Biotic Pump Miscellaneous: Jean-André Deluc, Heinrich Hertz, Meteorological Crosswinds, and the Drinking Bird

How past scientists understood water vapor’s role in atmospheric circulation, whether those insights were lost, and what connects the drinking bird to the biotic pump concept

As more people become interested in the biotic pump and reach out with questions or invitations to speak, I try to develop narratives that resonate with different audiences. You never know what might strike a chord. Besides, it can get a bit dull repeating the same set of arguments every time.

The biotic pump is driven by air pressure gradients generated by water vapor condensation and gravity. Today’s post combines historical theoretical insights into condensation-induced atmospheric dynamics (CIAD) with a practical illustration: the drinking bird toy, which operates on the same principles—condensation and gravity—to produce motion. That’s for the mind. For the heart, there is also a short biotic pump video featuring beautiful forests at the end of the post.

Jean-André Deluc

While Napoleon’s Grande Armée was enduring the harsh Russian winter of 1812, the eminent Swiss physicist Jean-André Deluc (then 85) reflected on the origins of rain and wind. In a paper published that year (Deluc, 1812, Gilberts Ann. d. Phys., 41, 162–194), he wrote:

“If [the rain] falls locally (...), it is accompanied by more or less heavy gusts, which come into existence, (...) where the water vapour turns into rain a kind of air-free space originates. This is the reason for sudden changes in wind direction, which cease again when the air has reached its original density.”
(translated by Dr. Stefan Emeis)

In these words, Deluc captured one of the two key elements of condensation-induced atmospheric dynamics: when water vapor condenses, it is compressed into a liquid with a specific volume thousands of times smaller than that of the gas. This sharp volumetric reduction leads to partial rarefaction in the condensation zone, creating a localized drop in air pressure.

Notably, these insights came from a scientist who was among the first to investigate the properties of evaporating fluids and to introduce the concept of latent heat—the energy absorbed during phase transitions such as melting ice or evaporating water.

But pressure reduction upon condensation is only half the story. The second crucial element is a positive feedback between condensation and air motion. Atmospheric condensation is not just driven by temperature or humidity—it depends on vertical air movement. As moist air rises and cools, condensation intensifies. The stronger the updraft, the more intense the condensation, which in turn strengthens the pressure gradients that drive the wind. This feedback loop is at the heart of many powerful atmospheric phenomena.

Heinrich Hertz

From 1812, we now jump to 1885 to hear the inaugural lecture of another remarkable physicist, Heinrich Hertz, delivered at the Technische Hochschule in Karlsruhe—where he would later make his groundbreaking discoveries in electromagnetism. Interestingly, Hertz's lecture was not about electromagnetism, but rather about the Earth’s energy balance and atmospheric circulation.

This unpublished lecture survived as a 50-page handwritten manuscript and was published in English only in 1997 by J. F. Milligan and H. G. Hertz in the American Journal of Physics. I myself learned about it very recently.

In the lecture, Hertz (then 31) presented a remarkably accurate summary of the Earth's energy balance. Among other things, he correctly estimated global annual precipitation to be about one meter:

Every year, we may assume that on the average a water layer a meter high over the entire Earth is converted into water-vapor—at the poles indeed far less, but on the other hand in the vast tropical zone substantially more. Probably two-thirds of the Sun’s energy falling on the Earth’s surface is needed to vaporize this great amount of water … Now, then, we may not say of this amount [of energy] that it provides only for the warming [of the Earth]; it serves even more as the heat source for a gigantic steam engine, which provides the power for the movement of the clouds, the winds, and the oceans. We can say that it serves to maintain the meteorological working of the Earth. For, as meteorology progresses, it becomes ever more clear that the principal driving force of all the more vigorous atmospheric movements is to be sought in the latent heat of the water vapor contained in the atmosphere.

As Hertz mentioned a steam engine, it's worth noting that in such engines, work is performed by steam (water vapor) that evaporates in the boiler, expands to do mechanical work, and then cools and condenses back into liquid in the condenser. The amount of work performed is determined by the pressure difference between the vapor in the hot boiler and in the cold condenser. This pressure difference, in turn, depends on the temperature difference between the boiler and the condenser.

