sbrothy said:
This almost belonged in the "Today I Learned..." thread.
There are many organic-derived molecules that seed clouds. Each can be ionised by UV light, and may then play a part in the nucleation of liquid water in the atmosphere.
Towards the poles, there is more cloud, and less UV light, so the molecules must be lifted high into the troposphere before they are ionised or can react. At those altitudes it is cold, below -20 °C, so there is almost no water remaining in the air. From the poles, those molecules must fall into the clouds from above, or circulate, before they can participate in nucleation. During that time they are transported by the global circulation.
If the seeded rain evaporates before reaching the ground, the molecules remain near the cloud base, where they concentrate and are recycled. If the rain drop reaches the ground, the nucleating molecules of all the drops that formed it, become part of the soil or hydrology.
Here are some examples of extra-atmospheric molecules.
The smell of the sea; Dimethyl Sulfide, DMS.
https://en.wikipedia.org/wiki/Dimethyl_sulfide
There is also common salt in sea air, but we cannot smell NaCl. The presence of chlorine is indicated, as it promotes rust on iron, and corrodes aluminium, near the coast.
The smell of earth, Geosmin, is released continuously by microorganisms in the soil, but we really only notice it when the first raindrops fall on a hot and dry clay soil. The accumulated store of the terpene, is then ejected, as water preferentially enters the dry clay.
https://en.wikipedia.org/wiki/Geosmin
https://en.wikipedia.org/wiki/Petrichor
Air pollution, NOx, from cities.
Pollen from grasslands.
Volatile oils from pine forests.
The smell of the jungle, including fungal spores.
Flying a mass spectrometer, low over a forest, has been used to map the underlying geology as it changes the volatile molecules present in the air.
Clay dust and bacteria from deserts.
An example: where the Listeria bacteria, was windblown with dust, coming to rest in the skin of cantaloupes being grown in the field. When they were cut, the streaked listeria began to grow on the moist surface of the cut.
https://en.wikipedia.org/wiki/Listeriosis#2018_Australia_listeriosis_outbreaks
The list goes on, including the poop of all birds, including penguins.
