There are two main ways that rain forms, and while we're really good at forecasting one kind, we're really bad at predicting the other. The first kind, which makes up about 4096 of the rain we experience, is the long-lasting, gray-skies-all-day kind of rain. This comes from giant air masses called fronts, which move around the planet; when a cold one runs into a warm one, the warm one - which is less dense - gets lifted up. As it rises, it quickly cools off, which causes tons of water vapor in it to condense right where the two air masses meet, and then fall as rain...or snow or sleet. Since the fronts themselves are large and easy to track with radar systems, not only can meteorologists tell pretty much exactly where this type of rain will fall, they can also measure the size and temperature of the two air masses and use that to accurately predict how intense the rain will be and how long it will last. But the majority of the rain we experience comes from a process called convection. This type of rain happens when heat from the ground warms up a small mass of air, which then rises and cools, causing the water vapor in it to condense and fall to the ground. That process of rising, cooling and condensing is very similar to what happens with frontal rain. But because convective rain is triggered by heat from a small patch of ground, rather than the temperature difference between two large masses of air, it's much more complicated. It's hard to know how warm various parts of the ground will be at a particular time, whether that heat will be enough to lift a particular mass of air to a particular height, how much water vapor that particular mass of air holds, and how long that particular mass of air will stay over that particular patch of warm ground.