After a couple of months of heatwave in the northern hemisphere, people can be forgiven if their thoughts turn to rainmaking. Forget the old rain dances, is there a scientific method to create rain?
The answer is yes - but first you need a cloud. If there is high pressure and no clouds (like now) then there is no possibility of creating rain.
To understand how this works, first you need to understand how raindrops form.
Clouds are simply water vapour in gas form. To get the gas to condense into a raindrop (which is a liquid), it needs something to change it's state. Typically water is turned into gas by heat, and turned back into liquid by cooling. The cooling tends to occur when the gas encounters a solid particle, and it condenses on the surface of the particle. Think of what happens when you breathe on a mirror in the morning. The water vapour you are breathing out condenses on contact with the mirror.
Therefore what naturally triggers rainfall is dust entering clouds.
Part of the reason there is so much rain in the rain forests of the Amazon is that the trees expirate water vapour which forms clouds above the canopy, and then winds from the Sahara blow across the Atlantic towards the Amazon, carrying fine sand. When the sand particles blow into the clouds, the water condenses around them, and falls as raindrops.
Europe also normally gets a lot of rain because often the wind will turn and blow the Sahara dust north, triggering precipitation as the dust encounters the clouds. Places that are too far north to get the dust (or vapour) tend to be dry tundra deserts.
Rain also occurs when it encounters a mountain. The mountain forces the air upwards (where it is cooler and lowers the temperature of the gas) and the mountain itself acts as a solid that the water vapour condenses against. The Himalayas trigger the monsoon rains when winds carrying clouds from the Indian Ocean crash into it. The Rockies act in a similar way when the clouds coming in from the Pacific crash into the mountains.
Scientists figured out how to make rain mechanically in the 1940's (in General Electric's labs). Typically if clouds form, but there is no rain, they'll send up a plane to sprinkle silver dioxide into the cloud. The vapour in the cloud then condenses around the silver dioxide particles and falls to earth. The most recent time this was done in the USA was in 2016, during the El Nino which was plaguing California. When at last some clouds appeared in the sky, the Los Angeles County Department of Public Works took no chances. They hired a company to spray the cloud with silver dioxide to ensure that some rain fell on the parched earth.
But you need clouds in the first place. There are two ways to get clouds - Grow a lot of trees with deep roots. The trees will tap the water table and then expirate the water into the air, forming clouds. Or be near the coast (the sun evaporates sea water and the winds carry these clouds to the coastal areas - this is Britain's strategy).
Of course if there is high pressure stuck above you, acting like an obstacle preventing the clouds from the oceans from blowing in, you are stuffed.