A cold February day in the northern hemisphere may produce clear blue skies, yet, the cold silent ambience is a portent for change in the weather. Cirrus clouds could generate cirrocumulus within a few hours or two days depending on wind speeds. Luke Howard, an English chemist devised the cloud classification in 1803 that remains the present day system:
- High Level Clouds, called cirriform, exist in the Troposphere's highest regions;
- Medium Level Clouds, called altiform, form halfway up the troposphere;
- Low Level Clouds, called stratiform, form in the lowest part of the troposphere.
Cirrus Clouds
High altitude clouds are composed of ice crystals or frozen water droplets in a featherlike or thin tufted transparent form. High altitude clouds have the word cirrus or prefix cirro incorporated into their nomenclature.
Cirrus clouds are white streaked, delicately fibrous bands of thin clouds, which stretch many kilometres across a blue sky. The 6,000m altitude where these clouds are formed, and where moisture availability is negligible, ensures these clouds are thin and lack shadows, which may be due to ice crystals falling into zones of varying wind directions. The wind shear gives the appearance of horses' tails. The clouds are generally shifted horizontally by the wind but still fall vertically and reach a level at which they evaporate.
Cirrus clouds generally indicate an advancing weather front and may also be generated by high altitude jet condensation trails. See attached photograph.
Cirrocumulus Clouds
Cirrocumulus clouds are generated by high level turbulence and are shaped into small whitish globular rolls which have a ripple-like appearance similar to the sand ripples observed on a beach. This pattern has been described as resembling the scales of a mackerel fish. Masses of cirrocumulus clouds are termed 'mackerel sky'.
Cirrocumulus are quite rare and generally occur in northern latitudes where they forecast the approach of a cold front. See attached photograph.
Cirrostratus Clouds
A thin whitish veil of high altitude cloud which allows the sun or moon to be seen and forms a halo around them. Often associated with an approaching warm front and predicting rain within 24 hours.
These clouds are difficult to photograph.
Weather Forecasting through Observing the Clouds
Most weather forecasts provide a regional forecast. A local forecast is gleaned from a cursory observation of the clouds - rain laden cumulonimbus clouds (nimbus = rain) indicate rain is imminent, how imminent depends on Irish Admiral Beaufort's wind speed scale. A 60kph wind speed indicates rain due at observer's location in 15 minutes. Take shelter!
Sources
- From Weather Vanes to Satellites, An Introduction to Meteorology by H. Spiegel and A. Gruber John Wiley and Sons 1983
- Usborne Spotter's Guides, Weather by Julie Tatchell Usborne Publishing 2006
- Clouds Cloud Formations and Weather Forecasting
Michael Manning - ' The best lack all conviction while the worst are full of passionate intensity' W.B.Yeats
