The science of rain: James Hutton's legacy

"The rain it raineth every day" (Shakespeare's 'Twelfth Night'). It has been claimed that rain in Scotland produces too much water (floods), sometimes too little water (droughts), and water that is too dirty (polluted rivers). But there is little doubt that rain is a never-ending topic of conversation and public interest throughout Scotland.

Around 250 years ago, James Hutton published two articles entitled 'The Theory of Rain' in the 'Transactions' of the recently established Royal Society of Edinburgh. In 2026, the tercentenary of Hutton's birth, it is especially fitting that the National Library of Scotland have an exhibition about 'Rain'. Hutton's understanding of rain in relation to how the earth operates as a system is still relevant to life in Scotland today.

Who was James Hutton?

James Hutton was born in Edinburgh in 1726 and became known as the 'Father of Modern Geology' thanks to his role in establishing geology as a science.

A prominent figure in the Scottish Enlightenment, along with peers like the economist Adam Smith and the philosopher David Hume, Hutton was a classic polymath of his time. He had fingers in geology, medicine, farming, natural history, and more.

How Hutton explained rain

Hutton's two 'Theory of Rain' articles significantly advanced our understanding of why it rains. He correctly speculated that rain and snow are caused by the mixing of two differently heated airs with the resulting cooling producing condensation. His key point was that rain only occurs when, following cooling, invisible water vapour in the air condenses to form small visible water droplets.

Hutton illustrated this by noting that, on cold winter days, if the door of a warm room is opened, condensation occurs. He explained that this is because the two bodies of air have mixed. The same process happens when we see steam spurting from a boiling kettle: the hot water vapour suddenly cools.

This was a significant advance in understanding why it rains. Although, a fuller explanation would require later findings on the crucial role of dust and salt particles as condensation nuclei, which leads to the formation of rain drops falling from clouds.

In addition to being what we would now call a 'meteorologist', Hutton was also a farmer. In his 'Elements of Agriculture', written at the end of his life, he elaborated on the role of rain in promoting successful farming. He was especially aware of the role of rain in promoting soil erosion on his farm in Berwickshire.

The local soil there is very sandy and easily eroded. It seems highly likely that watching the rain wash away his soil into nearby burns and ultimately into the North Sea was the trigger for him later to write 'Theory of the Earth' (1788).

Although not widely appreciated at the time, this work is now seen as fundamental to our understanding of what geologists call 'deep time' and the realisation that the surface of Earth is billions of years old and subject to repeated renewal.

Rain, climate and farming

As a farmer, Hutton understood that, alongside soil, seed and labour, climate played a key role in successfully bringing crops to harvest. Alas, in the 1760s, accurate measurement of rainfall was very challenging and early measurements were prone to error. When compiling an annual heat index for plant growth in 'Elements of Agriculture', Hutton lists rainfall as one of the key meteorological observations in his inventory. But he qualifies this by adding "for the rain, of which I had no measure." And even by 1826, rainfall was only reported at 17 locations across Scotland.

This lack of precise measurement did not diminish Hutton's appreciation of the impact rain had on successful farming. He clearly understood both the role of water in the soil (replenished by rainfall), and how the subsoil promoted drainage (following heavy rain) and water retention (during droughts).

Until now, Hutton's 'Elements of Agriculture' has remained as an unpublished manuscript held at the National Library of Scotland, on loan from the Royal Society of Edinburgh. But I have recently compiled an edition of the manuscript to be published in 2027 by Boydell and Brewer as 'James Hutton's 'Elements of Agriculture' 1797: the Science and Art of Farming'.

Although unfinished when he died in 1797, the final draft written by Hutton's amanuenses (scribes and assistants), is presented in elegant eighteenth-century copperplate script. Like in this section about the role of climate in promoting plant growth.

As many Hutton scholars have observed, his writings are generally dense and challenging to follow, but Hutton seems to have been aware of his stylistic failings. When compared with 'Theory of the Earth', his 'Elements of Agriculture' is easier to read, with Hutton helpfully providing brief summaries at the end of some of the chapters.

The endless cycles of nature

Hutton's passion for agriculture was an enduring one as evidenced by him saying "I write for my pleasure on what has been, in a manner, the study of my life". But now Hutton is best known for his claim that mountains are constantly being eroded by the combined effects of rain, wind and ice, with rivers subsequently transporting the detritus to the coast and seafloor.

In turn, tectonic forces in the earth's crust raise the seabed, creating new mountain ranges which are then subject to the processes of weathering, erosion and destruction. Hutton envisaged this cyclic process repeating itself endlessly, famously asserting that "we find there no vestige of a beginning, no prospect of an end."

Cyclical thinking can also be found in other parts of Hutton's writings. For example, he notes that 71% of the earth's surface being ocean and 29% being land ensures there is sufficient rainfall to sustain plant and animal life. He suggests that changes in these proportions would upset a delicate balance. This implies the operation of what we now know as the 'water cycle': the planet's system of water moving down from the clouds to the earth's surface as rain, followed by runoff to the oceans, and then evaporation from the land and sea, enabling clouds once again to form in the atmosphere.

The cyclical nature of maintaining soil quality was uppermost in Hutton's thinking about farming at a time when modern chemical fertilisers were yet to be developed. He was very aware of the risk of soil fertility being diminished by successively planting the same crop. He addressed this by rotating his crops; offsetting soil acidity by applying marl to his fields; and by keeping 'muck heaps' from animal manure and rotted crops to use to return minerals and organic matter to his fields.

By re-cycling animal and plant waste products he maintained soil fertility. Today, as we promote more sustainable agriculture and seek to retain carbon in our soils, Hutton's ethos of promoting soil fertility by natural means underpins current advances in agricultural science.

Today we have a much more sophisticated understanding of the interconnected cycles embracing the atmosphere, land, soil and water. And rain is a key component in many of these cycles. Hutton, the 'unknown polymath' of the Scottish Enlightenment, had key insights into how these cycles operate for the well-being of humankind.