Modern weather forecasts rely on complex computer simulators. These simulators use all the physics equations that describe the atmosphere, including the movement of air, the sun’s warmth, and the formation of clouds and rain.
But you don’t need a supercomputer to predict how the weather above your head is likely to change over the next few hours—this has been known across cultures for millennia. By keeping an eye on the skies above you, and knowing a little about how clouds form, you can predict whether rain is on the way.
And moreover, a little understanding of the physics behind cloud formationhighlights the complexity of the atmosphere, and sheds some light on why predicting the weather beyond a few days is such a challenging problem.
So here are six clouds to keep an eye out for, and how they can help you understand the weather.
On a sunny day, the sun’s radiation heats the land, which in turn heats the air just above it. This warmed air rises by convection and forms Cumulus. These “fair weather” clouds look like cotton wool. If you look at a sky filled with cumulus, you may notice they have flat bases, which all lie at the same level. At this height, air from ground level has cooled to the dew point. Cumulus clouds do not generally rain—you’re in for fine weather.
While small Cumulus do not rain, if you notice Cumulus getting larger and extending higher into the atmosphere, it’s a sign that intense rain is on the way. This is common in the summer, with morning Cumulus developing into deep Cumulonimbus(thunderstorm) clouds in the afternoon.
Cumulonimbus are often flat-topped. Within the Cumulonimbus, warm air rises by convection. In doing so, it gradually cools until it is the same temperature as the surrounding atmosphere. At this level, the air is no longer buoyant so cannot rise further. Instead it spreads out, forming a characteristic anvil shape.
But if you notice that Cirrus begins to cover more of the sky, and gets lower and thicker, this is a good indication that a warm front is approaching. In a warm front, a warm and a cold air mass meet. The lighter warm air is forced to rise over the cold air mass, leading to cloud formation. The lowering clouds indicate that the front is drawing near, giving a period of rain in the next 12 hours.
Our final two cloud types will not help you predict the coming weather, but they do give a glimpse of the extraordinarily complicated motions of the atmosphere. Smooth, lens-shaped Lenticular clouds form as air is blown up and over a mountain range.
And lastly, my personal favourite. The Kelvin-Helmholtz cloud resembles a breaking ocean wave. When air masses at different heights move horizontally with different speeds, the situation becomes unstable. The boundary between the air masses begins to ripple, eventually forming larger waves.