To my utter surprise, Hertz estimated the temperature difference for the Earth’s boiler and condenser to be around 15 K:

First of all, this steam engine works between very small temperature limits, and that is always a difficulty. If we consider the evaporated surface of the sea as the boiler, then we must take as the condenser the layer of air in which the clouds are formed. This latter layer is cooler than the boiler, and it must be so, for otherwise absolutely no conversion of heat into any kind of work would be possible. The difference [in temperature] is, however, not great; we can hardly assume that the difference between the temperature of the Earth and that of the cloud-covering layer amounts to more than 15 °C on the average. But a steam engine, whose condenser is only 15 °C colder than its boiler can, even with the most perfect design, convert at most about 1/20 of the heat provided into useful work.

In comparison, using modern climate re-analyses data, we estimated the mean temperature difference between the surface and the height where condensation occurs to be around 18 K (Makarieva et al. 2017, Table 1). This is very close!

The biotic pump and some confusion in atmospheric science

To readers unfamiliar with meteorology or climate science, it might not be obvious what makes these insights from past scientists so remarkable. What stands out is that the idea of water vapor as the atmosphere’s main working gas seems to have been set aside without thorough investigation. Instead, the prevailing view became that atmospheric motion is driven mainly by the Sun’s uneven heating of the Earth’s surface.

Yet as early as the 19th century, Hertz pointed out something crucial:

If the atmosphere were dry, the temperature differences existing in it would by themselves give rise merely to movements of minor significance.

Let us see how this matters for the biotic pump concept. The biotic pump concept says that forests drive atmospheric moisture transport by winds to meet their water needs, and that the physical mechanism behind this pumping is the pressure gradients caused by condensation and precipitation of water vapor.

Accordingly, arguments against the biotic pump can be made at two levels.

• First, that forests do not drive atmospheric moisture transport at all, but merely exist where geophysical conditions happen to favor their growth.

• Second, that forests may indeed influence moisture transport, but through a different mechanism—specifically, by warming the air through the latent heat released during condensation, rather than by pressure gradients caused by the removal of water vapor from the gas phase.

I will consider the second objection on another occasion, but today I want to illustrate the lack of agreement among atmospheric scientists on the first point, whether forests (and vegetation in general) can drive winds that bring in moisture.

Meesters et al. 2009 in their critique of the biotic pump concept (see our published response here) wrote

The atmospheric circulation is driven by pressure gradients together with the Coriolis force (due to the rotation of the Earth). Pressure differences are caused by the different weights of the air column in different places, which are caused in turn by temperature differences mostly. The latter have two principal origins: (1) surface differential heating, and (2) condensation (a process in which heat is released). The second process occurs mainly in air that is being lifted already by thermally-driven circulations. …

On a somewhat longer term, the circulation of water can have a dampening effect on the atmospheric circulation: clouds diminish surface heating (during daytime) and cooling (at night), and evaporation from the surface consumes energy at the expense of the sensible heat flux. Hence condensation and evaporation both influence atmospheric circulation in several ways, but their influence is secondary compared to that exerted by differential heating at the surface.

In contrast, Li & Fu 2004 who investigated the onset of the wet season in the Amazon rainforest, concluded exactly the opposite:

Thus the thermal forcing due to the temperature gradient over Amazonia is probably too weak [see Hertz’s quote above — AM]. Secondly the continent-ocean surface temperature gradient decreases as the northerly cross-equatorial flow and moisture convergence increase during the developing phase. This decrease suggests that the transition of the large-scale circulation is not controlled by the continent-ocean surface temperature gradient. The numerical experiments of Rind and Rossow (1984) have shown that heating in the middle troposphere more effectively forces the largescale atmospheric circulation than does thermal forcing near the surface. Thus, the increase of local rainfall can more effectively force the transition of the large-scale circulation over Amazonia than can the increase of continent-ocean surface temperature gradient.

