The Standardised Precipitation Index (SPI)
It is commonly accepted that drought is a multi-scalar phenomenon. McKee et al. (1993) clearly illustrated this essential characteristic of droughts through consideration of usable water resources including soil moisture, ground water, snowpack, river discharges, and reservoir storages. The time period from the arrival of water inputs to availability of a given usable resource differs…
SPEI (Standardised Precipitation-Evapotranspiration Index) – definition and brief description
The SPEI fulfils the requirements of a drought index since its multi-scalar character enables it to be used by different scientific disciplines to detect, monitor and analyze droughts. The SPEI can measure drought severity according to its intensity and duration, and can identify the onset and end of drought episodes. The SPEI allows comparison of…
Undulatus clouds – definition
Clouds in patches, sheets or layers, showing undulations. These undulations may be observed in fairly uniform cloud layers or in clouds composed of elements, separate or merged. Sometimes a double system of undulations is in evidence. This term applies mainly to Cirrocumulus, Cirrostratus, Altocumulus, Altostratus, Stratocumulus and Stratus.
Opacus clouds – definition
An extensive cloud patch, sheet or layer, the greater part of which is sufficiently opaque to mask completely the Sun or Moon. This term applies to Altocumulus, Altostratus, Stratocumulus and Stratus. In Altocumulus, often, the base is even and its apparent subdivision into merged elements results from the irregularity of its upper surface, sometimes the…
Stratiformis clouds – definition
Stratiformis clouds is a cloud Species which spread out in an extensive horizontal sheet or layer of separate or merged elements. They are seen in Altocumulus, the most frequently occurring species, Stratocumulus and, occasionally, Cirrocumulus clouds.
Cirrostratus – definition
Cirrostratus are transparent, whitish cloud veil of fibrous (hair-like) or smooth appearance, totally or partly covering the sky, often producing halo phenomena.
Cumulus mediocris – definition
Cumulus mediocris are clouds of moderate vertical extent, with small protuberances and sproutings at their tops. Cumulus mediocris generally produce no precipitation.
Altocumulus clouds – definition
Altocumulus clouds are white, grey or both white and grey, they can be seen in patch, sheet or layer of cloud, generally with shading, composed of laminae, rounded masses, rolls, etc., which are sometimes partly fibrous or diffuse and which may or may not be merged; most of the regularly arranged small elements usually have…
Cumulonimbus – definition
Heavy and dense cloud, with a considerable vertical extent, in the form of a mountain or huge towers. At least part of its upper portion is usually smooth, or fibrous or striated, and nearly always flattened; this part often spreads out in the shape of an anvil or vast plume. Under the base of this…
Mammatus (Mamma) – definition
Mamma, also known as Mammatus, is a cloud with hanging protuberances, like udders, on the under surface of a cloud.