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Forecasting from the Clouds
They float majestically above our heads, but clouds can give the sailor plenty of information about the weather, including that which is likely to occur over the coming hours. Some of the names of clouds may be familiar to you: cumulus, cirrus, cumulonimbus. This nomenclature was first suggested by a young chemist called Luke Howard at a lecture entitled “On the Modification of Clouds”, in 1802. The story of how Howard named clouds is a fascinating one and is wonderfully described in a book by Richard Hamblyn entitled The Invention of Clouds.
Today we use many of the same names, with minor changes, as those suggested by Howard. Over the following pages we consider how clouds affect the weather and can help the sailor predict future conditions.
We can break down clouds into three main categories; high, medium and low:
• High clouds: above 18,000 ft
• Medium clouds: 6000 to 18,000 ft
• Low clouds: below 6000 ft
Clouds have been assigned official designations depending upon the height at which their bases occur:
• High clouds: cirrus, cirrostratus, cirrocumulus
• Medium clouds: altocumulus, altostratus, nimbostratus
• Low clouds: stratus, cumulus, stratocumulus, cumulonimbus
Nimbostratus is a strange cloud in that it falls between low and medium level clouds, as its base can occur at either of these levels. Whether its base is low or medium, however, nimbostratus always brings rain.
Let’s now look at each of the clouds mentioned above.
High Clouds: Cirrus
Cirrus clouds, sometimes known as mare’s tales, are the familiar high, wispy clouds we see on a fine summer day. They are easy to identify and occur mainly above 20,000 ft. Cirrus clouds are useful in identifying how mobile the weather situation is and are usually the first sign that a warm front is on its way.
Cirrus Indicating a Mobile, Changeable Situation
If the cirrus clouds are in long streaks, it may indicate a jet stream, in which case the weather is very mobile and likely to change. Should cirrus increase and thicken into cirrostratus, one might expect rain within 7 to 10 hours.
Cirrus Indicating Showers and Troughs
If cirrus is a long way in the distance, perhaps just on the horizon, watch carefully.
It may be that you are seeing the top of a large cumulonimbus cloud, which could
mean that a trough or thunderstorm is on the way (this is more relevant in the tropics).
Cirrus Indicating Little Change and Fine Weather
If cirrus is in thin wisps and alters little over a couple of hours, changes in the weather are likely to be slow, and fine weather will probably persist. Watch the barometer for falling pressure to indicate that the weather may be changing.
