Fluffy spring clouds: The “fair-weather” cumulus

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As the days continue to lengthen, and the sun becomes stronger with the passing of every day, so the new life and regrowth of nature which began in the first days of spring gathers apace as we head into the summer.

From a meteorological perspective, it is perhaps the best time of year to look-up and appreciate once again our heavenly landscape, and observe what is indeed growing in the sky itself!

And there is one particular cloud that arises more commonly than others at this time of year – it is the ubiquitous cumulus, or “fluffy cotton wool” cloud (see photo).

Despite their beauty (or perhaps why we find them beautiful), cumulus clouds are very transient phenomena - they are born of warm, moist thermals rising into the atmosphere on a sunny morning.

Once arisen, the air parcels quickly saturate in the still-cold spring atmosphere. However, no sooner have they risen and saturated, they turn over upon themselves and sink back down into the atmosphere, evaporating quickly as they do so – with the result that each cumulus cloud may have a lifetime of only 5-10 minutes.

However, just as a Phoenix may arise from its ashes, so a new cumulus cloud will usually be born again a short distance away – and so the process is repeated many times over until late in the evening, just before the sun sets.

A keen observer of the sky will be able to view and capture such ephemeral movements, even without the use of time-lapse photography.

For centuries, mariners have used cumulus clouds as navigational guides – for they often lie above remote islands in Tropical Polynesia (Pacific Ocean) during daytime, revealing the presence of land for distances of up to one hundred miles or more away, even though actual surface of the island remains well hidden below the horizon.

In Scotland, cumulus clouds usually form over land only during the summer-half of the year, from approximately early March to late September, when the Sun’s rays are strongest.

Their close relative, the towering cumulonimbus (the biggest “grand-daddy” of all clouds) can be seen around our shores during most of the year, and are indeed most powerful during the winter months.

For April weather statistics see this week’s edition of the Stornoway Gazette out on May 8th.