocking anticyclone has formed somewhere, then it is drought inside and heavy precipitation outside that area.

The desert is blocked forever. There is no evaporation in the desert. The descending air flow extrudes the dry air from the desert without generating rains at its borders. The question is how a blocking anticyclone can form over non-deserts. If we know the answer, we will also know why heavy rains and floods, hurricanes and tornadoes can originate at the outskirts of a blocking anticyclone.

2. Evaporation, condensation and wind

The answer is as follows. Evaporation and condensation are the main drivers of air motion. Three major factors account for that.

1) On Earth, two thirds of which are covered by the liquid hydrosphere (the oceans), the air cannot remain dry. The atmospheric air is moist; it contains water vapor, which is saturated immediately at the ocean-atmosphere interface. (Saturated concentration is the maximum concentration of water vapor the air can contain at a given temperature).

2) Moist saturated air cannot be motionless in the gravitational field. Any small occasional displacement upwards of an air volume results in some cooling. (Indeed, a certain part of the kinetic energy of air molecules is converted to the potential energy in the gravity field. A related process - the vertical velocity of a stone, if the latter is thrown up, decreases with growing height; ultimately the stone ceases to ascend and starts falling down.) As the moist air cools, the water vapor condenses and quits the gas phase. This causes a drop of air pressure. There appears an unbalanced pressure gradient that makes air rise further, already in a regular manner.

3) The rate of evaporation is limited by solar radiation. On average, about one half of solar flux that reaches the surface can be spent on evaporation. Therefore, evaporation rate can vary by no more than twofold. In contrast, condensation rate is set by the vertical velocity of the ascending air masses. Condensation rate can exceed evaporation rate by many times. It can also turn to zero if the air is descending. This inherent difference in the physics of evaporation and condensation determines the rich spectrum of circulation patterns observed on Earth.

In order that precipitation be approximately equal to evaporation, it is necessary that the vertical velocity of the ascending air is determined by evaporation. A simple calculation shows that this condition determines a vertical velocity of 3 mm/sec. Indeed, we observe that on Earth as a whole precipitation and evaporation rates are equal. When averaged over a long period, the amounts of moisture evaporated from and precipitated over the planetary surface coincide. (It does not rain in deserts, but there is no evaporation there either). The mean global precipitation rate is 1 m liquid water per year. With 3 x 107 second in a year, we obtain a rate of 3 x 10–5 mm/sec for the liquid water. We have to take into account that air density is a thousand times (factor 103) smaller than water density and that water vapor constitutes about 1% of atmospheric air (another factor 102). Consequently, to raise liquid water at a rate of 1 m/year moist air must ascend with velocity 3 mm/s. Hide This is a very small velocity which we do not notice. We feel the wind if it is stronger than 1 m/sec.

So, it would be in principle possible to arrange that moisture precipitated in the same place where it evaporated, just going up and down. However, the dry air components which do not condense (nitrogen and oxygen) must move along close trajectories which contain both vertical and horizontal parts. There must be two vertical parts of the trajectory, one for the ascending and another for the descending air motion. In the upper and lower horizontal parts of the trajectory the air flows in the opposite directions.

Thus, precipitation occurs where the moist air ascends. Where the air descends, there is no precipitation: as far as the descending air warms, no condensation can take place. Wind velocities in the vertical and horizontal parts are approximately equal if the linear sizes of the vertical and horizontal parts of the circulation approximately coincide. Everybody who traveled on jet planes knows that the ascending air flow associated with condensation does not reach beyond 10 km altitude (there are practically no clouds above that height). If such 10 km-sized eddies occur chaotically, they take the form of squalls accompanied by heavy showers or thunderstorms.

3. Forest moisture pump

Normal conditions of human life and other terrestrial organisms correspond to a situation when the rates of condensation (precipitation) and evaporation approximately coincide. In such a stationary state condensation rate exceeds the evaporation rate by an amount equal to the river runoff. That is, precipitation is always equal to evaporation plus runoff. If this equation holds, there are no floods, droughts, fires, hurricanes or tornadoes. However, to maintain this equation a complex control of the water regime on land is necessary. Such a control is performed by the biota (living organisms) of natural forest ecosystems. It was called the biotic pump of atmospheric moisture. Prior to the evolutionary origin of forests which switched the biotic pump on land, the continents remained a lifeless desert.

Vladimir Mayakovsky, a great Russian poet, wrote addressing the eternal topic of what is good and what is bad in our life:

– If the roofs
                     are smashed by squall,
If it hails
               and thunders,
People know –
                        not a good
time to go
                  for a stroll.
Gentle rain
                  is over.
             world is sunlit.
This is
            very good indeed, –
Adults say
                 as well as kids.
"What is good and what is bad", 1925.

This is good indeed, but for such an idyll to persist the chaotic, violent eddies must be tamed; heavy, unpredictable showers must be turned into gentle regular drizzling. Two physical problems have to be solved to this end:

1) Land loses some part of precipitation in the form of runoff. This part evaporates back over the ocean, not on land. Thus, it is necessary to return this moisture back to land.

