Nature is unpredictable, and the weather is no big exception to that rule. It's tough to figure out, just based on the appearance of a day, what might happen in an hour or two. If you're trying to figure out whether rain or other poor weather is incoming, however, you do have one big ally (and impending enemy): the clouds in the sky.
Clouds are to rain what stomach rumblings are to indigestion. They're an often tell-tale sign that something's going on in the atmosphere. At the very least clouds can tell you that there's water droplets and/or ice crystals lingering in the troposphere, the lowest section of the atmosphere, and if those two elements become too copious or are altered in some way rain, at the very least, may be incoming. Just seeing a cloud doesn't mean rain is on its way, however, so how can you tell?
The obvious sign of an incoming weather problem is that of dark clouds, and one that most people can appreciate since these ominous formations so often tell the truth. Dark clouds are often the result of a heavy concentration of water droplets in the bottom of the cloud, droplets that will eventually get pulled back to the Earth by gravity. The droplets are dense enough that the light from the sun cannot pierce through the bottom of the cloud or reflect back as it does at the top, where ice crystals dominate.
The nice thing about these clouds is that they'll also often tell you how severe a shower will be. A long, unbroken line of gray stratus clouds, for example, typically only mean that you're in for some rain. Not a ton, but enough to mess up your day. You can carry about your life with little more to defend you than an umbrella. In the case of a beefy cumulus cloud with a dark bottom, however, you'll probably want to stay indoors altogether, as the high white caps of these clouds often bring with them severe thunderstorms - or worse.
It's also good to note that you can sometimes tell what's going to be coming down by the looks of the clouds, not just how much of it. Clouds thick to bursting with rain will appear quite dark, as the water droplets inside them don't allow the sunlight above to pierce through the bottom. Clouds filled with ice crystals that become hail, sleet or snow, on the other hand, take on a gray color and will predictably show up on colder days. Combine these with your knowledge of the time of month and you can usually tell when something bad is going to happen and what it will be.
Not always, of course. It takes a true meteorological expert to predict the weather, and as anyone who's read the newspaper or watched a weather report on the news can attest, they're not always right. Knowing a bit about the science behind it can't hurt, however, and even if you're driven a bit paranoid by every passing gray cloud you'll at least be prepared when your prediction winds up being accurate.