What determines if it will rain? How do meteorologist forecast this?
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Air can only hold so much humidity. Warm air is better at it, it can hold more humidity than cold air. It’s also less dense, thus lighter, meaning it tends to rise. When warmer and colder air meet, warmer air rises on top of it. This process makes it lose pressure, which causes it to cool down. As it cools it needs to expel its water content, because as we said earlier cold air holds less humidity, so all the H2O condenses into tiny suspended droplets, or sometimes ice crystals if it’s cold enough, and this is what a cloud is. When the rise of air is strong enough, condensation is so intense that it leads to larger and larger droplets which fall as rain (or snow if it’s cold enough). So all you need to know to predict when and where such condensation happens is the location of warm and cold airmasses, how they are moving respective to each others, and what the humidity content in them is.
This process doesn’t occur randomly, there are usually areas around the world where cold and warm air meet frequently, thus generating lots of storms. For example the polar front, the boundary between hot subtropical air and cold polar air, which lies at approximately 60° latitude N and S, is where most non-tropical rising/condensation activity occurs. If you keep an eye out for any systems that might be forming along the polar front, such as a cold front (cold air pushing into warm air) or a warm front (warm air pushing into cold air), or better yet a mid-latitude cyclone (basically a cold front chasing a warm front), you can predict where and when they’re going to hit based on the direction and speed of the winds that are carrying these systems around. You can also predict when and where exactly such systems are going to form in the first place, for example if there’s a ridge of warm air migrating poleward you can expect it to hit the polar front and trigger some front activity.
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