What the Clouds Are Telling Us: A Little Guide to DIY Forecasts

The sky can tell us what the weather will be like in the next few hours, just knowing how to read the signs. Here is a small guide to replace the weather forecasting services (and maybe get it right)

  clouds-today-42 An altostratus. Clouds in this conformation herald bad weather to come

How many times have you felt fooled by weather apps? Did you leave the house with an umbrella, waiting for downpours that didn't arrive? Have you changed your plans to avoid showers that exist only in the forecasts of your mobile phone? Let's face it right away: meteorology is a complex science, and simplifications are difficult to reconcile with a chaotic system such as the atmospheric one. Predicting the weather beyond the next 48-72 hours is a chimera, as is the illusion of knowing precisely from an app, in a given location and hour by hour, if it will rain.

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DECISIVE IN SHORT-TERM FORECASTS - In this case it is much more useful to look up at the sky and observe the clouds carefully. Clouds, as they are more correctly called, are fundamental to the nowcasting , i.e. the very short-term forecasts. And even if no cloud is the same as another - ultimately it is hot and humid air that cools as it rises, forming masses of vapour, droplets and ice crystals, in continuous transformation - learning to recognize them gives us an idea of what is going to happen in terms of weather. We do it with the help of Vincenzo Levizzani, of the Institute of Atmospheric and Climate Sciences (Isac) of the Cnr, in Bologna. A physicist who has dedicated his life to the study of thunderstorms, and is the author of the Small manual for cloud seekers , out now for Il Saggiatore.

VERTICAL OR NOT - «The first thing is to understand if the clouds have an important vertical development, or not, explains Levizzani. «The other aspect to keep in mind is whether the clouds present themselves with horizontal stratification, and how marked it is. Finally, their color is decisive: white, light grey, dark gray or tending towards black». Here, it is advisable to start from this simple rule: the higher the cloud rises, with vertical growth, the greater the probability of rain in the immediate future. The towering clouds are called cumulonimbus, according to an English classification that dates back to the early 19th century and is still in use today.

THE DARKER THEY ARE THE EASIER IT TO RAIN - They are the storm clouds par excellence, and the darker their color – the dark reveals the presence of ice – the more imminent is the possibility of a downpour. To understand: they are piles of water and ice that reach heights of even more than 12,000 meters, pushed by hot and humid updrafts, until they collapse due to instability: and it is then that rain, snow and hail are generated. Cumulonimbus clouds not only herald impending thunderstorms, but also the danger of lightning. “A lot of ice forms inside these clouds, and it is the impact of water droplets on these icy structures that produces an electric charge,” continues the physicist. At that point it is absolutely necessary to avoid being outdoors, especially if in the vicinity of vertically developed objects (such as trees, umbrellas, lampposts), to seek shelter indoors, even in a car. A curiosity: the lightning you see doesn't come from the sky, but it's discharges that start from the ground and go towards the cloud. Only our eye cannot perceive it. Before becoming cumulonimbus clouds, the clouds can look like soft, white, very decorative cotton balls: they are cumulus clouds, low clouds that do not exceed 2,000 meters and rarely bring rain. Higher up, between 5 and 15,000 metres, you can see other clouds that form when the hot, humid air that rises and the turbulent currents at high altitude meet. They are cirrus clouds, which look like curls of hair, and cirrocumulus clouds, which have a plumper tuft shape, and they too are not associated with precipitation. So the proverb «sheep in the sky, buckets of water» is wrong? Not exactly, because 'these clouds are the indicator of an approaching cold air front and give us a clear indication of the rain that is coming in the next 8-10 hours', Levizzani points out again.

NEMBOSTRATI & CO. - If we lower our gaze, we can then encounter two types of clouds with horizontal stratification. Higher up there are the high layers with which the classic overcast sky manifests itself: uniform and wavy clouds, which can take on dramatic aspects, with bluish-grey colors, without however a drop of water falling. Closer to home you can instead see the nimbostratus, thick and very dark clouds, which bring snowfalls and even sustained rains, but not thunderstorms, hailstorms or lightning. And finally, always at low altitudes, there are the stratocumulus clouds, «large and rounded cloud masses, which cover a large part of the oceans, protecting us from excessive solar radiation», concludes Levizzani. They are the clouds of the so-called 'gloomy weather', but apart from a light drizzle they do not create any other problems.

Source: oggi.it