I was wondering what exactly triggers clouds to rain and how heavy the rain is. I live in a mountain region, so i heart the terms of clouds being too heavy and low, so they get stuck and rain off. But that doesn’t feel to be entirely true and more of a folks tale.
So how does it happen?
In: Physics
Clouds are masses of condensed droplets of liquid water (or deposited crystals of solid ice, or condensed and then frozen solid ice) falling through air that’s simultaneously rising (powered in part by the latent heat of water vapour being released as it condenses).
The speed at which these droplets fall at is determined by their mass and drag, which in turn is roughly proportional to their volume and surface area. As they grow larger from more water condensing/depositing on them, their volume grows *much* faster than their surface area (an effect known as the square-cube law).
When the mass of a drop is too large for its drag and the vertical winds within/below a cloud to hold up, it falls as rain (or other precipitation). Under some conditions it can evaporate before hitting the ground.
Clouds are just masses of water vapour. If the temperature, humidity and air pressure conditions are right where that vapour is, then that water is going to condensate into rain (or other kinds of precipitate we all know and love.. snow, hail, sleet, ice crystals, etc.) and fall on you.
To add some more information: Clouds getting ‘stuck’ in a valley and raining off is actually a thing. Clouds will rise along the slopes of the mountains if the wind is blowing into the right direction. This will lead to cooling, more condensed water, larger cloud droplets and ultimately rain (or other precipitation).
This is called orographic lifting and is quite important in mountainous regions. You might have heard of the Foehn effect, where really warm wind is coming from the mountains. This effect is a result of clouds raining off at mountain slopes.
Clouds are water vapor suspended in air. The amount of water vapor air can hold depends on the temperature and pressure of the air. As the air raises, the temperature and pressure (generally) decrease. As temperatures decrease, the air can hold less water vapor. As pressure decreases, the air can hold more water vapor. Rain is caused when those properties change in such a way that the air currently holds more vapor than it is capable of, causing condensation.
It rains on the rainward side of a mountain because as elevation rises, temperatures drop so quickly it overpowers the effects of the drop in pressure. Once the air gets over the mountain, to the leeward side, it begins to warm, so it can hold more water vapor and stops condensing.
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