2) 2) It is necessary not to allow the wind to accelerate to squall-like velocities. Indeed, over its path from the ocean to land moist air is under the action of the pressure gradient force, which accelerates the air according to Newton's law. It is easy to see that in the absence of sufficient friction air velocity in the chaotic eddies would be around several dozen meters per second. It is a hurricane velocity! In the meantime, as we have seen, to reach the idyll one needs a very low vertical velocity of just a few millimeters per second. Indeed, if there is nothing to impede this acceleration, vertical (and, consequently, horizontal) wind velocity u in the end of the ascent at 10 km would be determined from the equality between kinetic energy of the wind, ρu2/2 (here ρ — is air density), and potential energy of condensation. The latter is equal to the partial pressure of water vapor — all water vapor has disappeared (condensed) by 10 km altitude. Partial pressure pv of water vapor at the surface makes about 2% of total air pressure. Total air pressure at the surface is equal to the weight of atmospheric column, p = ρgh, g = 9.8 m/sec2, h ≈ 10 km. We thus obtain wind velocity u from the equalityρu2/2 = 2 x 10-2 ρgh, which gives u = 0,2 (gh)1/2 ≈ 60 m/sec. Hide

Both problems are resolved by the natural forest which features (a) large linear size of the order of a few thousand kilometers and (b) large height of trees of the order of a few dozen meters. The forest draws a very long "train" of moist air from the ocean towards itself; the length of this "train" can be several thousand kilometers. Close canopy of high trees does not allow the train to accelerate to dangerous velocities and dampens the acceleration. Complex and largely unstudied processes in natural forests regulate both evaporation (in particular, via leaf transpiration and intercept) and precipitation (in particular, via emission of biogenic condensation nuclei).

Evaporation is higher over natural forest than over the ocean. This difference maintained over several thousand kilometers creates a zone of high condensation rate and low pressure over the forest. Air pressure decreases from the oceanic coast inland. Thus, the ocean becomes the region where the air descends, the pressure is high, and condensation rate is low, while the forest-covered continent becomes the region where the air ascends, the pressure is low and condensation rate is high. In the result, there appears a horizontal flux of moist air from ocean to land. This flux brings to land moisture that evaporated from the ocean and thus compensates for the loss of moisture by land that occurs via the river runoff. Earth rotation modifies the air motion that is induced by the forest moisture pump. Air fluxes curve in the horizontal plane forming cyclones over the forest and anticyclones over the ocean. This is the idyll.

Forest pump in Siberia

Forest moisture pump in action. Siberia, 1974.

Evaporation from the forest canopy keeps water vapor concentration close to the saturated value, despite the decrease of total air pressure with growing distance from the ocean. Local forest evaporation is compensated by local condensation; it rains. This process produces a regular local eddy of linear size about 10 km. In the vertical direction the acceleration is prevented by friction associated with falling rain drops, so that the squall-like velocities never form. In the lower part of this eddy the air moves in the same direction as the large-scale ocean-to-land air flux. This flux prevents local fluctuations of condensation and precipitation. Complex functioning of the natural forest ecosystem ensures that the river runoff is accurately compensated: that is, the amount of moisture brought from the ocean and precipitated over land is neither smaller nor larger than the runoff. In the result, in a natural forest there are no extensive floods, droughts, hurricanes or tornadoes.

Why the HEAT, what is going on? Forest moisture pump is being destroyed

Siberian forests, including the forests of the Far East, are unique in that they draw moisture from three oceans: the Atlantic, Arctic and Pacific. This allowed the forests to persist even after European forests have been largely destroyed by humans. (In contrast, the continental forests of Australia, Arabia and Sahara did not survive the elimination of the coastal forest zone.) Thanks to their unimpeded access to the moisture stores of the Arctic and Pacific oceans, Siberian forests were able to continue draw moisture from the Atlantic Ocean past the deforested Europe. The zone of low pressure maintained over the forested Siberia contributed to the persistence and regularity of moist western winds over Europe. Europe has not become a desert due to Siberian forests and the remaining forests of Eastern Europe.

Elimination of natural forests over large areas in Europe has made the western moist air flow extremely irregular. Today deforestation marches rapidly over Eastern Europe. This deforestation is the cause of the anomalous heat in July 2010. Elimination of forests and the associated drop in continental evaporation and condensation turn the continent into the region where the air descends. This region is surrounded by regions of ascending air and precipitation, but these may fall on the ocean. If the forest moisture pump were in action, the area of air ascent would remain over Europe preventing extreme heat and drought. What is going on now can be viewed as the beginning of European desertification. June was relatively cool as the secondary forests with high evaporation drew moisture inland from the Arctic Ocean. In return, they warmed the ocean with the reverse land-to-ocean air flow in the upper atmosphere. In July the warm ocean became the zone of ascent. Now the ocean is steeling the rains that are badly needed on land from a major part of the European subcontinent.

Europe marches to desert

Europe on its way to the desert. Forest logging in Leningrad district, Russia, 2010.

To know more about physics and ecology of the biotic pump go here.