In other words, Li and Fu (2004) propose, apparently in contrast to Meesters et al. (2009), that moisture convergence over the Amazon is not initiated by differential surface heating, but rather by increased local rainfall and the associated release of latent heat. According to their view, enhanced forest transpiration causes winds to reverse direction, drawing moisture from the ocean toward the forest.

One might have expected that scientists critiquing a new concept involving forests and moisture transport would be familiar with the broader literature—particularly a prominent study like that of Fu and colleagues, which was featured by the American Geophysical Union. Yet their work was not referenced by Meesters et al. (2009), nor by any of the biotic pump critics who took part in the discussions in Hydrology and Earth System Sciences Discussions (2007) and Atmospheric Chemistry and Physics Discussions (2010–2011).

At the time, my colleagues and I were new to the field, and access to international literature from Russia was not always straightforward. I must acknowledge that we were unaware of the earlier work of Fu and colleagues until the publication of Wright et al. (2017), after which we began citing their work regularly.

In 2010, another study emphasized the role of vegetation in driving winds through latent heat release. After analyzing vegetation–rainfall relationships in Southern Africa, Chikoori and Jury (2010) concluded in Earth Interactions, a journal of the American Meteorological Society:

It is postulated that an earlier rainfall event and subsequent “greening” results in a moisture flux that promotes the next rainfall event. This is reflected in a composite analysis of low-level velocity potential. The vegetation draws airflow toward itself in a self-sustaining way.

Notably, Chikoori and Jury (2010) did not cite the work of Fu and colleagues (neither did they cite our HESS paper), even though their findings are consistent with it.

It seems that the substantial initial opposition to the biotic pump concept may have discouraged further exploration of the potential active role vegetation could play in driving atmospheric moisture transport. I will explore this topic further in subsequent posts. Your feedback is welcome.

The drinking bird

To dilute theoretical considerations with something more visually engaging, let us now discuss the drinking bird toy. It illustrates the physical principles behind the biotic pump, as both harness condensation and gravity to generate motion.

According to Wikipedia, Einstein and his wife were enchanted by this toy when they saw it in Shanghai in 1922. It really is a remarkable little device. I’d always wanted to get one myself, and finally did, just a few days ago.

The toy has a sealed glass body containing a liquid that's in equilibrium with its vapor. That green liquid is usually dichloromethane—a substance that boils at around 40 °C (103 °F) and requires about 25% less energy to evaporate than water. These properties make its vapor pressure highly sensitive to even small temperature changes.

The bird starts moving once you moisten its head and beak. Water begins to evaporate from the head, which cools it down. As a result, the dichloromethane vapor inside the bird condenses, causing a drop in pressure.

This pressure drop draws the liquid dichloromethane up into the neck. As the liquid rises, the bird becomes top-heavy. Once it reaches a tipping point, the bird leans forward and 'drinks' from the cup.

If there's a cup of water in front of it, the bird’s head gets wet again, allowing the cycle to repeat - so long as the room’s relative humidity stays below 100%, so evaporation from the bird’s head can continue.

We can learn two things from this bird. First, if the air is saturated and water can’t evaporate, the bird won’t move. A temperature difference between the bird’s head and body is needed to make the vapor inside condense and drive the motion.

But evaporation alone isn’t enough. Without the bird, the imbalance between the water in the cup and the dry air wouldn’t cause any noticeable motion. What makes the bird move is the sealed vessel filled with vapor and liquid that responds to the temperature difference, turning that thermal imbalance into mechanical movement under the action of gravity.

When forests transpire and release water vapor into the atmosphere, they create both the imbalance needed to drive motion and the mechanism that generates it. As air rises, it cools—losing internal energy as it gains potential energy in Earth’s gravitational field. This cooling creates the necessary temperature gradient.

Because water vapor is a condensable gas, it responds to the drop in temperature by condensing and precipitating, which lowers surface pressure. As a result, air from surrounding regions begins to flow toward the area where precipitation occurs—similar to how the green fluid in the drinking bird rises toward the cooled head.

But unlike in the toy, where the rising fluid doesn’t contribute to the energy cycle, the incoming air over forests brings additional moisture. This helps sustain the water cycle, including the condensation-induced air inflow, and enables forests to meet their ongoing water needs.