Clouds are an ever-present feature of much of Earth's geography. Floating in the sky, often harmlessly, clouds don't have much bearing on our lives - aside from dictating when we need to pull out our jackets or umbrellas, or in some cases run for cover (in which case they play a HUGE role in our lives). But how much thought does the average person put into clouds?
Think about it. Clouds are a little extraordinary. Despite being a gas that cannot really be touched clouds are always there, floating miraculously in the atmosphere, watching over anything and everything we do. And they come and go seemingly in the blink of an eye. What causes these massive formations in the sky? What are clouds?
Answering that question requires looking at a cycle that most probably haven't bothered studying since they were children: the water cycle. The water cycle is a natural process by which heat from the sun causes water on the surface of the planet - from sea water to rivers to ice to snow to any number of other things - to change forms from a liquid to a gas. This process, known as evaporation, carries the water vapor into the air, above other, cooler pockets of air.
The higher this air goes the cooler it gets, and as it cools the water vapor begins to change back into water through a reverse process known as condensation. This rising process typically ends somewhere in the troposphere, the lowest portion of the Earth's atmosphere, the still changing water vapor becoming either water droplets if it achieved saturation in the lower parts of the troposphere or ice crystals at higher altitudes. The type of cloud formed typically depends on the altitude, and its 'species' is determined by the position of its base rather than its peak. Its color depends on the density of the water droplets or ice crystals making up the cloud and their ability to reflect or absorb light.
The cloud process does not end there, however. Clouds are rather volatile natural phenomena, and if the conditions are right - sufficient water vapor, for example, or an incoming cold or warm front - the clouds can generate storms, tornadoes, hurricanes and other such problems that plague the planet on a regular basis. Normally harmless clouds suddenly become harbingers of death and destruction for the planet below - though they also typically become dark and forbidding, giving people below time to prepare. Clouds may also simply disappear if the weather is warm enough to turn the water droplets or ice crystals back into water vapor.
One last thing to note: this whole process takes little time to complete. A previously cloudless day may be enshrouded in the things within hours as they either form or blow in from somewhere else. Consequently you can't expect one day to look like the last, not with any absolute certainty, as clouds are as unpredictable as nature itself.
Lloyd, Julie. A Pocket Guide to Weather. Parragon Books Ltd., 2007.
Whyzz.com - Why Do Clouds Disappear?
PhysicalGeography.net - Cloud Formation Process